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1. a vent 1 9 38 a 0 0 print a a results in 0 2 3 Thus don t use the prev operator on array or record variables that have their parts individually assigned elsewhere You can also prev entire expressions Thus Rey oe y lamda x x y z lambda x x y Z are both true under normal circumstances 6 2 Program Composition You can compose programs together with the operator on programs or the more general compose operator This has the effect of unioning programs together You can also specify that some of the variables used in the programs you compose are to be considered the same or shared while the rest of the variables are local to the programs Here is an example include standard ccl program plant a b x0 delta needs u x x0 y xX true x x delta x ax xx tb u yi X print x x n a program controller k needs y u 0 0 true u k y 16 program system x0 a b plant a b x0 0 05 controller 2 a b sharing u y program main system 3 141 5 5 1 1 Here a simple one dimensional linear system is declared in plant which takes some parameters Then the program controller is defined By themselves they don t do much the value of x in the plant just explodes But combined they do The program system is composed of instances of the plant and controller programs It takes2. It does not block external timestamp int from string data a udp_get_data int The argument to this function is a valid sid The return value is a record that has three fields an integer timestamp field that can be used to order messages from the same client in terms of the time they were sent a string valued from field containing the host name of the client that sent the data and a data field whose type depends on what the client sent An example server that simply prints to the terminal the messages it receives is as follows 25 include standard ccl include udp ccl program server port sid udp_new_server port udp_is_ready sid print got udp_get_data sid on port port r program main server 8000 The other two udp ccl functions are for clients external int udp new client string int The first argument to this function is a string containing the hostname to which to send data and the second argument is the UDP port to ad dress The return value is the client id cid that should be used in calls to udp_send_data external unit udp_send_data int a The first argument to this function is a valid cid The second argument is the data of the message and may of any type Once again care should be used with the data field as ccli can not type check whether the client s idea of the type of the data is consistent with the server
3. and testes for equality of its argu ments It does not perform variable assignment as in some other languages e oy Assignments are done with the operator 3 2 Type Errors The ccli interpreter will not let you enter expressions that use types in correctly For example you can not compare integers and strings as in the following 1 one produces the error Type error in example ccl on line 1 could not compare or arguments 1 has type integer while one has type string with information about what caused ccli to complain 3 3 Arithmetic Operations Integers and reals can be added subtracted multiplied divided mod ed and raised to each other If the arguments to the operation are both integers the result will be too Otherwise ccli with give you areal Standard precedence rules apply You may use parentheses wherever you need to Here are some examples integer artihmetic C1 1 75 7 8 mixed 2 073 5 0 1 23E 3 1 that evaluate to 33 and 38 990160 respectively 3 4 String Operations Besides comparing strings with and you can concatenate them with the lt gt operator The following yields the value true for example abc lt gt def abcdef 3 5 List Operations Besides comparing lists with and you can add elements to lists and concatenate them Adding elements is done with the operator and con catenating elements is done with the o
4. s idea of what it is An example client that can communicate with the above server is as follows include standard ccl include udp ccl program client host port period cid udp_new_client host port count 0 time dclock 26 dclock gt time period time dclock udp_send_data cid str count count this some data count count 1 hostname if ARGC gt 2 then ARGV 2 else localhost end program main client hostname 8000 0 25 7 6 Graphics windows ccl The library 1ib windows ccl provides a simple interface to the GDK GTK X Windows toolkit see http www gtk org To create a window use one of the following int new_window string int new_window_custom string real real real The return value is an opaque window identifer needed for future references to the window so assign a variable to it The string argument to both functions will be the window title In the second function the last three arguments are the scale factor and the width and height of the window in pixels Thus w i new_window_custom data 10 0 100 100 creates a new window 100 x 100 pixels big named data The scale factor of 10 0 means that if you draw a line of length 1 with will be approximately 10 pixels long The origin of the window is the center x coordinates go from left to right y coordinates fo from bottom to top To draw somet
