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Hugs 98 - Haskell

1. Note that you will need to use quotes around the prompt string if you want to include spaces or special characters Any s in the prompt will be replaced by the current module name The default prompt is s gt Set repeat string r string Hugs allows the user to recall the last expression entered into the interpreter by typing the characters as part of the next expression Prelude gt map 1 1 10 2 3 4 5 6 7 8 9 10 11 Prelude gt filter even 2 4 6 8 10 Prelude gt A r str option can be used to change the repeat string the symbol used to recall the last expression to str For example users of Standard ML might be more comfortable using Prelude gt set rit Prelude gt 6 7 42 Prelude gt it it 84 Prelude gt Another reason to change the repeat string is to avoid clashes with uses of the same symbol in a particular program for example if is defined as an operator in a program Note that the repeat string must be a valid Haskell identifier or symbol although it will always be parsed as an identifier If the repeat string is set to a value that is neither an identifier or symbol for example r0 then the repeat last expression facility will be disabled Set preprocessor string F cmd Although it is not needed very often there are sometimes applications where 24 it useful to arrange for input files to be preprocessed before they are p
2. ERROR Type error in application x expression sum a term gt 7a type Char does not match a Prelude gt sum 1 n ERROR Undefined variable n Prelude gt Another common problem occurs if there is no show function for the expression entered that is if its type is not an instance of the Show class For example suppose that a module defines a type T without a Show instance module Test where data T A B 27 With just these definitions any attempt to evaluate an expression of type T will cause an error Test gt A ERROR Cannot find show function for expression A of type ST Test gt To avoid problems like this you will need to add an instance of the Show class to your program One of the simplest ways to do that is to request a derived instance of Show as part of the datatype definition as in module Test where data T A B deriving Show Once this has been loaded Hugs will evaluate and display values of type T Test gt A A Test gt take 5 cycle A B A B A B A Test gt You should also note that the behaviour of the evaluator can be changed while the interpreter is running by using the set command to modify option settings View or change settings set options Without any arguments the set command displays a list of the options and their current settings The following output shows the settings on a typical machine Pre
3. Bibliography 1 2 10 11 R Bird and P Wadler Introduction to functional programming Prentice Hall 1988 K Chen P Hudak and M Odersky Parametric type classes extended abstract In ACM conference on LISP and Functional Programming San Francisco CA June 1992 B R Gaster and M P Jones A polymorphic type system for extensi ble records and variants Technical Report NOTTCS TR 96 3 Computer Science University of Nottingham November 1996 P Hudak and J Fasel A gentle introduction to Haskell ACM SIGPLAN No tices 27 5 May 1992 Also available as Research Report YALEU DCS RR 901 Yale University Department of Computer Science April 1992 G Hutton and E Meijer Monadic parser combinators Available from http www cs nott ac uk Department Staff gmh bib html 1996 M Jones The implementation of the Gofer functional programming sys tem Research Report YALEU DCS RR 1030 Yale University New Haven Connecticut USA May 1994 Available on the World Wide Web from http www cse ogi edu mpj pubs html M P Jones Simplifying and improving qualified types In International Conference on Functional Programming Languages and Computer Architec ture pages 160 169 June 1995 M P Jones Exploring the design space for type based implicit parameteri zation July 1999 S Peyton Jones and J Hughes editors Report on the Programming Lan guage Haskell 98 A Non strict Purely Functional L
4. Hugs 98 A functional programming system based on Haskell 98 User Manual Mark P Jones John C Peterson Oregon Graduate Institute Department of Computer Science P O Box 91000 Yale University Portland OR 97006 New Haven CT 06520 8285 USA USA Revised version September 10 1999 Contents IL Introductionl 1 2 A technical summary of Hugs 98 3 3 Hug beg 5 Ou oa ae Be oo ee eee eee eae 5 B2 OM Bo 4 Se ce So de hs eS a ED he eve BS 7 3 PROSTAMA 4 4 5 pias enh BORG a e a T 4 g Hug 10 4 nvironment optiong o a e a a 11 san gh eee eh Gah we A AG ke D hw eee 2 12 et search path 2 2 o 13 A e seno ae he Be oe ee Boe oe beac ee Yee 13 EPrni Staistled oo a ad eae dee RA A 14 Poke Ging Ge ees ee eee es oe 15 EA erro 4 ache aye ae RR RS 15 oad Rea ee Ale A te 16 E Literate MOMs eiii We REE we 17 Display dots while loading 2 ee 18 Display nothing while loading 004 19 EList fles loaded ys iria GH Be Ee Gare RG AS 19 E Detailed Kind errorg 26 ce ea ede ee es 19 r be AM ee A ee ee 20 ue he peek be ee ee ee 21 A ese gee o epee a ee St aces ee 23 geben Ax dee o Suk ts hae cds ee Ge BS 24 EDIL PAE eh ee he 24 el preprocessor string e 24 E Set constraint COLA a o A we A we e 25 5 Hugs commands 26 A i 42 002 dk oR Aare eed See a we 26 a ony es es ee eee ea 26 Bae ea ee se ae ee ace Bees 28 E
5. String gt a gt a traceShow trace Show a gt String gt a gt a 43 When called trace prints the string in its first argument and then returns the second argument as its result The traceShow function is a variant of trace that generates its output message by concatenating the supplied String argument with the result of applying show to its value argument These functions are not referentially transparent and should only be used for debugging or for monitoring execution You should also be warned that unless you understand some of the details about the way that Hugs programs are executed results obtained using trace can be rather con fusing For example the messages may not appear in the order that you expect Even ignoring the output that they produce adding calls to trace can change the semantics of your program Consider this a warning e Trex This library supports Trex extensible records These can only be used when Hugs is compiled with Trex support using the enable TREX configuration option Trex is described in more details in Section 7 2 e HugsInternals This library provides primitives for accessing Hugs inter nals for example they provide the means with which to implement sim ple error recovery and debugging facilities in Haskell They should be re garded as an experimental feature and may not be supported in future versions of Hugs They can only be used if hugs was configured with the enable inte
6. c 12 Int a True b Hello These examples show simple ways to construct record values We can also inspect the values held in a record using selector functions These are written with a character followed immediately by the name of a field For example Prelude gt a a True b Hello c 12 Int True Prelude gt b a True b Hello c 12 Int Hello Prelude gt c a True b Hello c 12 Prelude gt 12 Int Note howevever that there is a conflict here with the syntax of Haskell 98 that you should be aware of if you are running in Hugs mode with an infix operator and with support for records enabled Under these circumstances an expression of the form f g will parse as f g the application of a function f to a selector function tg and not as f g the application of an infix operator to two arguments f and g To obtain the second of these interpretations there must be at least one space between the and g tokens Record values can also be inspected by using pattern matching with a syntax that mirrors the notation used for constructing a record For example Prelude gt a x c y b _ gt y x a True b Hello c 12 Int 12 True Prelude gt The order of fields in a record pattern is significant because it determines the order from left to right in which they are matched In the following ex ample an attempt to match the pattern a x b True ag
7. BT B3 prompt 6 24 vi editor I 79
8. For example the following program module Main where data Tree a Leaf a Tree a Tree a type Example Tree Int Bool 19 will cause an error ERROR Main hs line 3 Illegal type Tree Int Bool in constructor application The problem here is that Tree is a unary constructor of kind gt but the definition of Example uses it as a binary constructor with at least two arguments and hence expecting a kind of the form gt gt k for some kind k By default Hugs reports problems like this with a simple message like the one shown above However if the k option is selected then the interpreter will print a more detailed version of the error message including details about the kinds of the type expressions that are involved ERROR Main hs line 3 Kind error in constructor application x expression Tree Int Bool constructor Tree kind OR gt xxx does not match gt a gt b In addition if the k option is used then Hugs will also include information about kinds in the information produced by the info command Prelude gt info Tree type constructor with kind gt data Tree a constructors Leaf a gt Tree a 7 Tree a gt Tree a gt Tree a instances instance Eval Tree a Prelude gt Use show to display results u u In normal use Hugs displays the value of each expression entered into the inter preter by ap
9. For the examples above we get Prelude gt set u f Prelude gt 1 div 0 value is bottom primDivInt 1 0 Prelude gt 1 div 0 2 primDiviInt 1 0 2 value is bottom 2 Prelude gt Reading an expression in braces as bottom the output produced here shows the correct values according to the semantics of Haskell Of course it is not possible to detect all occurrences of bottom like this such as those produced by a nonterminating computation Prelude gt last 1 C Interrupted nothing printed until interrupted Prelude gt Note that the basic method of evaluation is the same with both the f and f options all that changes is the way that the printing mechanism deals with certain kinds of runtime error Garbage collector notification g g It is sometimes useful to monitor uses of the garbage collector and to determine how many cells are recovered with each collection If the g option is set then the interpreter will print a message of the form Gc num each time that the garbage collector is invoked The number after the colon indicates the total number of cells that are recovered 16 As a simple application we can use garbage collector messages to observe that an attempt to sum an infinite list although non terminating will at least run in constant space Prelude gt set g Prelude gt sum 1 Gc 95763 Gc 95760 Gc 95760 Gc 95760 Interru
10. use project file edit file edit last module set module for evaluating expressions evaluate expression print type of expression display this list of commands set command line options help on command line options list names currently in scope describe named objects browse names defined in lt modules gt edit module containing definition of name shell escape change directory force garbage collection print Hugs version exit Hugs interpreter Change module module module A module module command changes the current module to one given by module This is the module in which evaluation takes place and in which ob jects named in commands are resolved The specified module must be part of the current program If no module is specified then the last module to be loaded is assumed Note that the name of the current module is usually displayed as part of the Hugs prompt cd directory Change directory A cd dir command changes the current working directory to the path given by dir If no path is specified then the command is ignored 30 Force a garbage collection gc A gc command can be used to force a garbage collection of the interpreter heap and to print the number of unused cells obtained as a result Prelude gt gc Garbage collection recovered 95766 cells Prelude gt Exit the interpreter Quit The quit command terminates the current Hugs
11. 4 or earlier will need only minor modifications before they can be loaded and used from Hugs in Haskell 98 mode Note however that some of the demo programs included in the standard Hugs distribution will not work in Haskell 98 mode e Hugs mode This enables a number of advanced Hugs features such as type system extensions restricted type synonyms etc Most of these fea tures are described in more detail in the following sections The underlying core language remains as in Haskell 98 mode For example the member function of the Functor class is still called fmap there is no Eval class fixity declarations can appear anywhere that a type signature is permitted comprehension syntax is still restricted to lists and so on The choice between the two modes is made when the interpreter is started and it is by design not possible to change mode without exiting and restarting Hugs The default mode is usually Haskell 98 this can also be set explicitly by starting Hugs with the command line option 98 To select the Hugs mode you should start the interpreter with the command line option 98 The mode in which the interpreter is running is displayed as part of the startup banner and is also included in the information produced by using the set command without any arguments The intention here is that beginners will get Haskell 98 mode by default while more experienced users will be able to set up alias batch or script files or file associati
12. Haskell 98 the de facto standard for non strict functional programming languages This manual should give you all the information that you need to start using Hugs However it is not a tutorial on either functional programming in general or on Haskell in particular The first two sections provide introductory material e Section PF A brief technical summary of the main features of Hugs 98 and the ways that it differs from previous releases e Section B A short tutorial on the concepts that you need to understand to be able to use Hugs The remaining sections provide reference material including e Section Ml A summary of the command line syntax environment variables and command line options used by Hugs e Section B A summary of commands that can be used within the interpreter e Section 6 An overview of the Hugs libraries e Section A description of Hugs extensions e Section Information about other ways of running Hugs programs e Section J A list of differences between Hugs 98 and standard Haskell e Section LO Pointers to further information Whether you are a beginner or a seasoned old timer we hope that you will enjoy working with Hugs and that if you will pardon the pun you will use it to embrace functional programming Acknowledgements The development of Hugs has benefited considerably from the feedback suggestions and bug reports provided by its users There are too many people to name here but thanks are due f
13. Or to put it an other way the type relations that are defined by a collection of class and instance declarations are often too general for practical applications where programmers might expect stronger dependencies between parameters In the rest of this sec tion we will describe these problems in more detail We will also describe the mechanisms introduced in the September 1999 release of Hugs that allow pro grammers to declare explicit dependencies between parameters avoiding these difficulties in many cases and making multiple parameter classes more useful for some important practical applications Ambiguity problems During the past ten years many Haskell users have looked into the possibility of building a library for collection types using a multiple parameter type class that looks something like the following class Collects e ce where empty ce insert e gt ce gt ce member e gt ce gt Bool The type variable e used here represents the element type while ce is the type of the container itself Within this framework we might want to define instances of this class for lists or characteristic functions both of which can be used to rep 46 resent collections of any equality type bit sets which can be used to represent collections of characters or hash tables which can be used to represent any col lection whose elements have a hash function Omitting standard implementation details this would lead
14. Shell escap i ca mos peene eu ea e k k EG e E do ai 29 EList commands obesas abra dere 30 ange directory orce a garbage collection 2 22 004 xit the interpreter e soe a reaa kpag b oading and editing modules and projects Print type of expression 0 0 0 0 o Display information about nameg Display names defined in modules 008 6 library overvie l standard Libraries 0 4 Hugs specific Libraries oaoa a a o An overview of Hugs extensions pe class extensiong oaoa o ea a a a a 1 Multiple parameter classes SL Overlapping Instances ee 2 PASIG GONCE PU uv ae e a a A Gwe Other type system extensiong 00004 3 nhanced polymorphic recursion LO pe annotations in patterns 10 4 xistential types 2 a a a LO Restricted type Synonyms 84 J mplicit parameters Ss O Hugs prog il 40 40 40 41 43 45 46 46 ol 52 53 53 54 57 60 60 60 63 64 65 67 69 Hue W 111 71 71 12 73 74 TT Copyright and License The Hugs 98 system is Copyright Mark P Jones Alastair Reid the Yale Haskell Group and the Oregon Graduate Institute of Science and Technology 1994 1999 All rights reserved It is distributed as free software under the following license Redistribution and use in source and binary