5. by its argument Note that this may not be especially accurate in a multitasking operating system like UNIX 7 2 Math math ccl This library defines as external functions all the trigonometric functions logarithms square root floor and ceiling It also defines the symbol pi to be an approximation of 7 20 The math libary also defines a few lambda functions as follows abs x Evaluates to the absolute value of the integer or real value of x max xX y Returns the maximum of x and y min X y Returns the minimum of x and y sign x Returns 1 if x is positive 1 if it is negative and 0 if it is zero dot x y Returns the dot product of the lists x and y which should be integer or real lists of equal length mmult A B Returns the matrix product of A and B which should each be lists of lists of integers of reals To use dot or mmult you would write for example include math ccl include list ccl x i 1 2 3 dot x x A 1 3 5 gt 3 2 AAA OAN No w wey gt y mmult A tocol x The function tocol converts the row vector x into a column vector and is defined list ccl discussed later External vs Lambda If you look at CCL_ROOT 1ib math ccl you will notice that mmult is defined as 21 external real matrix_mult real list list real list list libcclmath so ccl_matrix_mult mmult A B matrix_mult A B That is the lambda abstraction mm
6. column vector It is equivalent to map Ax x v 7 4 Interprocess Communication iproc ccl The ccli iproc ccl library provides three functions and some behind the scenes functionality that allow programs to send messages to each other or to themselves via mailboxes The library keeps a list of such mailboxes internally in reality they are queues of messages and associates an integer id to each one Messages have type to int from int where the to field defines what mailbox the message is to be sent to or what mailbox the message was received from The from field is used to indicate what agent the message was sent from Typically each program in a composition of programs is associated with an id that it uses in the from field when it sends messages and that it expects in the to field when it receives them Additional fields defining the message content may be included in messages The iproc ccl functions are external unit send to int from int This function is used to add enqueue a message to the end of the mailbox queue associated with the integer identifier to external to int from int recv int This function is used to remove dequeue a message from the mailbox queue associated with the integer identifier to If that mailbox is empty a message with the from field equal to 1 is returned The call is non blocking external bool inbox int This
7. execution returns to the current program with possibly unintended results That is x does not refer to the value of x at the previous time step It is only the variable s previous value As a general rule only use the prev operator on local variables To compose more than two programs you could do for example 17 p 1 p 2 sharing x p 3 sharing x p 4 sharing x program main but you could accomplish the same thing with program main compose i in 1 2 3 4 p i sharing x which is a bit more concise The general form for the compose operator is program q param1 param_2 compose v in L p expr sharing vari var2 The variable 7 ranges over the values in the list L and may appear free in expr It is very important to note that in the present version of ccli the list L is evaluated in the global scope and can not refer to the parameters param1 param2 in the parameter list of q So you can not say program q n compose i in range n p i because n is not defined in the global scope in this example anyway The range function is defined in list ccl It takes an integer n and returns 0 n 1 Here s a more complete example that you could extend to do a multi agent simulation include list ccl include standard ccl program agent i print Hello from agent i n etc Fs program quit true exit n atoi ARGV 2 prog
8. external bool input ready This functions takes no arguments and returns true if and only if a key has been pressed but not read from the keyboard More exactly it returns true if and only if the standard input buffer is nonempty external string get_char This function takes no arguments and waits for the next key to be pressed It returns a length 1 string with the ASCII representation of the key pressed To be used in a non blocking sense guard the function with input_ready 19 external unit exit This function kills ccli external int atoi string This function converts its argument which is supposed to be a string repre sentation of an integer into an int external real atof string This function converts its argument which is supposed to be a string repre sentation of an real into a real external string tostring a This function takes an argument of any type and converts into a string external int uclock mclock dclock These functions return the number of microseconds milliseconds and sec onds respectively since ccli was started Here is an example program that uses all three time functions include standard ccl program main print Press any key to print the clock values n input_ready get_char print rms mclock us uclock os dclockQ r T external unit usleep int Sleeps for the number of microseconds specified
9. function is used to check whether the mailbox associated with its integer 23 argument has any messages in it If it does true is returned otherwise false is returned Two considerations need to be taken into account when using the iproc ccl library First be careful that your programs do not send significantly more messages to a mailbox than will be received from that mailbox If a pro gram keeps sending messages without receiving them it will use up all the available memory on the system The iproc ccl helps you be careful about this by issuing annoying warnings every time a message is sent to a mailbox with more than some maximum number of messages in it 100 in the last distribution Second ccli can not type check messages sent with the li brary That is one program might send a message with a string field named msg Another program may receive the message and then try to access a field named msg but use it with type int Or it may try to access a field that was not defined in the message when it was sent Both of these errors will not be caught by ccli at compile time and will issue runtime errors Here is an example to study It defines n agents numbered 0 to n 1 When agent 7 receives a message from agent i 1 it sends a message to agent i 1 Agents 0 and n 1 behave a little differently Note that the program prints out the messages in increasing order of agent id regardless of how the commands in the main composition ar
10. main xX o2 1 x gt 0 X s ab ds print x n F3 which is a ccli program If you make a file with the program above in it and then run ccli on the file you will get 2 OORU ad infinitum press cntl C to halt ccli It starts by setting x to 1 and then it continuously executes the guarded command over and over again The general syntax for a program declaration is program p param_1 param_n statement_1 statement_2 statement_m where the statements are either assignments of values to variables and there fore initializers or they are guarded commands Guarded commands have the form 13 boolexpr command_i command_2 command_k which means to ccli anyway that while executing the program main if boolexpr is true then execute all the commands in the body of the guarded command In each iteration of the program ccli considers the guarded commands in order and then in those whose guards are true executes the assignments in order as well A command is either an assignment or an expression In the latter case the expression is evaluated and its result is thrown out which is useful when the expression has side effects as with the external function print Initializer statements can also have the form needs X y Z which tells ccli that the enclosing programs needs the values of x y and z to be initialized elsewhere This will make more sense after you read Section 6 2
11. program that makes a spinning red square with a black border 28 include windows ccl include math ccl fun make_square r theta let c r cos theta s r sin theta in de Gy Sk Sy Ee A Sy SB Sy HS Sec FF end program spin r delta theta 0 0 w new_window_custom spin 10 0 30 r 30 r true erase w setcolor w red poly w make_square r theta true setcolor w black poly w make_square r theta false refresh w theta theta delta Fs program p r delta spin r delta window_manager program main p 10 0 05 Another function that might be useful if you want to save a graphic or make an animation for example is the function wW filename external unit tofile int string libcclwindow so ccl_tofile which saves the image in the window identified by w into a file named filename The file name should end with the graphics format you want jpg png ps or ppm 29 8 Defining External Functions in C You can define you own external functions in ccli This is very useful for interfacing ccli with other APIs such as the GTK or to low level hardware code In this section we outline how to do this The procedure is roughly 1 Define a C function 2 Compile it into a shared object library 3 Delcare it within a ccli file It is easiest to explain with an example The rest of this section shows
12. the initial value of x and the parameters a and b and passes them to plant and passes 2a b to controller the gain in the control law Most importantly it says the the variables u the control input to the plant and y are shared that is assignments in either the plant part or the controller part of system will be to the same memory location Meanwhile the variable x remains local to plant and invisible to other programs The needs y declaration in the controller program is used to specify that the variable y needs to be initialized by some other program in this case by plant A similar situation holds for the needs u declaration in the plant program Any variable declared as needed should be included in a shared part of a composition for it to be visible to the program that initializes it Programs may initialize the same shared variables if they do not use the needs delcarator The order of composition is important This is because ccli will execute the initializers in order of appearance Thus if program p initializes x to 1 and program q initializes x to 2 then program p q sharing x initializes x to 2 ccli infers the types of all variables appearing in a program and makes sure that if a variable is shared between two programs it has the same type in each Warning Be wary of accessing x for any variable that is shared between multiple programs The variable x may be assigned multiple times elsewhere before
13. CCL The Computation and Control Language User s Manual Eric Klavins Electrical Engineering Department University of Washington Seattle WA 98195 klavins at washington edu 1 Introduction This manual describes the CCL language and associated tools To get started with CCL make sure that you have CCL built and installed and that the CCL_ROOT and LD_LIBRARY_PATH environment variables are set correctly see the CCL web page for details Here is how to say hello world in CCL First make your own directory for your examples mkdir code cd code In your new directory create a file called hello ccl with the following text in it include standard ccl program main str Hello world true print str n exit F Run this program by executing ccli hello ccl on the command line Note that ccli is the name of the CCL interpreter The result of executing the above should be that Hello world is printed and then the program exits If you get this to work you have CCL properly installed on your system Here s how hello ccl works The first line includes some standard func tion definitions In particular we need the print function and the exit function The rest of the file declares a program called main The CCL interpreter expects that a program called main will be defined If it finds one it will execute it If it does not it will just quit Within this particular main program th