15. are neither polymorphic or recursive 68 8 Other Hugs programs The Hugs interpreter is available in two other guises a stand alone system that executes programs in a load and go style without the surrounding command system and a Windows user interface layered on top of the basic Hugs system 8 1 Stand alone program execution Once a program has been developed and debugged the Hugs command loop can be eliminated and the program can be executed immediately without any com mand to run it A slightly modified version of the interpreter called runhugs loads the literate program specified as its first argument and runs main in module Main Unlike the standard Hugs system runhugs makes command arguments available to the running Hugs system The first argument specifying the program is removed from the argument list On Unix systems executable programs may be created by placing runhugs in the first line of an executable file like so hugs runhugs gt module Main where gt main putStr Hello World n Because runHugs uses literate Haskell only the line starting with is viewed as a comment Stand alone programs can import other modules using import chasing these modules need not be literate The runhugs program uses the same environment variables to set Hugs options as the standard Hugs systems However runhugs does not set options from the command line all command line options are passed into the executing Hug
16. because the type of f does not allow the two arguments to have different types This then is an example of a multiple parameter class that does actually work quite well in practice without ambiguity problems There is however a catch This version of the Collects class is nowhere near as general as the original class seemed to be only one of the four instances in Section 7 1 1 can be used with this version of Collects because only one of them the instance for lists has a collection type that can be written in the form c e for some type constructor c and element type e Adding dependencies To get a more useful version of the Collects class Hugs provides a mechanism that allows programmers to specify dependencies between the parameters of a multiple parameter classl lFor readers with an interest in theoretical foundations and previous work The use of dependency information can be seen both as a generalization of the proposal for parametric 48 To start with an abstract example consider a declaration such as class C a b where which tells us simply that C can be thought of as a binary relation on types or type constructors depending on the kinds of a and b Extra clauses can be included in the definition of classes to add information about dependencies between parameters as in the following examples class Dab a gt b where class Ea b a gt b b gt a where The notation a gt b used
17. definition of the corresponding datatype definition e If the name represents an internal Hugs function then the cursor will be positioned at the beginning of the standard prelude file Note that names of infix operators should be given without any enclosing them in parentheses Thus f starts an editor on the standard prelude at the first line in the definition of If a given name could be interpreted both as a type constructor and as a value constructor then the former is assumed 5 3 Finding information about the system List names names pattern The names command can be used to list the names of variables and functions whose definitions are currently loaded into the interpreter Without any argu ments names produces a list of all names known to the system the names are listed in alphabetical order The names command can also accept one or more pattern strings limiting the list of names that will be printed to those matching one or more of the given pattern strings Prelude gt n fold foldl foldl foldl11 foldr foldri 5 names listed Prelude gt Each pattern string consists of a string of characters and may use standard wild card syntax matches anything matches any single character c matches exactly the character c and ranges of characters of the form a zA Z etc For example Prelude gt n map Fflile lt gt appendFile map mapM mapM_ readFile writ
18. each one are fixed and there is no apparent relationship between records of different type In fact all record values and record types in Trex are built up incrementally starting from an empty record and extending it with additional fields one at a time It is for this reason that Trex values are often referred to as extensible records In the simplest case any given record r can be extended with a new field labelled 1 provided that r does not already include an 1 field For example we can con struct a True b Hello by extending a True with a field b Hello Trex gt b Hello a True a True b Hello Trex gt Alternatively we can construct the same result by extending b Hello with a field a True Trex gt a True b Hello a True b Hello Trex gt The syntax of the current implementation allows us to add several new fields at a time the corresponding syntax for pattern matching is also supported Trex gt a True b Hello c 12 Int b1 World a True b Hello bi World c 12 Trex gt On the other hand a record cannot be extended with a field of the same name even if it has a different type The following examples illustrate this Trex gt a True a False ERROR Repeated label a in record a True a False Trex gt a True r where r a 12 Int ERROR a Int already includes a a field Trex gt Notice that Hugs produced two different kinds of
19. error message here In the first case the presence of a repeated label was detected syntactically In the 57 second example the problem was detected using information about the type of the record r Much the same syntax can be used in patterns to decompose record values Trex gt b bval r gt bval r a True b Hello Hello a True Trex gt In the previous examples we saw how a record could be extended with new fields As this example shows we can use pattern matching to do the reverse operation removing fields from a record We can also use pattern matching to understand how selector functions like a b and so on are implemented For example the selector x is equivalent to the function x value _ gt value A selector function like this is poly morphic in the sense that it can be used with any record containing an x field regardless of the type associated with that particular component or of any other fields that the record might contain Trex gt x value _ gt value x True b Hello True Trex gt x value _ gt value name Hugs age 2 x None None Trex gt To understand how this works it is useful to look at the type that Hugs assigns to this particular selector function Trex gt type x value _ gt value x value _ gt value r x gt Rec x a r gt a Trex gt There are two important pieces of notation here that deserve further
20. explanation e Rec x a r is the type of a record with an x component of type a The row variable r represents the rest of the row that is it represents any other fields in the record apart from x This syntax for record type extension was chosen to mirror the syntax that we have already seen in the examples above for record value extension e The constraint r x tells us that the type on the right of the gt symbol is only valid if r lacks x that is if r is a row that does not contain an x field If you are already familiar with Haskell type classes then you may like to think of x as a kind of class constraint written with postfix syntax whose instances are precisely the rows without an x field 58 For example if we apply our selector function to a record x True b Hello of type Rec b String x Bool then we instantiate the variables a and r in the type above to Bool and b String respectively In fact the built in selector functions have exactly the same type as the user defined selector shown above Prelude gt type x tx bx gt Rec x a b gt a Prelude gt The row constraints that we see here can also occur in the type of any func tion that operates on record values if the types of those records are not fully determined at compile time For example given the following definition average r tx r y r 2 Hugs infers a principal type of the form average Fractional a b
21. file has been reloaded then we can use the comb function like any other built in operator Fact gt reload Reading file fact hs Hugs session for Hugs lib Prelude hs Fact hs Fact gt comb 5 2 10 Fact gt 4 Starting Hugs On Unix machines the Hugs interpreter is usually started with a command line of the form hugs option file On Windows 95 NT Hugs may be started by selecting it from the start menu or by double clicking on a file with the hs or 1hs extension This manual assumes that Hugs has already been successfully installed on your system Hugs uses options to set system parameters These options are distinguished by a leading or and are used to customize the behaviour of the interpreter When Hugs starts the interpreter performs the following tasks e Options in the environment are processed The variable HUGSFLAGS holds these options On Windows 95 NT the registry is also queried for Hugs option settings e Command line options are processed e Internal data structures are initialized In particular the heap is initialized and its size is fixed at this point if you want to run the interpreter with a heap size other than the default then this must be specified using options on the command line in the environment or in the registry e The prelude file is loaded The interpreter will look for the prelude file on the path specified by the P option If the prelude located in the file Prel
22. forms with or without modification are permitted provided that the following conditions are met Redistributions of source code must retain the above copyright notice this list of conditions and the following disclaimer Redistributions in binary form must reproduce the above copyright notice this list of conditions and the following disclaimer in the documentation and or other materials provided with the distribution Neither name of the copyright holders nor the names of its contributors may be used to endorse or promote products derived from this software without specific prior written permission THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND THE CONTRIBUTORS AS IS AND ANY EXPRESS OR IMPLIED WAR RANTIES INCLUDING BUT NOT LIMITED TO THE IMPLIED WAR RANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED IN NO EVENT SHALL THE COPYRIGHT HOLDERS OR THE CONTRIBUTORS BE LIABLE FOR ANY DIRECT IN DIRECT INCIDENTAL SPECIAL EXEMPLARY OR CONSEQUENTIAL DAMAGES INCLUDING BUT NOT LIMITED TO PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES LOSS OF USE DATA OR PROFITS OR BUSINESS INTERRUPTION HOWEVER CAUSED AND ON ANY THE ORY OF LIABILITY WHETHER IN CONTRACT STRICT LIABILITY OR TORT INCLUDING NEGLIGENCE OR OTHERWISE ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE 1 Introduction Hugs 98 is a functional programming system based on
23. here between the and where symbols not to be con fused with a function type indicates that the a parameter uniquely determines the b parameter and might be read as a determines b Thus D is not just a relation but actually a partial function Similarly from the two dependencies that are included in the definition of E we can see that E represents a partial one one mapping between types More generally dependencies take the form x1 xn gt y1 ym where x1 Xn and y1 yn are type variables with n gt 0 and m gt 0 meaning that the y parameters are uniquely determined by the x parameters Spaces can be used as separators if more than one variable appears on any single side of a dependency asint gt a b Note that a class may be annotated with multiple dependencies using commas as separators as in the definition of E above Some dependencies that we can write in this notation are redundant and will be rejected by Hugs because they don t serve any useful purpose and may instead indicate an error in the program Examples of dependencies like this include a gt a a gt a a a gt etc There can also be some redundancy if multiple dependencies are given as in a gt b b gt c a gt c and in which some subset implies the remaining depen dencies Examples like this are not treated as errors Note that dependencies appear only in class declarations and not in any other part of the language In particul
24. hugs One advantage of this approach is that individual users do not have to worry about setting the environment variables themselves In addition to 11 the E and P options other options such as s in this example can be set It is easy for more advanced users to copy and customize a script like this to suit their own needs e Users of DOS or Windows 3 1 might add the following line to autoexec bat set HUGSFLAGS P hugs lib hugs libhugs E vi d 4s The setting for the path assumes that the system has been installed in a top level hugs directory and will need to be modified accordingly if a different directory was chosen In a similar way the setting for the editor will only work if you have installed the editor program in this case vi that it refers to e On Windows 95 NT the setup program initializes the environment and this can be changed subsequently on these systems only by using either the set command or a registry editor The InstallShield script that performs the installation initializes the path using the installation directory other directories can be added using P Installed options are stored under the HKEY_LOCAL_MACHINE key changes to these options using set are placed under HKEY_CURRENT_USER so that different users do not alter each other s options For completeness we should also mention the other environment variables that are used by Hugs e The SHELL variable on a Unix machine or the CO
25. modules that includes facilities for Win32 programming e User interface enhancements particularly the import chasing and search path features which were motivated by a greater emphasis on the role of libraries in Haskell 1 3 and later versions of the language e Small improvements in runtime performance and more reliable space usage thanks to the use of non conservative garbage collection during program execution e A graphical user interface for the Hugs systems that runs on the Windows operating system There have also been a number of other enhancements and fixes for bugs in previous releases 3 Hugs for beginners This section covers the basics that you need to understand to start using Hugs Most of the points discussed here will be familiar to anyone with experience of previous versions of Hugs or Gofer To begin with we need to start the inter preter the usual way to do this is by using the command hugs which produces a startup banner something like the following TEISS Hugs 98 Based on the Haskell 98 standard CS IiE I A Copyright c 1994 1999 1 1 ___ World Wide Web http haskell org hugs Report bugs to hugs bugsChaskell org Version September 1999 __ Haskell 98 mode Restart with command line option 98 to enable extensions Reading file Hugs lib Prelude hs Hugs session for Hugs lib Prelude hs Type for help Prelude gt The file Hugs 1ib Prelude hs mentioned here contains s
26. that dependencies between modules are documented within individual modules leaving the system free to determine the order in which the files should be loaded For example if the Main module in the exam ple above actually needs the definitions in Types and Basics then this will be documented by import statements and the whole program could be loaded with a single load Main command 33 Edit file edit file The edit command starts an editor program to modify or view a module file On Windows 95 NT the editor and interpreter are executed as independent processes On other systems the current Hugs session will be suspended while the editor is running Then when the editor terminates the Hugs session will be resumed and any files that have been changed will be reloaded automatically The E option should be used to configure Hugs to your preferred choice of editor If no filename is specified then Hugs uses the name of the last file that it tried to load This allows the edit command to integrate smoothly with the facilities for loading files For example suppose that you want to load four files f1 hs 2 hs 3 hs and f4 hs into the interpreter but the file 3 hs contains an error of some kind If you give the command load f1 f2 f3 f4 then Hugs will successfully load f1 hs and f2 hs but will abort the load com mand when it encounters the error in f3 hs printing an error message to describe the problem th
27. the system carries on and loads the module as normal On the other hand if the module includes import statements for modules that have not already been loaded then the interpreter postpones the task of reading the current module until all of the specified imports have been successfully loaded This explains why Demo hs and STArray hs are read twice in the example above first to determine which imports are required and then to read in the rest of the file once the necessary imports have been loaded The list of directories and filenames that Hugs tries in an attempt to locate the source for a module Mod named in an import statement can be specified by dir Mod suf dir lt a path 07 suf lt pr hs Ths The search starts in the directory d where the file containing the import statement was found then tries each of the directories in the current path as defined by the P option represented here by path and ends with which gives a search relative to the current directory The fact that the search starts in d is particularly important because it means that you can load a multi file program into Hugs without having to change to the directory where its source code is located For example suppose that tmp contains the files A hs B hs and C hs that B imports A and that C imports B Now regardless of the current working directory you can load the whole program with the command load tmp C the import in C wi
28. this gives more general types and potentially more efficient handling of overloading 7 2 Extensible records Trex Hugs supports a flexible system of extensible records sometimes referred to as Trex The theoretical foundations for this and a comparison with related work is provided in a report by Gaster and Jones B This section provides some background details for anybody wishing to experiment with the implementation of extensible records that is supported in the current distribution of Hugs Please note that support for this extension in any particular build of the Hugs system is determined by a compile time setting If the version of Hugs that you are using was built without including support for extensible records then you will not be able to use the features described here The current implementation does not use our prefered syntax for record opera tions too many of the symbols that we would like to have used are already used in conflicting ways elsewhere in the syntax of Haskell 98 53 7 2 1 Basic concepts In essence records are just collections of values each of which is associated with a particular label For example a True b Hello c 12 Int is a record with three components an a field containing a boolean value a b field containing a string and a c field containing the number 12 The order in which the fields are listed is not significant so the same record value could also be written as
29. type appears recursively on the left side of the constructor then the read instance will loop Hugs does not allow the use of qualified names in instance declarations Hugs does not use the Unicode character set yet Characters are currently drawn from the ISO Latin 1 set Two adjacent dashes start a one line comment for strictly technical reasons the change from Haskell 1 4 to Haskell 98 to use maximal munch for such comments has not yet been implemented The floating point printer is not exactly as defined in the report The printed form of a floating point number may re read as a slightly different number 71 e Derived instances for large tuples are not supplied Instances for tuples larger than 5 3 in the 16 bit PC system are not in the Prelude e When using getArgs only the stand alone system passes arguments to the executing program The interactive system always uses an empty argument list when runnning a program e The syntax of sections is slightly different For example the Haskell ex pression 2 3 must instead be written as 2 3 e Instead of 10 hIsEOF hugs provides 10 hugsHIsEOF Whereas hIsEOF should tell you if the next call of hGetChar would raise an EOF error hugsHIsEOF tells you if the last call of hGetCHar raised an EOF error the same as ANSI C s feof e We ignore entity lists in qualified imports but unqualified imports are treated correctly For example you can write import qualifi
30. we can define instances such as instance Eq a gt Eq Stack a where isEmpty s1 isEmpty s2 isEmpty s2 isEmpty sl otherwise top si top s2 amp amp pop s1 pop s2 si s2 As a convenience Hugs allows the type signatures of functions mentioned in the 66 type synonym declaration to be specified within the definition Thus the above example could also have been written as type Stack a a in emptyStack Stack a push a gt Stack a gt Stack a pop Stack a gt Stack a top Stack a gt a isEmpty Stack a gt Bool emptyStack If a type signature is included as part of the definition of a restricted type syn onym then the declaration should not be repeated elsewhere in the module Hugs will reject any attempt to do this by complaining about a repeated type signature 7 4 Implicit parameters Hugs supports an experimental implementation of Implicit Parameters which provides a technique for introducing dynamic binding of variables into a language with a Hindley Milner based type system This is based on as yet unpublished work by Jeff Lewis Erik Meijer and Mark Shields The prototype implementa tion and much of the following description was provided by Jeff Lewis A variable is called dynamically bound when it is bound by the calling context of a function and statically bound when bound by the callee s context In Haskell all variables are statically bound Dynamic b
31. y b x gt Rec y a x a b gt a However any of the following more specific types could be specified in a type declaration for the average function average Fractional a gt Rec x a y a gt a average rx r y gt Rec x Double y Double r gt Double average Rec x Double y Double gt Double average Rec x Double y Double z Bool gt Double Each of these types is an instance of the principal type given above These examples show an important difference between the system of records described here and the record facilities provided by SML In particular SML prohibits definitions that involve records for which the complete set of fields cannot be determined at compile time So the SML equivalent of the average function described above would be rejected because there is no way to determine if the record r will have any fields other than x or y SML programmers usually avoid such problems by giving a type annotation that completely specifies the structure of the record But of course if a definition is limited in this way then it also less flexible With the current implementation of our type system there is an advantage to knowing the full type of a record at compile time because it allows the compiler to generate more efficient code However unlike SML the type system also offers the extra flexibility of polymorphism and extensibility over records if that is needed 99 7 3
32. 0 represents the list of integers between 1 and 10 inclusive and sum is a prelude function that calculates the sum of a list of numbers So the result obtained by Hugs is 1 2 3 4 5 6 7 8 9 10 55 In fact we could have typed this sum directly into Hugs Prelude gt 1 2 3 4 4 5 6 7 8 9 10 55 Prelude gt Unlike many calculators however Hugs is not limited to working with num bers expressions can involve many different types of value including numbers booleans characters strings lists functions and user defined datatypes Some of these are illustrated in the following example Prelude gt not True False False Prelude gt reverse Hugs is cool looc si sguH Prelude gt filter even 1 10 2 4 6 8 10 Prelude gt take 10 fibs where fibs 0 1 zipWith fibs tail fibs 0 1 1 2 3 5 8 13 21 34 Prelude gt You cannot create new definitions at the command prompt these must be placed in files and loaded as described later The definition of fib in the last example above is local to that expression and will not be remembered for later use Also the expressions entered must fit on a single line Hugs even allows whole programs to be used as values in calculations For exam ple putStr hello is a simple program that outputs the string hello Combining this with a similar program to print the string world gives Prelude gt putStr hello gt gt putStr world hello world
33. Bool c Collects Char c gt c gt c Notice that the type for f allows the two parameters x and y to be assigned different types even though it attempts to insert each of the two values one after the other into the same collection If we re trying to model collections that contain only one type of value then this is clearly an inaccurate type Worse still the definition for g is accepted without causing a type error As a result the error in this code will not be flagged at the point where it appears Instead 47 it will show up only when we try to use g which might even be in a different module An attempt to use constructor classes Faced with the problems described above some Haskell programmers might be tempted to use something like the following version of the class declaration class Collects e c where empty ce insert e gt ce gt ce member e gt c e gt Bool The key difference here is that we abstract over the type constructor c that is used to form the collection type c e and not over that collection type itself represented by ce in the original class declaration This avoids the immediate problems that we mentioned above e empty has type Collects e c gt c e which is not ambiguous e The function f from the previous section has a more accurate type f Collects e c gt e gt e gt ce gt ce e The function g from the previous section is now rejected with a type error as we would hope
34. Ge ahi te bee Viele Hugs session for Hugs lib Prelude hs Hugs lib Array hs Prelude gt This setting can also be useful on very slow machines where the growing line of dots provides confirmation that the interpreter is making progress through the various stages involved in loading a file You should note however that the mechanisms used to display the rows of dots can add a substantial overhead to the time that it takes to load files in one experiment a particular program took nearly five times longer to load when the option was used In this case users might prefer to use the q option described below Display nothing while loading q q The q quiet option supresses the messages used to indicate progress while Hugs is loading files If this option is turned off using the q then the format of output messages is determined by the current or setting List files loaded tw W By default Hugs prints a complete list of all the files that have been loaded into the system after every successful load or reload command The w option can be used to turn this feature off Note that the info command without any arguments can also be used to list the names of currently loaded files Detailed kind errors k k Haskell uses a system of kinds to ensure that type expressions are well formed for example to make sure that each type constructor is applied to the appropriate number of arguments
35. Haskell 98 mode the type for runST requires support for rank 2 polymorphism which is only available in Hugs mode See Section for further details The specifications and implementations of all of these libraries are still evolving and are subject to change 6 3 Portable Libraries These libraries are not part of the Haskell standard but can be ported to most Haskell systems e ListUtils This module provides list functions that were removed from the Prelude in the move from Haskell 1 2 to Haskell 1 3 module ListUtils where sums products Num a gt a gt a subsequences a gt a permutations a gt a e ParseLib This module provides a library of parser combinators as de scribed in the paper on Monadic Parser Combinators by Graham Hutton and Erik Meijer 5 e Interact This library provides facilities for writing simple interactive pro grams module Interact where type Interact String gt String end Interact readChar peekChar Interact gt Char gt Interact gt Interact pressAnyKey Interact gt Interact unreadChar Char gt Interact gt Interact writeChar Char gt Interact gt Interact writeStr String gt Interact gt Interact ringBell Interact gt Interact readLine String gt String gt Interact gt Interact An expression e of type Interact can be executed as a program by evalu ating run e 41 e AnsiScreen This librar
36. MSPEC variable on a DOS machine determines which shell is used by the command e The EDITOR variable is used to try and locate an editor if no editor option has been set Note however that this variable does not normally provide the extra information that is needed to be able to start the editor at a specific line in the input file 4 2 Options The behaviour of the interpreter particularly the read eval print loop can be customized using options For example you might use hugs i g h30K to start the interpreter with the i option import chasing disabled the g option garbage collector messages enabled and with a heap of thirty thousand cells 12 As this example suggests many of the options are toggles meaning that they can either be switched on by preceding the option with a character or off by using a character Options may also be grouped together For example hugs stf le is equivalent to hugs s t f 1 e Option settings can be specified in a number of different ways the HUGSFLAGS environment variable the Windows registry the command line and the set command but the same syntax is used in each case To avoid any confusion with filenames entered on the command line option settings must always begin with a leading or character However in some cases the h p r P and E options the choice is not significant With the exception of the heap size option h all options can be changed whil
37. Other type system extensions In this section we describe several other type system extensions that are currently available in Hugs mode 7 3 1 Enhanced polymorphic recursion As required by the Haskell 98 report Hugs supports full polymorphic recursion even for functions with overloaded types This means that Hugs will accept definitions like the following p Eq a gt a gt Bool p x x x amp amp p x Note that the type signature here is not optional In fact Hugs goes further than is implied by the Haskell 98 report by using programmer supplied type signatures to reduce type checking dependencies within individual binding groups For example the following definitions are acceptable even though there is no explicit type signature for the function q p Eq a gt a gt Bool p x x x amp amp q x q x x x amp p x This is made possible by the observation that we can calculate a type for q without needing to calculate the type of p at the same time because the type of p is already specified 7 3 2 Rank 2 polymorphism Hugs provides a facility that allows the definition of functions that take poly morphic arguments This includes functions defined at the top level in local definitions in class members and in primitive declarations In addition Hugs allows the definition of datatypes with polymorphic and qualified types The following examples illustrate the syntax that is used amazed fora
38. Prelude gt Just as there are standard operations for dealing with numbers so there are standard operations for dealing with programs For example the gt gt operator used here constructs a new program from the programs supplied as its operands running one after the other Normally Hugs just prints the value of each ex pression entered But as this example shows if the expression evaluates to a program then Hugs will run it instead Hugs distinguishes programs from other expressions by looking at the type of the expression entered For example the expression putStr world has type IO which identifies it as a program to be executed rather than a value to be printed 3 2 Commands Each line that you enter in response to the Hugs prompt is treated as a command to the interpreter For example when you enter an expression into Hugs it is treated as a command to evaluate that expression and to display the result There are two commands that are particularly worth remembering e q exits the interpreter On most systems you can also terminate Hugs by typing the end of file character e prints a list of all the commands which can be useful if you forget the name of the command that you want to use Like most other commands in Hugs these commands both start with a colon The full set of Hugs commands is described in Section Bl Note that the interrupt key control C or control Break on most systems can be used to abandon