14. about program composition As we have noted ccli looks for a program called main to execute So if you want something to happen you need a main program defined some where The parameters can be used to create a whole family of programs one for each set of values of the parameters For example you might write include standard ccl program p x0 delta x x0 x gt 0 x x delta print x n program main p 5 35 0 01 which produces the output 14 ad infinitum The program main should not have parameters 6 1 Variable Memory After a variable is assigned ccli can remember its previous value For a variable is x the expression x read prev x refers to its previous value Note that x is the same as x That is ccli only remembers one step back This is useful in programs where you might need to know the difference between the current value of a variable and its value the last time through For example here is one way to write the Fibonacci sequence include standard ccl program main x sao true print CE kee J Ant X sax t Executing fib 1 produces gt gt oaonwne v w w ew 13 13 21 21 34 KAA AAA KY OoOnwnNnrer ad infinitum Warning Assignments such as a i xorr a xare designed to be as efficient as possible and for technical reasons do not preserve the variable memory structure Thus 15 a
15. ard math functions like cosine and sine To use them you write for example include math ccl x sin 0 123 1 but not include math ccl x sin 0 123 1 which will produce a type error saying something like you can t apply the value sin as though it were a lambda expression This is because sin is declared as an external function type which means that to use it you have to put its arguments in parentheses Another example is the print function which takes any number of arguments of any types and prints them include standard ccl b s 0 53 y 1 2 print Wy ii X and y ti y n 11 which does about what you would expect Note that every expression in CCL evaluates to a value of some type Functions like print evaluate to the type unit which ccli prints as a period as in You aren t supposed to use the result of a print call but it has to have a some sort of value this is like void in C The math library and other libraries are described in Section 7 External functions are declared with return types are argument types Types are denoted by type expressions which you have seen in ccli s type error reporting Type expressions are defined by the following grammar typeexpr variable atomic listexpr rec func variable a b atomic unit bool int real string listexpr typeexpr list rec var typeexpr func typeexpr g
16. ause without access to its C definition ccli is not able to infer the type of the function Furthermore the declaration tells ccli to find the object code for the function in the file libcclmath so which should be either in the current directory or in a direc tory listed in the LD_LIBRARY_PATH environment variable The last argument gives the name of the C function in this case ccl_cos within the shared object library For more information see the code in the standard and graphics direc tories Also learn about shared object libraries by reading the dlopen man page i e man dlopen 9 Invoking ccli The ccli command has the following general form ccli file ccl lt args options gt That is the first argument to ccli is a file containing CCL code The rest of the arguments are either arguments to your CCL program or if they begin with a leading a dash or minus sign they are options to ccli The arguments to your program are available via the ARGV and ARGC variables as discussed in Section 4 The options are stripped before being put in the in ARGV array and do not count toward the size of ARGC The options are put as strings into a global variable called OPTIONS just in the off chance that your program might need to know about them At present ccli takes the following options pstring Defines the name of the main program to be executed to be string If this option is not given then the name main is used as discu
17. e scheduled try running it with and without the r option include iproc ccl include standard ccl include list ccl program agent i n got_mesg if i 0 then true else false end sent_mesg false got_mesg amp sent_mesg amp i lt n 1 send to iti from i msg hi from lt gt tostring i sent_mesg true inbox i amp i n 1 print recv i a Dy 24 got_mesg true inbox i amp i n 1 print recv i na exit program main compose i in range 10 agent i n 7 5 UDP Datagrams udp ccl The udp cc1 library allows CCL programs to communicate with other CCL programs running as truely separate processes started with a different call to ccli possibly on other machines The library is similar to the iproc ccl library except that explicit servers and clients are used The library consists of five functions The first three are used by UDP servers external int udpnew_server int The argument to this function is the UDP port the server should listen to Port numbers in the 7000s are good for experimental programs The return value is an opaque integer server id sid that is used in udp_is_ready and udp_get_data external bool udp_is_ready int The argument to this function is the sid of a server started with udp_new_server It returns true if and only if there is data ready to receive in the server s buffer
18. eger defined to be the number of arguments on the command line to ccli excluding options to ccli The second ARGV is a list of the arguments on the command line interpreted as strings For example if you call ccli like this ccli file ccl 10 unix then you get ARGC 4 ARGV ccli file ccl 10 unix Note To convert the string 10 into the integer 10 use the standard ccl function atoi as in include standard ccl n atoi ARGV 2 Recursion Having variables allows us to write recursive functions ccli provides the keyword fun for declaring recursive functions Here s the hoary chestnut fun fact n if n lt 0 then 1 else n fact n 1 end fact 5 10 that you see in every programming language manual The code above sets the variable fact to a representation of the factorial function and then applies it to the value 5 to give 120 Note you can t do fact lambdan if n lt 0 then 1 else n fact n 1 end because ccli will complain that fact is an unbound variable which it is in this case the right hand side of an assignment is always evaluated before the left hand side unless you use the fun keyword 5 External Functions and Parentheses ccli provides some externally defined functions and you can define your own as described later These functions are not lambda expressions and thus they use a different syntax For example the file CCL_ROOT 1ib math ccl defines some stand
19. ere is one variable initializer for the variable str and one guarded command The initializer just sets the variable str to the string Hello world The guarded command has a guard true in this case and a 2 set of commands The commands print the value of str and then exit The CCL interpreter starts executes programs by first initializing variables and then executing the guarded commands over and over again This guarded command exits the first time it is executed Otherwise it would print Hello world repeatedly until the interpreter is killed with Cnt1 C try removing the exit line to see this behavior Note Another way to evaulate expressions with ccli is to put them on a line by themselves outside of a program definition Thus the above could have been written include standard ccl str Hello world print str na and essentially the same output would be produced 2 Values CCL supports the following types unit boolean integer real floating point string list record abstract functions lambda external functions Lists are homogeneous All elements of a given list must have the same type Here is an example program that demonstrates the different types booleans true false integers 1 2 reals 1 0 3 1415927E 2 strings abcdefg n lists 1 2 3 4 ta b c records x 5 y z ccl is cool the identity function
20. hing in a window use one of the following functions W x1 yl x2 y2 unit line int real real real real W points filled unit poly int real list list bool W x y r filled Zt unit circle int real real real bool W x y wid hi al a2 filled unit arc int real real real real real real bool W X y text unit text int real real string The first argument to each of these functions is a window identifier created with one of the new window functions The rest of the arguments are unique to the functions and should be apparent from the annotations above After drawing lines or polygons you need to refresh the window before you will see anything Do this using the function unit refresh int which takes a window identifier as an argument If you want to draw in some color other than black the default use unit setcolor int string which takes a window identifier and a string that represents the color cur rently black white red blue green or yellow To erase a window use unit erase int and then refresh To process events in a window like exposures mouse clicks etc use unit update_windows You usually don t use this function but instead compose your windows progams with the following program which is in lib windows ccl program window_manager true update_windows For example here is a
21. how the cos function is defined First in standard ccl_math cc we define the C function include lt math h gt include SymbolTable hh extern C Value ccl_cos list lt Value gt args return new Value cos args gt begin gt num_value All external function definitions must be declared this way The external C part tells the C compiler to use C naming conventions important for making shared object code usable by other modules The return value must be Value a pointer to a Value object The Value class is defined in SymbolTable hh The argument is a list of values to be computed from within ccli when the function is called We use the STL list template to define lists The body of the function makes a new value using the Value double constructor and the C cos function The argument to the cos function is the numerical value of the first value in the argument list To compile you do something like g shared o ccl_math so ccl_math cc I CCL_ROOT base where ccl_math cc is the name of the file containing the aboce function Look at standard Makefile for details From within a ccl file you declare the cos function as in external real cos real libcclmath so ccl_cos 30 This declaration states that the symbol cos should be considered to be an external function that takes a single real value and returns a single real value This type information is needed by ccli bec