39. Trex Trex gt a True b Hello c 12 Int a True b Hello c 12 Trex gt c 12 Int a True b Hello a True b Hello c 12 Trex gt Note that the fields are always displayed with their labels in alphabetical order The fact that the fields appear in a specific but frankly arbitrary order is very important show is a normal function so its output must be uniquely determined by its input and not by the way in which that input value is written The records 59 used in the example above have exactly the same value so we expect exactly the same output for each In a similar way it is sometimes useful to test whether two records are equal by using the operator Any program that requires this feature can obtain the necessary instances of the Eq class by importing the Trex library as shown above Of course like all other values in Haskell records have types and these are written using expressions of the form Rec r where Rec is a built in type constructor and r represents a row that associates labels with types For example Trex gt t c 12 Int a True b Hello a True b Hello c 12 Rec a Bool b Char c Int Trex gt The type here tells us unsurprisingly that the record a True b Hello c 12 has three components an a field containing a Bool a b field containing a String and a c field of type Int As with record values themselves the order of the components in a
40. a gt a gt Appl Because the variable a does not appear in the result type the choice of a in any particular use of MkApp1 will be hidden As a result when a MkApp1 constructor is used in a pattern match we must be careful that the hidden type does not escape into the result type or into the enclosing assumptions For example the following definitions are acceptable good1 MkAppl f x i f x good2 MkAppl f x i map f iterate i x but the next two definitions are not badi MkAppl f x i x bad3 y let g MkAppl f x i length x y 1 in True The facilities for type annotations in patterns that were described in Section 7 3 3 can be used in conjunction with existentials as in the example good MkAppl f x a i map f iterate i x a In this case the typing annotations are redundant although they do still provide potentially useful information for the programmer 64 A datatype whose definition involves existentially quantified variables cannot use the standard Haskell mechanisms for deriving instances of standard classes like Eq and Show If instances of these classes are required then they must be provided explicitly by the programmer It is possible however to attach type class constraints to existentially quantified variables in a datatype definition For example we can define a type of show able values using the definition data Showable forall a Show a gt MkShowable a This w
41. a and so the type assigned to g is actually monomorphic e Type signatures do not introduce bindings for type variables but may in volve type variables bound in an enclosing scope For example there is no direct relation between the variable t appearing in the type signature and the variable t appearing in the pattern annotation in the following code pair 2 t gt s gt t s pair x y t x y t 63 The explanation for this is that the type signature for pair which might in practice be separated from the definition is not in the scope of the binding of the variables x and y e In the current implementation pattern type annotations that include vari ables are allowed on the left hand side of a pattern binding but scope only over the right hand side of the binding 7 3 4 Existential types Hugs supports a form of existential types in datatype definitions in the style originally suggested by Perry and by Laufer Existentially quantified type vari ables must be bound by an explicit forall construct preceding the name of the constructor in which the existentially quantified variables appear The appar ently counterintuitive use of forall to capture existentially quantified variables becomes clearer when we look at an example data Appl forall a MkAppl a gt Int a a gt a and consider that the MkApp1 constructor defined here does indeed have a fully polymorphic type MkAppl a gt Int gt a gt
42. ad1 and listMonad2 have types Monad1 Monad2 Note that an expression like MkMonad2 x gt x will not be allowed because by the rules above the constructor MkMonad2 can only be used when both argu ments are provided An attempt to correct this problem by eta expansion such as b gt MkMonad2 x gt x b will also fail because the new variable b that this introduces is now lambda bound and hence the type that we obtain for it will not be as general as the MkMonad2 constructor requires We can however use an auxiliary function with an explicit type signature to achieve the desired effect halfListMonad forall a b a gt a gt b gt b gt Monad2 halfListMonad b MkMonad2 x gt x b In the current implementation the named update syntax for Haskell datatypes in expressions like exp field newValue cannot be used with datatypes that include polymorphic components The runST primitive that is used in work with lazy state threads is now handled using the facilities described here to define it as a function runST forall s STs a gt a 62 As a result it is no longer necessary to build the ST type into the interpreter to make use of these facilities a program should instead import the ST library or it s lazier variant LazyST A further consequence of this is that the ST and LazyST libraries cannot be used when Hugs is running in Haskell 98 mode because that prevents the
43. ainst the record 54 b undefined a fails because x does not match the empty list but a match against a 2 b True succeeds binding x to 2 Prelude gt x a x b True lt b undefined a a 2 b True 2 Prelude gt Changing the order of the fields in the pattern to b True a x forces match ing to start with the b component But the first element in the list of records used above has undefined in its b component so now the evaluation produces a run time error message Prelude gt x b True a x lt b undefined a a 2 b True Program error undefined Prelude gt Although Hugs lets you work with record values it does not by default allow you to print them More accurately it does not automatically provide instances of the Show class for record values So a simple attempt to print a record value will result in an error like the following Prelude gt a True b Hello c 12 Int ERROR Cannot find show function for expression a True b Hello c 12 kk of type Rec a Bool b Char c Int Prelude gt The problem here occurs because Hugs attempts to display the record by applying the show function to it and no version of show has been defined If you do want to be able to display record values then you should load or import the Trex module which is usually included in the 1ib hugs directory of the Hugs distribution Prelude gt load
44. aling with programs that involve multiple modules It works in a natural way using the information in import statements at the beginning of modules and is particularly useful for large programs or for programs that use standard Hugs libraries For example consider a module Demo hs that requires the facilities provided by the STArray library This dependency might be reflected by including the following import statement at the beginning of Demo hs import STArray Now if we try to load this module into Hugs then the system will automatically search for the STArray library and load it into Hugs before Demo hs is loaded In fact the STArray library module also begins with some import statements import ST import Array So Hugs will actually load the ST and Array libraries first then the STArray library and only then will it try to read the rest of Demo hs Prelude gt load Demo Reading file Demo hs 21 Reading file hugs libhugs STArray hs Reading file hugs libhugs ST hs Reading file hugs lib Array hs Reading file hugs libhugs STArray hs Reading file Demo hs Demo gt Initially the interpreter reads only the first part of any module loaded into the system upto and including any import statements Only one module is allowed in each file files with no module declaration are assumed to declare the Main module If there are no imports or if the modules specified as imports have already been loaded then
45. and will be used by subsequent executions of Hugs Shell escape E command A cmd command can be used to execute the system command cmd without leaving the Hugs interpreter For example 1s or dir on DOS machines can be used to list the contents of the current directory For convenience the command can be abbreviated to a single character The command without any arguments starts a new shell e On a Unix machine the SHELL environment variable is used to determine which shell to use the default is bin sh e Onan DOS machine the COMSPEC environment variable is used to determine which shell to use this is usually COMMAND COM Most shells provide an exit command to terminate the shell and return to Hugs 29 List commands The command displays the following summary of all Hugs commands Prelude gt LIST OF COMMANDS Any command may be abbreviated to c where c is the first character in the full name load lt filenames gt load also lt filenames gt reload project lt filename gt edit lt filename gt edit module lt module gt lt expr gt type lt expr gt set lt options gt set names pat info lt names gt browse lt modules gt find lt name gt command cd dir ge version quit Prelude gt load modules from specified files clear all files except prelude read additional modules repeat last load command
46. anguage February 1999 Available from http www haskell org definition S Peyton Jones and J Hughes editors Standard libraries for the Haskell 98 programming language February 1999 Available from http www haskell org definition S Peyton Jones M Jones and E Meijer Type classes Ex ploring the design space In Proceedings of the Second Haskell 76 Workshop Amsterdam June 1997 Available on the web from http www cse ogi edu mpj pubs multi html 12 The Hugs GHC Team The Hugs GHC Extension Libraries January 1999 Available from http www haskell org libraries TT Index options 7 e e f f 8 78 i i k k 1 1 q 79 8 8 t t u u KU commands 7 it 1 also BY browse ted edit Bl find gc Bl info load BI 84 module names BJ project BJ quit 7 BI reload Bl BJ set 3 E3 type version PY emacs editor 4 LS evaluator 26 garbage collection 16 BT Gofer Al 73 Haskell Haskell98 I heap size 0 13 23 HUGSPATH import chasing A PT BJ interrupt key 7 I0 monad Bl 13 27 libraries AnsilInteract AQ AnsiScreen Array ET B2 IOExtensions 3 Interact HI ListUtils MI Number 3 ParseLib I STArray RI ST 2 Trace Trex p4 Win32 p4 modules BT literate options 2 28 B3 t t E P g h prelude B 7 B LO 27
47. ar the syntax for instance declarations class constraints and types is completely unchanged By including dependencies in a class declaration we provide a mechanism for the programmer to specify each multiple parameter class more precisely The compiler on the other hand is responsible for ensuring that the set of instances that are in scope at any given point in the program is consistent with any declared dependencies For example the following pair of instance declarations cannot type classes that was put forward by Chen Hudak and Odersky 2 or as a special case of the later framework for improvement E of qualified types The underlying ideas are also discussed in a more theoretical and abstract setting in a manuscript 8 where they are identified as one point in a general design space for systems of implicit parameterization 49 appear together in the same scope because they violate the dependency for D even though either one on its own would be acceptable instance D Bool Int where instance D Bool Char where Note also that the following declaration is not allowed even by itself instance D a b where The problem here is that this instance would allow one particular choice of a to be associated with more than one choice for b which contradicts the dependency specified in the definition of D More generally this means that in any instance of the form instance D t s where for some partic
48. assed to the Hugs interpreter This might be used for example to make use of filters to support conditional compilation language extensions literate programming systems or format conversion The F option can be used to set a particular command string cmd as the name for a preprocessor If set then for any source file say file hs that the user tries to load into Hugs the interpreter will use the output from the command cmd file hs instead of the contents of the file file hs itself Note that the F option is system dependent and is not supported on all plat forms Set constraint cutoff limit c num The c parameter controls the complexity of constraint satisfaction searches in the Hugs type checker This is a technical mechanism to ensure that type checking terminates and can safely be ignored by most users However in programs that make significant use of complex type class hierarchies it may sometimes be necessary to increase the setting for the c option to enable the Hugs type checker to explore a larger search space The usual default for this setting is 40 which corresponds to the command line option c40 and we have not yet seen any examples of valid Hugs programs that are rejected with this setting It is possible to construct artificial programs that do require higher values but such examples are pathological and they do not seem to appear in practice There is no practical benefit in choosing a lower val
49. at occured Now if you use the command edit then Hugs will start up the editor with the cursor positioned at the relevant line of f3 hs whenever this is possible so that the error can be corrected and the changes saved in f3 hs When you close down the editor and return to Hugs the interpreter will automatically attempt to reload f3 hs and then if successful go on to load the next file f4 hs So after just two commands in Hugs the error in f3 hs has been corrected and all four of the files listed on the original command line have been loaded into the interpreter ready for use Find definition find name The find name command starts up the editor at the definition of a type constructor or function specified by the argument name in one of the files cur rently loaded into Hugs Note that Hugs must be configured with an appropriate editor for this to work properly There are four possibilities e If there is a type constructor with the specified name then the cursor will be positioned at the first line in the definition of that type constructor e Ifthe name is defined by a function or variable binding then the cursor will 34 be positioned at the first line in the definition of the function or variable ignoring any type declaration if present e If the name is a constructor function or a selector function associated with a particular datatype then the cursor will be positioned at the first line in the
50. available in other Haskell implementations All that you need to do to use libraries is to import them using an import declaration For example module MandlebrotSet where import Array import Complex Of course this assumes that HUGSPATH has been set to point to the directories where the libraries are stored Section P T and that import chasing is enabled The default search path includes the directories containing both the standard and unofficial libraries 6 1 Standard Libraries The Hugs 98 distribution includes the following standard libraries Array Char Complex 10 Ix List Locale Maybe Monad Numeric Prelude Random Ratio and System The libraries Directory Time and CPUTime are not currently sup ported The library report IO contains full descriptions of all of theses standard libraries Differences between the library report and the libraries supplied with Hugs are described in Section 9 6 2 The Hugs GHC Extension Libraries Hugs and GHC provide a common set of libraries to aid portability detailed spec ifications for these libraries are described elsewhere 2 The Hugs GHC modules included in the current distribution include Addr Bits Channel ChannelVar 40 Concurrent Dynamic Foreign IOExts Int GetOpt NumExts Pretty ST LazyST Weak and Word The Exception and Stable libraries are not currently supported Note that the ST and LazyST libraries cannot be used when the in terpreter is running in
51. ber n is the product of the numbers from 1 to n Before we can use this definition in a Hugs session we have to load Fact hs into the interpreter One of the simplest ways to do this uses the load command Prelude gt load fact hs Reading file fact hs Hugs session for Hugs lib Prelude hs Fact hs Fact gt Notice the list of filenames displayed after Hugs session for this tells you which module files are currently being used by Hugs the first of which is always the standard prelude The prompt is now Fact and evaluation will take place within this new module We can start to use the fact function that we have defined Fact gt fact 6 720 Fact gt fact 6 fact 7 5760 Fact gt fact 7 div fact 6 7 Fact gt As another example the standard formula for the number of different ways of choosing r objects from a collection of n objects is n r n r A simple and direct but otherwise not particularly good definition for this function in Hugs is as follows comb n r fact n div fact r fact n r One way to use this function is to include its definition as part of an expression entered in directly to Hugs Fact gt comb 5 2 where comb n r fact n div fact r fact n r 10 Fact gt The definition of comb here is local to this expression If we want to use comb several times then it would be sensible to add its definition to the file Fact hs Once this has been done and the Fact hs
52. ble with the Win32 libraries or with programs that use them such as Conal Elliot s Fran system or Paul Hudak s Graphics library In addition the current imple mentation uses a compute intensive polling process to detect certain events and this can incur a fairly substantial performance penalty For these reasons the Hugs for Windows front end is not recommended for work on large projects 70 9 Conformance with Haskell 98 A number of Haskell 98 features are not yet implemented in Hugs 98 All known differences between the specification and implementation are described in this section although there are bound to be some unintentional omissions 9 1 Haskell 98 features not in Hugs Mutually recursive modules are not supported Some library functions have been moved into the Prelude This is necessary because the Prelude and the standard libraries as defined in the Haskell 98 report are mutually recursive This mutual recursion has been avoided by moving the following functions into the Prelude From Ix Ix range index inRange rangeSize From Char isAscii isControl isPrint isSpace isUpper isLower isAlpha isDigit isOctDigit isHexDigit isAlphanum digitToInt intToDigit toUpper toLower ord and chr From Ratio Ratio Rational 4 numerator denominator and approxRational Derived Read instances do not work for some infix constructors If an infix constructor has left associativity and the
53. d a type constructor depending on context then the output for both possibilities will be displayed Display names defined in modules browse module The browse command can be used to display the list of functions that are exported from the named modules List gt browse Maybe module Maybe where mapMaybe a gt Maybe b gt a gt b catMaybes Maybe a gt a listToMaybe a gt Maybe a maybeToList Maybe a gt a fromMaybe a gt Maybe a gt a fromJust Maybe a gt a isNothing Maybe a gt Bool isJust Maybe a gt Bool List gt Only the names of currently loaded modules will be recognized Display Hugs version version The version command is used to display the version of the Hugs interpreter Prelude gt version Hugs Version September 1999 Prelude gt This is the same information that is displayed in the Hugs startup banner 39 6 Library overview Haskell 98 places much greater emphasis on the use of libraries than early versions of the language Following that lead the Hugs 98 distribution includes most of the official libraries defined in the Haskell Library Report 0 The distribution also includes a number of unofficial libraries which fall into two categories portable libraries which are implemented using standard Haskell or widely implemented Haskell extensions and Hugs specific libraries which use features that are not
54. definition and use of values like runST that require rank 2 types 7 3 3 Type annotations in patterns Hugs allows patterns of the form pat type to be used as type anno tations in the style of Standard ML To allow effective type inference the type specified here must be a monotype no forall part or class constraints are allowed but it may include variables which with one exception noted below have the same scope as the patterns in which they appear For ex ample the term x Int gt x has type Int gt Int while the expression x a xs a gt xs x has typea gt a gt a Use of this fea ture is subject to the following rules e It is an error for a variable to be used in a type where a more specific type is inferred For example x a gt not x is not a valid expression e It is an error for distinct variables to be used where the types concerned are the same For example the expression x a y b gt x y is not valid e Type variables bound in a pattern may be used in type signatures or further pattern type annotations within the scope of the binding For example f x a let g a gt a gy x y in gx In current versions of Haskell there is no way to write a type for the local function g in this example because of the convention that free type variables are implicitly bound by a universal quantifier In this example the variable is instead bound in the pattern x
55. e restricted type synonym definition In the definition of any other value T is treated as if it had been introduced by a definition of the form data Tai am 65 For a simple example of this consider the following definition of a datatype of stacks in terms of the standard list type type Stack a a in emptyStack push pop top isEmpty emptyStack Stack a emptyStack push a gt Stack a gt Stack a push pop Stack a gt Stack a pop error pop empty stack pop _ xs xs top Stack a gt a top error top empty stack top x _ x isEmpty Stack a gt Bool isEmpty null The type signatures here are particularly important For example because emptyStack is mentioned in the definition of the restricted type synonym Stack the definition of emptyStack is type correct The declared type for emptyStack is Stack a which can be expanded to a agreeing with the type for the empty list However in an expression outside the binding group of these functions the Stack a type is quite distinct from the a type emptyStack 1 ERROR Type error in application x Expression emptyStack 1 Term emptyStack Type Stack b Does not match a The binding group of a value is to the set of values whose definitions are in the same mutually recursive group of bindings In particular this does not extend to class and instance declarations so
56. e the interpreter is running using the set command The same command can be used without any arguments to display a summary of the available options and to inspect their current settings The complete set of Hugs options is described in the sections below The only omission here is the 98 and 98 options that are used to set the Haskell 98 compatability mode These are discussed in Section 7 Set search path P path The P path option changes the Hugs search path to the specified path The search path is usually initialized in the environment and should always include the directory containing the Hugs prelude and the standard libraries When an unknown module is imported Hugs searches for a file with the same name as the module along this path The current directory is always searched before the path is used Directory names should be separated by colons or on Windows DOS machines by semicolons Empty components in the path refer to the prior value of the path For example setting the path to dir dir on Windows DOS would add dir to the front of the current path Within the path Hugs refers to the directory containing the Hugs libraries so one might use a path such as Hugs lib Hugs lib hugs Set editor E cmd A E cmd option can be used to change the editor string to the specified cmd while the interpreter is running The editor string is usually initialized from the environment when the interp
57. e types for both ce and e without ambiguity More gen 50 erally we need only regard a type as ambiguous if it contains a variable on the left of the gt that is not uniquely determined either directly or indirectly by the variables on the right Dependencies also help to produce more accurate types for user defined func tions and hence to provide earlier detection of errors and less cluttered types for programmers to work with Recall the previous definition for a function f f x y insert x y insert x insert y for which we originally obtained a type f Collects ac Collects b c gt a gt b gt c gt c Given the dependency information that we have for Collects however we can deduce that a and b must be equal because they both appear as the second parameter in a Collects constraint with the same first parameter c Hence we can infer a shorter and more accurate type for f f Collects a c gt a gt a gt c gt c In a similar way the earlier definition of g will now be flagged as a type error Although we have given only a few examples here it should be clear that the addition of dependency information can help to make multiple parameter classes more useful in practice avoiding ambiguity problems and allowing more general sets of instance declarations 7 1 2 More flexible instance declarations Hugs mode does not place any syntactic restrictions on the form of type expression or class con
58. eFile 17 names listed Prelude gt 35 Print type of expression type expr The type command can be used to print the type of an expression without evaluating it For example Prelude gt t hello world hello world String Prelude gt t putStr hello world putStr hello world 10 Prelude gt t sum 1 10 sum enumFromTo 1 10 Num a Enum a gt a Prelude gt Note that Hugs displays the most general type that can be inferred for each expression For example compare the type inferred for sum 1 10 above with the type printed by the evaluator using set t Prelude gt set t Prelude gt sum 1 10 55 Int Prelude gt The difference is explained by the fact that the evaluator uses the Haskell default mechanism to instantiate the type variable a in the most general type to the type Int avoiding an error with unresolved overloading Display information about names info name The info command is useful for obtaining information about the files classes types and values that are currently loaded If there are no arguments then info prints a list of all the files that are currently loaded into the interpreter Prelude gt info Hugs session for Hugs lib Prelude hs Demo hs Prelude gt If there are arguments then Hugs treats each one as a name and displays in formation about any corresponding type constructor class or function The fol
59. ed Prelude foo even though foo is not exported from the Prelude and you can write module M where import qualified Prelude import nothing x Prelude length abcd e The Double type is implemented as a single precision float this isn t for bidden by the standard but it is unusual 9 2 Libraries The following libraries are not yet available Directory Time CPUTime Bit Nat and Signed In the IO library these functions are not defined handlePosn ReadWriteMode hFileSize hIsEOF isEOF hSetBuffering hGetBuffering hSeek hIsSeekable hReady and hLookahead The following non standard functions are exported hugsGetCh 10 Char getchar without echoing to screen hugsHIsEOF Handle gt IO Bool same semantics as C s feof different from Haskell s hIsEOF hugsIsEOF IO Bool same semantics as C s feof stdin hPutStrLn String gt 10 corresponds to Prelude putStrLn T2 9 3 Haskell 98 extensions In addition to the features described in Section 7 Hugs 98 supports some modest extensions to the Haskell language e Import declarations may specify a file name instead of a module name e The T syntax is allowed for type synonyms in import and export lists 73 10 Pointers to further information Hugs The full distribution for Hugs is available on the World Wide Web from http haskell org hugs The distribution includes source code demo programs library fil
60. ed option settings in the environ ment rather than specifying them on the command line they will then be used automatically each time the interpreter is started The method for setting these options depends on the machine and operating system that you are using and on the way that the Hugs system was installed The following examples show some typical settings for Unix machines and PCs e The method for setting HUGSFLAGS on a Unix machine depends on the choice of shell For example a C shell user might add something like the following to their cshrc file set HUGSFLAGS P usr Hugs lib usr Hugs libhugs E vi d s The P option is used to set the search path and the E is used to set the editor The string quotes are necessary for the value of the E option becauses it contains spaces The setting for the path assumes that the system has been installed in usr local Hugs and will need to be modified accordingly if a different directory was chosen The editor specified here is vi which allows the user to specify a startup line number by preceding it with a character The settings are easily changed to accommodate other editors If you are installing Hugs for the benefit of several different users then you should probably use a script file that sets appropriate values for the environment variables and then invokes the interpreter bin sh HUGSFLAGS usr Hugs lib usr Hugs libhugs E vi d s s export HUGSFLAGS exec usr local bin
61. eral non standard facilities for Hugs programmers Other Haskell implementations may provide similar features but this is not guaranteed and there may be significant differences in organization naming semantics or functionality e Number This library defines a numeric datatype of fixed width integers whatever Int supplies However unlike the built in Int type overflows are detected and cause a run time error To ensure that all integer arith metic in a given module includes overflow protection you must include a default declaration for Number module Number where data Number instance instance instance instance instance instance instance instance instance Eq Ord Show Enum Num Boun Real Ix Inte fixed width integers Number class instances Number Number Number Number ded Number Number Number gral Number This library cannot be used when Hugs is running in Haskell 98 mode because it requires features that are only supported in full Hugs mode e I0Extensions This module provides non standard extensions to the IO monad module IOExtensions where readBinaryFile FilePath gt 10 String writeBinaryFile FilePath gt String gt I0 appendBinaryFile FilePath gt String gt 10 O openBinaryFile FilePath gt I0Mode gt 10 Handle getCh IO Char argv String e Trace This library provides a single function that can sometimes be useful for debugging module Trace where
62. es user docu mentation and precompiled binaries for common platforms There is a mailing list for Hugs users at hugs usersChaskell org and another for bug reports at hugs bugsChaskell org Admin requests for example to subscribe or unsubscribe should be sent to majordomo haskell org For more detailed instructions just send a message to this address with help in the body An overview of nearly all Haskell related resources can be found at http haskell org Functional programming The usenet newsgroup comp lang functional provides a forum for general dis cussion about functional programming languages A list of frequently asked ques tions FAQs and their answers is available from The FAQ list contains many pointers to other functional programming resources around the world Further reading As we said at the very beginning this manual is not intended as a tutorial on either functional programming in general or Haskell in particular For these things our first recommendations would be for the Introduction to Functional 74 Programming by Bird and Wadler I and the Gentle Introduction to Haskell by Hudak Peterson and Fasel A respectively Note however that there are several other good textbooks dealing either with Haskell or related languages For those with an interest in the implementation of Hugs the report about the implementation of Gofer 6 Hugs predecessor should be a useful starting point 75
63. es to load is exactly the same filename entered by the user However if the named file cannot be accessed then the system will try adding a hs suffix and then a 1hs suffix and then it will repeat the process for each directory in the path until either a suitable file has been located or otherwise until all of the possible choices have been tried 31 For example this means that you do not have to type the hs suffix to load a file Demo hs from the current directory provided that you do not already have a Demo file in the same directory In the same way it is not usually necesary to include the full pathname for one of the standard Hugs libraries For example provided that you do not have an Array Array hs or Array lhs file in the current working directory you can load the standard Array library by typing just load Array Load additional files also filename The also command can be used to load module files without removing any that have previously been loaded However if any of the previously modules have been modified since they were last read then they will be reloaded automatically before the additional files are read If successful a command of the form load f1 fn is equivalent to the se quence of commands load also f1 also fn In particular also uses the same mechanisms as load to search for modules Repeat last load command reload The reload command can be u