22. in the value 10 Note that lambda x lambda y x y isa function that takes an argument x and returns another lambda expression Thus lambda x lambda y x y 10 returns another lambda expression as a value namely lambda y 10 y You can apply the whole expression to two numbers as in lambda x lambda y x y 10 11 which evaluates to false The function lambda x lambda y x y is polymorphic mean ing that it can be applied to any two arguments of the same type In fact ccli considers its type to be 2a gt a gt boolean where a is a variable type i e it stands for any type If you apply the function to a string as in lambda x lambda y x y ccl you get a function of type string gt bool That is putting a string as the first argument adds the constraint to the ccli type checker that the second argument should also be a string Thus lambda x lambda y x y ccl 5 produces the type error Type error in hello ccl on line 23 could not apply function to argument lambda x lambda y x y ccl has type string gt boolean while 5 has type integer noting the problem You also have to be careful about parentheses For example the following produces an error lambda x lambda y x y 10 11 because it parses as lambda x lambda y x y 10 11 which makes no sense you can t subtract an integer from a function Thus u
23. lambda x X you can also write the above as xX X More interesting expressions are covered in the next section External functions are covered later as well print and exit are examples Remember that expressions appearing outside a program terminated by a semicolon are evaluated as they are encountered by ccli and ccli prints each expression and its valuation Thus the output of the above program will look a bit repetitive Also note that lines beginning with are treated as comments Comments may also appear between and as in C The unit type is reserved for external functions that do not return values Don t worry about it for now 3 Expressions Values are atomic expressions not made up of other shorter expressions In this section we describe all the kinds of expressions you can build up from shorter expressions in CCL 3 1 Boolean Operations Here is how you write and not and or simple boolean expressions true amp false true true false You can also compare integers and reals with lt gt lt and gt as in 1 0 lt 2 3 0 gt 7 amp 3 0 gt 5 0 false liberally mixing integers and reals and putting in parentheses Finally you can test for equality and inequality of booleans integers reals strings and lists as in 1 2 a I pn 1 2 3 _ 4 5 Test the equality of other types in not supported Warning In ccli means equals
24. o this is for example fun draw w line C W si O 0 1 1 line w 0 1 0 which draws a crosshairs in the window w It s return type is unit list which you can just throw out What if you want the function to return a value or the functions you want to call don t all return unit Then put the commands in a let expression as in 32 fun draw w let dummy line w 1 0 1 0 line w 0 1 0 1 in true end which returns true and could have any expression you want for the body of the let expression You would have multiple definitions in the let statement for functions return other types 33
25. perator For example to insert the value 1 into the beginning of the list 2 3 you do 10 2 3 which yields the list 1 2 3 To concatenate two lists you write 0 1 4 2 3 which yields 0 1 2 3 Note that you might find ccli printing out strange things like 00 10 26 No worries this is just how ccli stores the expression 0 1 2 The two expressions evaluate to the same thing 3 6 Record Operations If you want to get the value of certain field in a record you use the operator For example x 0 y 1 x evaluates to 0 You may construct new records from old using the lt lt operator For example x 0 y 1 lt lt x 1 evaluates to x 1 y 1 More generally the expression r lt lt s has all fields appearing in either r or s The value associated with the a particular field x in r lt lt s is s x if x exists in s and otherwise it is r x If r and s have a field name in common but do not assign that field a value of the same type a type error is produced 3 7 Lambda Expressions Applications and Currying A lambda expression is a way to write a function without really giving it a name For example here is a function that takes a number and negates it lambda x x and here is another that takes two elements are compares them lambda x lambda y x y You can apply these functions to arguments by juxtaposing them as in lambda x x 10 which results
26. ram main compose i in range n agent i quit If this code is in a file is called agent ccl then executing ccli agent ccl 5 results in 18 Hello from agent Hello from agent Hello from agent Hello from agent Hello from agent BwWNYNRP OO 7 Standard Libraries In this section we describe most of the functions defined in the core CCL libraries These include standard I O functions mathematical funtions list operations and graphics operations The list of functions here may not be complete Look at the actual library files in CCL _ROOT lib for a com plete list of functions available and brief descriptions of their use To use the functions in a library simply include it at the beginning of your code CCL automatically looks in CCL ROOT lib and then in the local directory for included files 7 1 Standard Functions standard ccl This library defines basic I O functions and other common functions Almost every program you write will need at least one of the functions in this library external unit print This function allows you to print to the terminal standard out It returns unit and so should not be used in assignments It takes any number of arguments of any type converts them to strings and prints them one after the other String arguments may contain standard escape sequences like n and t newline and tab This is similar to the UNIX printf function which print actually uses to print strings