64. following sections 5 1 Basic commands Evaluate expression expr To evaluate an expression the user simply enters it at the Hugs prompt This is treated as a special case without the leading colon that is required for other 26 commands The expression must fit on a single line there is no way to continue an expression onto the next line of input to the interpreter The actual behaviour of the evaluator depends on the type of expr e If expr has type IO t for some type t then it will be treated as a program using the I O facilities provided by the Haskell IO monad Any final result produced by the computation will be discarded Prelude gt putStr Hello world Hello world Prelude gt e In any other case the value produced by the expression is converted to a string by applying the show function from the standard prelude and the interpreter uses this to print the result Prelude gt Hello world Hello world Prelude gt Unlike some previous versions of Hugs there is no special treatment for values of type String to display a string without the enclosing quotes and special escapes you should turn it into a program using the putStr function as shown above The interpreter will not evaluate an expression that contains a syntax error a type error or a reference to an undefined variable Prelude gt sum 1 ERROR Syntax error in expression unexpected Prelude gt sum a
65. hs as a literate module If the 1 option is selected then any other file loaded into Hugs will be treated as a normal module Conversely if 1 is selected then these files will be treated as literate modules The effect of using literate modules can be thought of as applying a preproces sor to each input file that is loaded into Hugs This has a particularly simple definition in Hugs illiterate String gt String illiterate cs unlines xs gt xs lt lines cs 17 The system of literate modules that was used in Orwell is a little more complicated than this and requires the programmer to adopt two further conventions in an attempt to catch simple errors in literate modules e Every input file must contain at least one line whose first character is gt This prevents modules with no definitions because the programmer has forgotten to use the gt character to mark definitions from being accepted e Lines containing definitions must be separated from comment lines by one or more blank lines i e lines containing only space and tab characters This is useful for catching programs where the leading gt character has been omitted from one or more lines in the definition of a function For example gt map f O map f x xs f x map f xs would be treated as an error Hugs will report on errors of this kind whenever the e option is enabled the default setting The Haskell Report defines a sec
66. ill mean that all of the operations of the specified classes in this case just Show are available when a value of this type is unpacked during pattern matching For example this can be put to good use to define a simple instance of Show for the Showable datatype instance Show Showable where show MkShowable x show x This definition can now be used in examples like the following Main gt map show MkShowable 3 MkShowable True MkShowable a 3 True 12921 Main gt 7 3 5 Restricted type synonyms Hugs supports the use of restricted type synonyms first introduced in Gofer and similar to the mechanisms for defining abstract datatypes that were provided in several earlier languages The purpose of a restricted type synonym is to restrict the expansion of a type synonym to a particular set of functions Outside of the selected group of functions the synonym constructor behaves like a standard datatype More precisely a restricted type synonym definition is a top level declaration of the form type Tail am rhs in f1 fn where T is a new type constructor name and rhs is a type expression typically involving some of the distinct type variables a1 am The major difference with a normal type synonym definition is that the expansion of the type synonym can only be used within the binding group of one of the functions f1 fn all of which must be defined by top level definitions in the module containing th
67. inding of variables is a notion that goes back to Lisp but was later discarded in more modern incarnations such as Scheme Dynamic binding can be very confusing in an untyped language and unfortunately typed languages in particular Hindley Milner typed languages like Haskell only support static scoping of variables However by a simple extension to the type class system of Haskell we can sup port dynamic binding Basically we express the use of a dynamically bound variable as a constraint on the type These constraints lead to types of the form x t gt t which says this function uses a dynamically bound variable x of type t For example the following expresses the type of a sort function implicitly parameterized by a comparison function named cmp sort cmp a gt a gt Bool gt a gt al The dynamic binding constraints are just a new form of predicate in the type class system 67 An implicit parameter is introduced by the special form x where x is any valid identifier Use if this construct also introduces new dynamic binding constraints For example the following definition shows how we can define an implicitly pa rameterized sort function in terms of an explicitly parameterized sortBy func tion sortBy a gt a gt Bool gt a gt a sort cmp a gt a gt Bool gt a gt a sort sortBy cmp Dynamic binding constraints behave just like other type clas
68. ion 7 for details Hugs is implemented as an interpreter that provides e A relatively small portable system that can be used on a range of different machines from home computers to Unix workstations e A read eval print loop for displaying the value of each expression that is entered into the interpreter e Fast loading type checking and compilation of Haskell programs with facilities for automatic loading of imported modules e Integration with an external editor chosen by the user to allow for rapid development and for location of errors e Modest browsing facilities that can be used to find information about the operations and types that are available Hugs is a successor to Gofer an experimental functional programming system that was first released in September 1991 and users of Gofer will see much that is familiar in Hugs However Hugs offers much greater compatibility with the Haskell standard indeed the name Hugs was originally chosen as a mnemonic for the Haskell users Gofer system There have been many modifications and enhancements to Hugs since its first release on Valentines day February 14 in 1995 Some of the most obvious im provements include e Full support for new Haskell 98 features including the labelled field syntax do notation newtype strictness annotations in datatypes the Eval class ISO character set etc e Support for Haskell modules and a growing collection of library
69. ll a a gt a gt Bool Char amazed i i True i a twice forall b b gt f b gt a gt f f a twice f f f 60 There are a number of important points to note here e In Hugs mode forall is a reserved word e Quantified variables may be of any kind including types or gt unary type constructors as in the examples above e Variables quantified in a forall type must appear in the scope of the quantifier Unused quantified variables would serve no useful purpose and are perhaps most likely to occur as the result of mispelling a variable name e Nested quantifiers are not allowed and quantifiers can only appear in the types of function arguments not in the results e A function can only take polymorphic arguments if an explicit type signa ture is provided for that function Any call to such a function must have at least as many arguments as are needed to include the rightmost argument with a quantified type For example neither of the functions amazed or twice defined above can be partially applied e It is not necessary for all polymorphic arguments to appear at the beginning of a type signature For example the following type signature is valid eg Int gt forall a a gt a gt Int gt Int However as a consequence of the rules given above the eg function defined here must always be applied to at least two arguments even though the first of these does not have a poly
70. ll be taken as a reference to tmp B hs while the import in that file will be taken as a reference to tmp A hs Import chasing is often very useful but you should also be aware of its limitations e Mutually recursive modules are not supported if A imports B then B must not import A either directly or indirectly through another one of its imports e Import chasing assumes a direct mapping from module names to the names of the files that they are stored in If A imports B then the code for B must 22 be in a file called either B B hs or B 1hs and must be located in one of the directories specified above On rare occasions it is useful to specify a particular pathname as the target for an import statement Hugs allows string literals to be used as module identifiers for this purpose import TypeChecker Types hs Note however that this is a nonstandard feature of Hugs and that it is not valid Haskell syntax You should also be aware that Hugs uses the names of files in deciding whether a particular import has already been loaded so you should avoid situations where a single file is referred to by more than one name For example you should not assume that Hugs will be able to determine whether Demo hs and Demo hs are references to the same file Import chasing is usually enabled by default setting i but it can also be disabled using the i option Set heap size h size A h size option can be used t
71. lowing examples show the the kind of output that you can expect e Datatypes The system displays the name of the datatype the names and types of any constructors or selectors and a summary of related instance 36 declarations Prelude gt info Either type constructor data Either a b constructors Left a gt Either a b Right b gt Either a b instances instance Eq b Eq a gt Eq Either a b instance Ord b Ord a gt Ord Either a b instance Read b Read a gt Read Either a b instance Show b Show a gt Show Either a b instance Eval Either a b Prelude gt Newtypes are dealt with in exactly the same way For a simple example of a datatype with selectors the output produced for a Time datatype data Time MkTime 1 hours mins secs Int is as follows Time gt info Time type constructor data Time constructors MkTime Int gt Int gt Int gt Time selectors hours Time gt Int mins Time gt Int secs Time gt Int instances instance Eval Time Time gt e Type synonyms The system displays the name and expansion Prelude gt info String type constructor type String Char Prelude gt 37 The expansion is not included in the output if the synonym is restricted e Type classes The system lists the name superclasses members and in stance declarations for the specified class Prelude gt info Num ty
72. lude gt set TOGGLES groups begin with to turn options on off resp s Print no reductions cells after eval t Print type after evaluation f Terminate evaluation on first error g Print no cells recovered after gc 1 Literate modules as default e Warn about errors in literate modules Print dots to show progress q Print nothing to show progress W Always show which modules are loaded k Show kind errors in full u Use show to display results i Chase imports while loading modules 28 OTHER OPTIONS leading or makes no difference hnum Set heap size cannot be changed within Hugs pstr Set prompt string to str rstr Set repeat last expression string to str Pstr Set search path for modules to str Estr Use editor setting given by str cnum Set constraint cutoff limit Fstr Set preprocessor filter to str Current settings fewkui stgl q h250000 p s gt r c40 Search path P Hugs lib Hugs lib hugs Hugs lib exts Editor setting E vi d s Preprocessor 2 F Compatibility Haskell 98 98 Prelude gt Refer to Section 41 2 for more detailed descriptions of each of these option settings The set command can also be used to change options by supplying the required settings as arguments For example Prelude gt set st Prelude gt 1 3 4 Int 4 reductions 4 cells Prelude gt On Windows 95 NT all option settings are written out to the registry when a set command is executed
73. morphic type e In the definition of a function there must be at least as many arguments on the left hand side of the definition as are needed to included the rightmost argument with a quantified type Only variables or a wildcard _ can be used as arguments on the left hand side of a function definition where a value of polymorphic type is expected e Arbitrary expressions can be used for polymorphic arguments in a function call provided that they can be assigned the necessary polymorphic type For example all of the following expressions are valid calls to the amazed function defined above amazed let i x x in i amazed Ax gt x amazed id id id id amazed id id id id id 61 A similar syntax can be used to include polymorphic components in datatypes as illustrated by the following examples data Monadi m MkMonad1 uniti forall a a gt m a bindi forall ab m a gt a gt mb gt m b data Monad2 m MkMonad2 forall a a gt m a forall a b m a gt a gt m b gt m b listMonad1 MkMonad1 unit1 bindi x gt x gt Mx f gt concat map f x listMonad2 MkMonad1 x gt x x f gt concat map f x In this case MkMonad1 and MkMonad2 have types forall b b gt m b gt forall b c m b gt b gt m c gt m c gt Monadi m forall b b gt m b gt forall b c m b gt b gt m c gt m c gt Monad2 m respectively while ListMon
74. ndard type is an instance of both the Integral and the Floating classes Main gt describe 23 Int Integral Main gt describe 23 Float Floating Main gt Note that this experimental feature may not be supported in future releases 52 7 1 4 More flexible contexts Haskell 98 allows only class constraints of the form C a t1 tn to appear in the context of any declared or inferred type where C is a class a is a variable and t1 tn are arbitrary types n gt 0 Class constraints of this form are sometimes characterized as being in head normal form In many practical cases we have n 0 corresponding to class constraints of the form C a In Hugs mode these restrictions are relaxed and any type whether in head nor mal form or not is permitted to appear in a context For example the principal type of an expression x gt x is Eq a gt a gt Bool reflecting the fact that the equality function is used to compare lists of type a In previous versions of Hugs and in Haskell 98 an inferred type of Eq a gt a gt Bool would have been produced for this term The latter type can still be used if an explicit type signature is provided for the term assuming that an instance declaration of the form instance Eq a gt Eq a where is in scope For example the following program is valid f Eq a gt a gt Bool f x x Note that contexts are not reduced by default because
75. o request a particular heap size for the interpreter the total number of cells that are available at any one time when Hugs is first loaded The request will only be honoured if it falls within a certain range which depends on the machine and the version of Hugs that is used The size param eter may include a K or k suffix which acts as a multiplier by 1 000 For example either of the following commands hugs h25000 hugs h25K will usually start the Hugs interpreter with a heap of 25 000 cells Cells are generally 8 bytes wide except on the 16 bit Hugs running on DOS and Hugs allocates a single heap Note that the heap is used to hold an intermediate parsed form of each module while it is being read type checked and compiled It follows that the larger the module the larger the heap required to enable that module to be loaded into Hugs In practice most large programs are written and loaded as a number of separate modules which means that this does not usually cause problems Unlike all of the other options described here the heap size setting cannot be changed from within the interpreter using a set command However on Window 95 NT changing the heap size with set will affect the next running of Hugs since it saves all options in the registry 23 Set prompt p string A p str option can be used to change the prompt to the specified string str Prelude gt set p Hugs gt Hugs gt set p
76. ond style of literate programming in which code is surrounded by begin code and end code See Appendix C of the Haskell Report for more information about literate programming in Haskell Display dots while loading pi As Hugs loads each file into the interpreter it prints a short sequence of messages to indicate progress through the various stages of parsing the module dependency analysis type checking and compilation With the default setting the in terpreter prints the name of each stage backspacing over it to erase it from the screen when the stage is complete If you are fortunate enough to be using a fast machine you may not always see the individual words as they flash past After loading a file your screen will typically look something like this Prelude gt 1 Array Reading file Hugs lib Array hs Hugs session for Hugs lib Prelude hs Hugs lib Array hs Prelude gt On some systems the use of backspace characters to erase a line may not work properly for example if you try to run Hugs from within emacs In this case 18 you may prefer to use the setting which prints a separate line for each stage with a row of dots to indicate progress Prelude gt load Array Reading file Hugs lib Array hs Parsing dos do Raids Ga De Phos VOUS ie Ee Nia ee ea Dependency analysis A oe A PA Be LY PS checking onset a A one tee eats sarees Pace she Sears Compa ling es bes ia Sate Ga dee ad sel ie ed
77. ons etc to provide simple ways of invoking the interpreter in either mode On Win 32 machines for example one can set up file associations so that you can right click on a hs or 1hs file and get a choice of loading the file into either a Haskell 98 or Hugs mode session The remainder of this section sketches some of the extensions that are currently supported when the interpreter is running in Hugs mode 45 7 1 Type class extensions In Hugs mode several of the Haskell 98 restrictions on type classes are relaxed This allows the use of multiple parameter classes and more flexible forms of instance declarations 7 1 1 Multiple parameter classes Haskell 98 allows only one type argument to be specified for any given type class As a result each type class corresponds to a set of types For example a class constraint Eq t tells us that the type t is assumed or required to be an instance of the class Eq and the class Eq itself corresponds to the set of all equality types In Hugs mode this restriction is relaxed so that programmers can also define classes with multiple parameters each of which corresponds to a multi place relation on types Multiple parameter type classes seem to have many potentially interesting ap plications O However some practical attempts to use them have failed as a result of frustrating ambiguity problems This occurs because the mechanisms that are used to resolve overloading are not aggressive enough
78. or all of their contributions A special thank you also to our friends and colleagues at OGI Yale and elsewhere for their input to the current release 2 A technical summary of Hugs 98 Hugs 98 provides an almost complete implementation of Haskell 98 9 including e Lazy evaluation higher order functions and pattern matching e A wide range of built in types from characters to bignums and lists to functions with comprehensive facilities for defining new datatypes and type synonyms e An advanced polymorphic type system with type and constructor class overloading e All of the features of the Haskell 98 expression and pattern syntax in cluding lambda case conditional and let expressions list comprehensions do notation operator sections and wildcard irrefutable and as patterns e An implementation of the main Haskell 98 primitives for monadic I O with support for simple interactive programs access to text files handle based I O and exception handling e An almost complete implementation of the Haskell module system The primary omission is that mutually recursive modules are not yet supported Hugs 98 also supports a number of advanced and experimental extensions in cluding multi parameter classes extensible records rank 2 polymorphism exis tentials scoped type variables and restricted type synonyms By default these features can only be used if Hugs is started with the 98 command line flag See Sect
79. pair is strictly more specific than the other This facility has been introduced in a way that does not compromise the coherence of the type system However its semantics differs slightly from the semantics of overlapping instances in Gofer so users may some times be surprised with the results This is why we have decided to allow this feature to be turned on or off by a command line option the default is off If practical experience with overlapping instances is positive then we may change the current default or even remove the option If the command line option m is selected then a lazier form of overlapping instances is supported which we refer to as multi instance resolution The main idea is to omit the normal tests for overlapping instances but to generate an error message if the type checker can find more than one way to resolve overloading for a particular instance of the class For example with the m option selected then the two instance declarations in the following program are accepted even though they have overlapping in fact identical constraints on the right of the gt symbol class Numeric a where describe a gt String instance Integral a gt Numeric a where describe n Integral instance Floating a gt Numeric a where describe n Floating As it turns out these instances do not cause any problems in practice because they can be distinguished by the contexts on the left of the gt symbol no sta
80. pe class class Eq a Show a Eval a gt Num a where a gt a gt a a gt a gt a a gt a gt a negate a gt a abs a gt a signum a gt a fromInteger Integer gt a fromInt Int gt a instances instance Num Int instance Num Integer instance Num Float instance Num Double instance Integral a gt Num Ratio a Prelude gt e Other values For example named functions and individual constructor selector and member functions are displayed with their name and type Time gt info hours min a gt b gt c gt a gt e gt pb a gt a gt a data constructor hours Time gt Int selector function min Ord a gt a gt a gt a class member Time gt As the last example shows the info command can take several arguments and prints out information about each in turn A warning message is displayed if there are no known references to an argument Prelude gt info Unknown reference Prelude gt 38 This illustrates that the arguments are treated as textual names for operators not syntactic expressions for example identifiers The type of the operator can be obtained using the command info as above There is no provision for including wildcard characters of any form in the arguments of info commands If a particular argument can be interpreted as for example both a constructor function an
81. perations Main the main program If we load this into Hugs with a command project Demo prj then the inter preter will read the project file and then try to load each of the named files In this particular case the overall effect is essentially the same as that of load Types Basics Main Once a project file has been selected the project command without any argu ments can be used to force Hugs to reread both the project file and the module files that it lists This might be useful if for example the project file itself has been modified since it was first read Project file names may also be specified on the command line when the interpreter is invoked by preceding the project file name with a single character Note that there must be at least one space on each side of the Standard command line options can also be used at the same time but additional filename arguments will be ignored Starting Hugs with a command of the form hugs Demo prj is equivalent to starting Hugs without any arguments and then giving the command p Demo prj The project command uses the same mechanisms as load to locate the files mentioned in a project file but it will not use the current path to locate the project file itself you must specify a full pathname As has already been said import chasing usually provides a much better way to deal with multiple file programs than the old project file system The big ad vantage of import chasing is
82. plying the standard prelude function show Show a gt a gt String to it and displaying the resulting string of characters This approach works well for any value whose type is an instance of the standard Show class for 20 example the prelude defines instances of Show for all of the built in datatypes It is also easy for users to extend the class with new datatypes either by providing a handwritten instance declaration or by requesting an automatically derived instance as part of the datatype definition as in data Rainbow Red Orange Yellow Green Blue Indigo Violet deriving Show The advantage of using show is that it allows programmers to display the results of evaluations in whatever form is most convenient for users which is not always the same as the way in which the values are represented This is probably all that most users will ever need However there are some cir cumstances where it is not convenient for example for certain kinds of debugging or for work with datatypes that are not instances of Show In these situations the u option can be used to prevent the use of show In its place Hugs will use a built in printing mechanism that works for all datatypes and uses the repre sentation of a value to determine what gets printed At any point the default printing mechanism can be restored by setting u Import chasing i i Import chasing is a simple but flexible mechanism for de
83. pted Prelude gt Garbage collector messages may be printed at almost any stage in a computation or indeed while loading type checking or compiling a file of definitions For this reason it is often best to turn garbage collector messages off using set g for example if they are not required Literate modules 1 1 e e Like most programming languages Hugs usually treats source file input as a sequence of lines in which program text is the norm and comments play a sec ondary role In Hugs as in Haskell comments are introduced by the character sequences and An alternative approach using an idea described by Knuth as literate program ming gives more emphasis to comments and documentation with additional characters needed to distinguish program text from comments Hugs supports a form of literate programming based on an idea due to Richard Bird and originally implemented as part of the functional programming language Orwell In a Hugs literate module program lines are marked by a gt character in the first column any other line is treated as a program comment This makes it particularly easy to write a document which is both an executable Hugs module and at the same time without need for any preprocessing suitable for use with document preparation software such as TEX Hugs will treat any input file with a name ending in hs as a normal module and any input file with a name ending in 1
84. reter starts running Any occurrences of 4d and s in the editor option are replaced by the start line number and the name of the file to be edited respectively when the editor is invoked If specified the line number parameter is used to let the interpreter 13 start the editor at the line where an error was detected or in the case of the find command where a specified variable was defined Other editors can be selected For example you can use the following value to configure Hugs to use emacs E emacs d s More commonly emacsclient or gnuclient is used to avoid starting a new emacs with every edit On Windows DOS you can use Eedit for the standard DOS editor or Enotepad for the Windows notepad editor However neither edit or notepad allow you to specify a start line number so you may prefer to install a different editor Print statistics s s Normally Hugs just shows the result of evaluating each expression Prelude gt map x gt x x 1 10 1 4 9 16 25 36 49 64 81 100 Prelude gt 1 1 2 3 4 Interrupted Prelude gt With the s option the interpreter will also display statistics about the total number of reductions and cells the former gives a measure of the work done while the latter gives an indication of the amount of memory used For example Prelude gt set s Prelude gt map Ax gt x x 1 10 1 4 9 16 25 36 49 64 81 100 248 reduction
85. rnal prims flag e GenericPrint This library provides a generic or polymorphic print function in Haskell that works in essentially the same way as Hugs builtin printer when the u option is used The module HugsInternals is required e CVHAssert This library provides a simple implementation of Cordy Hall s assertions for performance debugging These primitives are an experimental feature that may be removed in future versions of Hugs They can only be used if hugs was configured with the enable internal prims flag e Win32 This library contains Haskell versions for many of the functions in the Microsoft Win32 library It is only available on Windows 95 NT The with plugins configuration option must be used in conjunction with this and the other Microsoft libraries Other libraries included in the standard distribution but not further documented here are Sequence Pretty HugsDynamic HugsLibs StdLibs and OldWeak 44 7 An overview of Hugs extensions The Hugs interpreter can be run in two different modes e Haskell 98 mode This should be used for the highest level of compatibil ity with the Haskell 98 standard known deviations from the standard are documented in Section H In this mode any attempt to use Hugs specific extensions should trigger an error message Although there are some fairly substantial differences between Haskell 1 4 and Haskell 98 our experience is that most programs written for Haskell 1
86. row is not significant Trex gt a True b Hello c 12 Rec b String c Int a Bool a True b Hello c 12 Trex gt However the type of a record must be an accurate reflection of the fields that appear in the corresponding value The following example produces an error because the specified type does not list all of the fields in the record value Trex gt a True b Hello c 12 Rec b String c Int ERROR Type error in type signature expression term a True b Hello c 12 type Rec a Bool b Char c a x does not match Rec b String c Int because field mismatch Trex gt Notice that Trex does not allow the kind of subtyping on record values that would allow a record like a True b Hello c 12 to be treated implicitly as having type Rec b String c Int simply by forgetting about the a field Finding an elegant efficient and tractable way to support this kind of implicit coercion in a way that integrates properly with other aspects of the Hugs type system remains an interesting problem for future research 56 7 2 2 Extensibility An important property of the Trex system is that the same label name can appear in many different record types and potentially with a different value type in each case However all of the features that we have seen so far deal with records of some fixed shape where the set of labels and the type of values associated with
87. s 429 cells Prelude gt 1 1 2 3 4 Interrupted 18 reductions 54 cells Prelude gt Note that the statistics produced by s are an extremely crude measure of the behaviour of a program and can easily be misinterpreted For example e The fact that one expression requires more reductions than another does not necessarily mean that the first is slower some reductions require much more work than others and it may be that the average cost of reductions in the first expression is much lower than the average for the second 14 e The cell count does not give any information about residency which is the number of cells that are being used at any given time For example it does not distinguish between computations that run in constant space and computations with residency proportional to the size of the input One reasonable use of the statistics produced by s would be to observe general trends in the behaviour of a single algorithm with variations in its input Print type after evaluation t t With the t option the interpreter will display both the result and type of each expression entered at the Hugs prompt Prelude gt set t Prelude gt map x gt x x 1 10 1 4 9 16 25 36 49 64 81 100 Int Prelude gt not True False Bool Prelude gt x gt x lt lt function gt gt a gt a Prelude gt Note that the interpreter will not display the type of an expression if i
88. s constraints in that they are automatically propagated Thus when a function is used its implicit parameters are inherited by the function that called it For example our sort function might be used to pick out the least value in a list least cmp a gt a gt Bool gt a gt a least xs fst sort xs Without lifting a finger the cmp parameter is propagated to become a parameter of least as well With explicit parameters the default is that parameters must always be explicit propagated With implicit parameters the default is to always propagate them However an implicit parameter differs from other type class constraints in the following way All uses of a particular implicit parameter must have the same type This means that the type of x x is x a gt a a and not x a x b gt a b as would be the case for type class constraints An implicit parameter is bound using an expression of the form e with binds or equivalently as dlet binds in e where both with and dlet dynamic let are new keywords These forms bind the implicit parameters arising in the body not the free variables as a let or where would do For example we define the min function by binding cmp min a gt a min least with cmp lt Syntactically the binds part of a with or dlet construct must be a collection of simple bindings to variables no function style bindings and no type signatures these bindings