27. se lambda x lambda y 10 11 or lambda x lambda y 1 x y x y 10 11 depending on your intentions 3 8 Conditionals You can write if then else statements that evaluate to values as in if 1 gt O then 1 else O end which evaluates to 1 The then and else parts of the statement must have the same types and the if part must be a boolean expression Thus the following produce type errors if 1 then 1 else O end if 1 lt 0 then 1 else oops end It is important to remember that if statements evaluate to values kind of like thea b c syntax in C So for example the following is perfectly legal ccl is lt gt if true then strange but beautiful else just strange end which you can probably evaluate in your head 3 9 Let Expressions With a let expression you can declare a local variable For example the expression let x 10 y x 2 in x y end evaluates to 15 Once again all expressions evaluate to a value in this case an integer Thus you can write for example 1 let x 10 y x 2 in x y end which evaluates to 16 4 Variables and Recursive Functions In ccli you can use variables in place of values Assignments are done with the operator For example we could write f lambda x lambda y x y 2 which results in the value 10 5 Special Variables Two variables are predefined by ccli The first is ARGC an int
28. ssed above r Tell ccli to executes the clauses in the main program in a random order Normally the clauses are executed in order of appearance in the main program With the r option each clause is executed once before any can be executed again A sequence of steps wherein each clause is executed once is called an EPOCH Each epoch has a different ordering Using this option allows you to see the effect of arbitrary orderings on 31 your program in case you are interested in modeling distributed interleaved systems d Tells ccli to execute the main program using a simple single stepping debugger The commands are executed in order After all variables are initialized a command prompt is printed along with a line number and file name of the next guard or command set to be executed At this prompt you may run the following commands s Execute the currently printed guard or command and move to the next p var Print the value of the variable var t n Print the symbols and their values in top n scopes T Print the symbols and their values in all scopes q Quit 1 Clear the screen h Print a very simple help message Note the r and d options cannot be used together 10 Tips and Tricks 10 1 Side Effects Some external function calls have side effects and return type such as print or many of the windows functions You might want to write a function that draws a picture involving several windows calls One way to d
29. t typeexpr The special symbol may appear in record expressions and as the last expression in the list of arguments to an external function It means there may be more arguments For example external x int func a list library so func declares the external function func It has a record return type that has the integer field x defined and may have other fields It takes at least one argument a list of any type and may take more arguments of any type Including Libraries We have already seen the include statement It is a preprocessor directive i e include is not a keyword it is a directive to the lexer part of ccli that reads in ccl files It tells ccli to read in the file noted in its argument before continuing with the preset file ccli provides several libraries in CCL_ROOT 1lib including standard input and output math functions list operations and simple graphics tools These are described in more detail in Section 7 6 Programs Programs are the heart of CCL A program consists of two parts A set of initializers and a set of guarded commands Programs are intended to be the guts of a while statement As in 12 x Hs while true do if x gt O then x x 1 end print x end while which is not a ccli program by the way In this psuedocode snippet the line x 1 is an initializer and the if statement is a guarded command In CCL you write include standard ccl program
30. ult is just a wrapper around the external function matrix_mult You can use either one The designers of ccli could put wrappers around all external functions that take a fixed number of argu ments and maybe they will in a future release For now the goal is to make things fast so computational overhead involved in computing the wrapper although small is significant after repeated calls 7 3 Common List Operations list ccl The list library contains a number of functions useful for manipulating lists and lists pretending to be arrays or vectors The functions map length table and range are the workhorses of ccli rev L Returns a list with the same elements as L but in reverse order map f L Returns a list obtained from L by applying f to each element of L length L The length of the list L zip AB Returns a list of pairs A 0 B 0O A 1 B 1 makelist n default Returns the list that is n elements long all of whose elements are equal to default sumlist L The sum of all the elements in L where L should be a list of numbers table f nm Returns the list f n f n 1 f m range n Returns the list 0 1 n 1 member x L Returns true if x appears in L and false otherwise 22 remove x L Returns a list identical to L except with all occurences of elements equal to x removed cross A B Returns the cross product of A and B taken as multisets tocol v Good for changing a row vector into a
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