89. s program The stand alone Hugs program may return an exit code On Windows 95 NT runhugs is invoked using a separate file extension that is set up to call runhugs rather than hugs Installation sets up the hsx extension for this purpose A hsx program will run when it is clicked on a console window will appear if the program writes to standard output or reads from standard input This window is closed immedately upon exiting the program There is no way to pass parameters to the hsx program when it is double clicked Windows 69 95 NT can also use runhugs to open files of a given type this involves setting the open command for the file type to call runhugs passing it the Haskell program to run and the file being opened The online documentation has some examples of this 8 2 Hugs for Windows Hugs for Windows winhugs offers a GUI front end to the Hugs interpreter on Microsoft Windows platforms The user interface features a scrolling console window that mimics the normal Hugs interface together with a menu and toolbar that provide additional facilities for browsing Haskell programs Most of the additional features are self explanatory although short descriptions of menu and toolbar choices are displayed in a status line Hugs for Windows uses the same command line options and environment registry variables as Hugs It also stores options in a ini file The Hugs for Windows front end is useful for beginners but is not compati
90. sed to repeat the last load command If none of the previously loaded files has been modified since the last time that it was loaded then reload will not have any effect However if one of the modules has been modified then it will be reloaded Note that modules are loaded in a specific order with the possibility that later modules may import earlier ones To allow for this if one module has been reloaded then all subsequent modules will also be reloaded This feature is particularly useful in a windowing environment If the interpreter is running in one window then reload can be used to force the interpreter to take account of changes made by editing modules in other windows Load project project project file Project files were originally introduced to ease the task of working with programs whose source code was spread over several files all of which had to be loaded at 32 the same time The facilities for import chasing usually provide a much better way to deal with multiple file projects but the current release of Hugs does still support the use of project files The project command takes a single argument the name of a text file contain ing a list of file names separated from one another by whitespace which may include spaces newlines or Haskell style comments For example the following is a valid project file A simple project file Demo prj Types datatype definitions Basics basic o
91. session 5 2 Loading and editing modules and projects Load definitions from module load filename The load command removes any previously loaded modules and then attempts to load the definitions from each of the listed files one after the other If one of these files contains an error then the load process is suspended and a suitable error message will be displayed Once the problem has been corrected the load process can be restarted using a reload command On some systems the load process will be restarted automatically after a edit command The exception occurs on Windows 95 NT because of the way that the interpreter and editor are executed as independent processes If no file names are specified the load command just removes any previously loaded definitions leaving just the definitions provided by the prelude The load command uses the list of directories specified by the current path to search for module files We can specify the list of directory and filename pairs in the order that they are searched using a Haskell list comprehension dir file suf dir lt path suf lt hs 1lhs The file mentioned here is the name of the module file that was entered by the user while path is the current Hugs search path The search starts with the directory which usually represents a search relative to the current working directory So the very first filename that the system tri
92. straints that can be used in an instance declaration Apart from the normal restrictions to ensure that such type expressions are well formed of course For example the following definitions are all acceptable instance Eq Tree a Eq a gt Eq Tree a where instance Eq a gt Eq Bool gt a where instance Num a gt Num String a where Compare this with the restrictions of Haskell 98 which allow only variables resp simple types as the arguments of classes on the left resp right hand side of the gt sign The price for this extra flexibility is that it is possible to code up arbitrarily complex instance entailments which means that checking entailments and hence calculating principal types is in the general case undecidable The setting for the c option described in Section 4 2 will cause the type checker 51 to fail if the complexity of checking of entailments rises above a certain level Usually this results from examples that would otherwise cause the type checker to go into an infinite loop It is possible that some syntactic restrictions on instance declarations might be introduced at some point in the future in a way that will offer much of the flexibility of the current approach but in a way that guarantees decidability 7 1 3 Overlapping instances The command line option o can be used to enable support for overlapping in stance declarations provided that one of each overlapping
93. tandard definitions that are loaded into Hugs each time that the interpreter is started the filename will vary from one installation to the next You may notice a pause while the interpreter is initialized and the prelude definitions are loaded into the system 3 1 Expressions In essence using Hugs is just like using a calculator the interpreter simply eval uates each expression that is entered printing the results as it goes Prelude gt 2 3 8 40 On Windows 95 NT the installation procedure normally adds Hugs to the start menu You can also start the interpreter by double clicking on a hs or 1hs file 21f Hugs does not load correctly and complains that it cannot find the prelude then Hugs has not been installed correctly and you should look at the installation instructions Prelude gt sum 1 10 55 Prelude gt The Prelude gt characters at the begining of the first third and fifth lines here form the Hugs prompt This indicates that the system is ready to accept input from the user and that it will use definitions from the Prelude module to evaluate each expression that is entered The Hugs prelude is a special module that contains definitions for the built in operations of Haskell such as and sum In response to the first prompt the user entered the expression 2 3 8 which was evaluated to produce the result 40 In response to the second prompt the user typed the expression sum 1 10 The notation 1 1
94. the process of compiling files or evaluating expressions When the interrupt is detected Hugs prints Interrupted and returns to the prompt so that further commands can be entered 3 3 Programs Functions like sum gt gt and take used in the examples above are all defined in the Hugs prelude you can actually do quite a lot using just the types and operations provided by the prelude But in general you will also want to define new types 7 and operations storing them in modules that can be loaded and used by Hugs A module is simply a collection of definitions stored in a file For example suppose we enter the following module module Fact where fact Integer gt Integer fact n product 1 n into a file called Fact hs By convention Hugs modules are stored in files ending with the characters hs The file name should match the name of the module it contains The product function used here is also defined in the prelude and can be used to calculate the product of a list of numbers just as you might use sum to calculate the corresponding sum So the line above defines a function fact that takes an argument n and calculates its factorial In standard mathematical notation fact n n which is usually defined by an equation n 1i 2 x n 1 n Once you become familiar with the notation you will see that the Hugs definition is really very similar to this informal mathematical version the factorial of a num
95. to the following declarations instance Eq e gt Collects e e where instance Eq e gt Collects e e gt Bool where instance Collects Char BitSet where instance Hashable e Collects a ce gt Collects e Array Int ce where All this looks quite promising we have a class and a range of interesting im plementations Unfortunately there are some serious problems with the class declaration First the empty function has an ambiguous type empty Collects e ce gt ce By ambiguous we mean that there is a type variable e that appears on the left of the gt symbol but not on the right The problem with this is that according to the theoretical foundations of Haskell overloading we cannot guarantee a well defined semantics for any term with an ambiguous type For this reason Hugs rejects any attempt to define or use such terms ERROR Ambiguous type signature in class declaration ambiguous type Collects a b gt b assigned to empty We can sidestep this specific problem by removing the empty member from the class declaration However although the remaining members insert and member do not have ambiguous types we still run into problems when we try to use them For example consider the following two functions f x y insert x insert y g f True a for which Hugs infers the following types f Collects a c Collects b c gt a gt b gt c gt c g Collects
96. ts evaluation is interrupted or fails with a run time error In addition the interpreter will not print the type IO of a program in the IO monad the interpreter treats these as a special case giving the programmer more control over the output that is produced Terminate on error i 1 In normal use the evaluation of an expression is abandoned completely if a run time error occurs such as a failed pattern match or an attempt to divide by zero For example Prelude gt 1 div 0 Program error primDivInt 1 0 Prelude gt 1 div 0 2 Program error primDivInt 1 0 Prelude gt This is often useful during program development because it means that errors are detected as soon as they occur However technically speaking the two expres 15 sions above have different meanings the first is a singleton list while the second has two elements Unfortunately the output produced by Hugs does not allow us to distinguish between the values The f option can be used to make the Hugs printing option a little more accurate this should normally be combined with u because the built in printer is better than the user defined show functions at recovering from evaluation errors With these settings if the interpreter encounters an irreducible subexpression then it prints the expression between a matching pair of braces and attempts to continue with the evaluation of other parts of the original expression
97. ude hs cannot be found in one of the path directories or in the current directory then Hugs will terminate Hugs will not run without the prelude file e Program files specified on the command line are loaded The effect of a command hugs f1 fnis the same as starting up Hugs with the hugs command and then typing load f1 fn In particular the interpreter will not terminate if a problem occurs while it is trying to load one of the specified files but it will abort the attempted load command The environment variables and command line options used by Hugs are described in the following sections 10 4 1 Environment options Before options on the command line are processed initial option values are set from the environment On Windows 95 NT these settings are added to the reg istry during setup On other systems the initial settings are determined by the HUGSFLAGS environment variable The syntax used in this case is the same as on the command line options are single letters preceeded by or and sometimes followed by a value Option settings are separated by spaces option values con taining spaces are encoded using Haskell string syntax The environment should be set up before the interpreter is used so that the search path is correctly defined to include the prelude The built in defaults however may allow Hugs to be run without any help from the environment on some systems It is usually more convenient to save preferr
98. ue for this parameter Note that if the value used is too low then Hugs will not be able to load some standard files and libraries including the prelude 25 5 Hugs commands Hugs provides a number of commands that can be used to evaluate expressions to load files and to inspect or modify the behaviour of the system while the interpreter is running Almost all of the commands in Hugs begin with the character followed by a short command word For convenience all but the first letter of a command may be omitted For example 1 s and q can be used as abbreviations for the load set and quit commands respectively Most Hugs commands take arguments separated from the command itself and from one another by spaces The Haskell syntax for string constants can be used to enter parts of arguments that contain spaces newlines or other special characters For example the command load My File will be treated as a command to load two files My and File Any of the following commands can be used to load a single file My File whose name includes an embedded space load My File load My SPFile load MyX File load My File You may wish to study the lexical syntax of Haskell strings to understand some of these examples In practice filenames do not usually include spaces or special characters and can be entered without surrounding quotes as in load fact hs The full set of Hugs commands is described in the
99. ular types t and s the only variables that can appear in s are the ones that appear in t and hence if the type t is known then s will be uniquely determined The benefit of including dependency information is that it allows us to define more general multiple parameter classes without ambiguity problems and with the benefit of more accurate types To illustrate this we return to the collection class example and annotate the original definition from Section 7 1 1 with a simple dependency class Collects e ce ce gt e where empty ce insert e gt ce gt ce member e gt ce gt Bool The dependency ce gt e here specifies that the type e of elements is uniquely de termined by the type of the collection ce Note that both parameters of Collects are of kind there are no constructor classes here Note too that all of the in stances of Collects that we gave in Section 7 1 1 can be used together with this new definition What about the ambiguity problems that we encountered with the original def inition The empty function still has type Collects e ce gt ce but it is no longer necessary to regard that as an ambiguous type Although the variable e does not appear on the right of the gt symbol the dependency for class Collects tells us that it is uniquely determined by ce which does appear on the right of the gt symbol Hence the context in which empty is used can still give enough information to determin
100. y defines some basic ANSI escape seqences for ter minal control module AnsiScreen where type Pos Int Int at Pos gt String gt String highlight String gt String goto Int gt Int gt String home String cls String The definitions in this module will need to be adapted to work with termi nals that do not support ANSI escape sequences e Ansilnteract This library includes both Interact and AnsiScreen and also contains further support for screen oriented interactive I O module Ansilnteract module Ansilnteract module Interact module AnsiScreen where import AnsiScreen import Interact clearScreen Interact gt Interact writeAt Pos gt String gt Interact gt Interact moveTo Pos gt Interact gt Interact readAt Pos gt start coords Int gt max input length String gt Interact gt continuation Interact defReadAt Pos gt start coords Int gt max input length String gt default value String gt Interact gt continuation Interact promptReadAt Pos gt start coords Int gt max input length String gt prompt String gt Interact gt continuation Interact defPromptReadAt Pos gt start coords Int gt max input length String gt prompt String gt default value String gt Interact gt continuation Interact 42 6 4 Hugs Specific Libraries These libraries provide sev

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