CEP GE: a full-screen structural editor
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1. x 1 DELETE lt statements gt repeat if lt condition gt then lt statement gt else type lt statement gt fa untilx 0 snd perce 7 e Cursor postion Q Function key selected by user FUNCTION KEY OnNOAIA WN Fig 1 Refinement procedure record etc Instead a menu allows him to choose between conditional procedure declaration record type declaration etc and the system produces the correct syntax for him routine tasks should be done by machines not people The only case when the keyboard apart from function keys is necessary is when the user has to supply a text that the system could not invent by itself such as an identifier or a comment The text window is used for this The text is written in it using the full screen text editor included in CEPAGE 2 3 BASIC FUNCTIONS The main functions offered by CEPAGE fall into the following categories 1 browse scan the document up or down within the hierarchy of syntax entities backwards or forwards within lists 2 construction modification creation of entities modification of earlier versions insertion and deletion within lists 3 copy transfer reproducing or moving an element of text using the definition operation as described below 4 save retrieve using a file in a suitable form to save the present state of a partially or completely refined document retrieval of a document previou2. Proceedings of the 5th International Confer ence on Software Engineering 422 429 1981 San Diego CA Kaiser 82 G E Katser and E Kant Incremental expression parsing for syntax directed editors Computer Science Report Carnegie Mellon University Pittsburgh PA Oct 1982 Meyer 81 B Meyer Ensorcel entr es et sorties sans format 1 re partie Atelier Logiciel 4 Electricit de France April 1981 Fourth Edition Meyer 82 B Meyer Principles of package design Communi cations of the ACM 25 7 419 428 July 1982 Meyer 83a B Meyer Towards a two dimensional program ming environment In Proceedings of the European Conference on Integrated Computing Systems ECICS 82 Stresa Italy 1 3 September 1982 P DeGANO and E SANDEWALL Eds 1983 North Holland Amsterdam Meyer 83b B MEvER and J M Nerson Showing programs on a screen Internal Report HI 4590 01 Electricit de France Sep 1983 Meyer 84a B MEYER A system description method In Workshop on Specification Languages Orlando FL March 1984 to appear Meyer 84b B MEYER and J M NeRson Showing programs on a screen submitted for publication 1984 Mikelsons 81 M MIKELSons Prettyprinting in an interactive programming environment Sigplan Notices 16 6 108 116 June 1981 Oppen 80 D C Oppen Prettyprinting ACM Transactions on Programming Languages and Systems TOPLAS 2 4 465 483 Oct 1980 Schroeder 83
3. a user of a full screen editor to go back to a line by line editor regardless of its other qualities In our experience this also goes for structural editors line by line type editors will not find favour amongst those who are used to full screen systems The objectives of CEPAGE derive from these con siderations It was a matter of combining the advantages of structural editors as regards security and power with the power of full screen text editors and the benefits of modern terminals The CEPAGE project was not intended to be a research project but rather an exercise in technology transfer intended to win industrial support for an idea structural editing which has been the subject of consider able research In fact we had to invent a little more than we had originally imagined The main sources of our inspiration were as far as structural editors were concerned GANDALF and toa lesser degree MENTOR and CPS SMALLTALK also influenced us as a man machine interface model By any objective standards CEPAGE is a small scale project Specification and design were carried out by the two authors of this article Its implementation was due almost entirely to Jean Marc Nerson in addition CEPAGE includes a small text editor written by N Triquet Initial discussions were held at the beginning of 1983 with the aim which was met of producing a working prototype by 20 December 1983 Programming consists of about 6000 lines
4. an abstraction applied to one or more sublists of a list as in 231 statements p expression 57 statements At each stage of the session the system determines the focus of the user s apparent attention after the last operations he has carried out and seeks to display as detailed as possible a view of a portion of the document around the focus and decides on whatever abstraction and collapsing is necessary It deduces the displayable form that is then transmitted to GESCRAN for eventual display on the screen Finding a good representation for display purposes proved to be an unexpectedly difficult task We were surprised by the lack of available documents if the brief reference in Barstow 84 is excluded the only published reference to the best of our knowledge is Mikelsons 81 which is difficult to use because of its lack of precision and the special characteristics of the environment des cribed The abundant literature on program formatting pretty printing is of little use here the basic assump tion though it is not generally stated explicitly cf in particular Oppen 80 is that though the length of individual lines may be limited the number of lines is not For screen formatting however columns and lines are severely limited resources We were consequently led to design algorithms we have described elsewhere Meyer 83b 84b which are beyond the scope of this paper These algorithms be
5. to supply at certain stages of a session e g identifiers comments 3 A Menu window provides a list of options available at any stage 4 A Type window gives the syntax type of undefined elements see discussion below 5 Reserved windows not shown in Fig 1 give information on documents or elements of documents other than the document being edited these windows are used to change document during a session as well as for copy and write operations 6 A Message window displays diagnostics 2 2 DIALOGUE At each stage of a CEPAGE session the system gives the user a choice between a certain number of options on amenu The menus are sufficient for the basic CEPAGE functions a user manual is not necessary once the user has a reasonable grasp of the basic concepts of the system In the current IBM version choices between the various elements of the menu are made using function keys on the terminal On more advanced terminals a mouse might be used The cursor is used whenever an element of the document has to be selected e g to indicate which terminal will be used for editing as in Fig 1 a by placing it over the element in question This is the only mode of access to a document the concept of a line number for example is not used A more rapid device such as a mouse would be particularly valuable There are some more advanced functions using com mands these commands are made up of a single w
6. A SCHROEDER Outils d analyse des programmes sous Mentor Globule AFCET no 4 1983 Shneiderman 83 B SHNEIDERMAN Direct manipulation a step beyond programming languages Computer IEEE 16 8 57 69 Aug 1983 Teitelbaum 81 T TerreLpAum and T Reps The Cornell Program Synthesizer a syntax directed programming environment Communications of the ACM 24 19 563 573 Sep 1981 Wilander 80 J WILANDER An interactive programming system for Pascal BIT 20 163 174 1980
7. CEPAGE a full screen structural editor Bertrand MEYER Computer Science Department University of California Santa Barbara California 93106 U S A Jean Marc NERSON CIMSA 10 Avenue de l Europe 78140 V lizy France ABSTRACT This paper describes CEPAGE an editor for structured documents designed for ease of use on modern terminals The design of CEPAGE is the result of work on syntax editors full screen editors and advanced software environments CEPAGE is a universal editor in which the language description is merely a parameter its external interface is designed for the children of the video age Although itself a prototype CEPAGE embodies the properties which a structural editor usable in an industrial environment should possess Keywords structural editors syntax editors human interfaces man machine communication programming environ ments program formatting ergonomics 1 Objectives CEPAGE is a structural editor a term we prefer to syntax editor whose human interface was designed with particular care It is completely parameterizable and can be applied to any language defined by a grammar a programming language or specification language but also languages for describing structured documents of any kind we shall from now on use the word documents for the objects the editor is used to construct CEPAGE belongs to a tradition of structural editors developed over the past few years Hansen 71
8. Out extends the element delimited to the structure immediately enclosing it syntactically e g if a statement has just been defined the whole of the block that contains it is included 2 In cancels the effect of the previous out function by returning to the next lower level CEPAGE A FULL SCREEN STRUCTURAL EDITOR 63 3 Extend left includes the immediately preceding element applicable when the current element is a sublist the three complementary operations are exclude left extend right exclude right 4 Accept the currently delimited element is accep ted 5 Cancel 2 5 LANGUAGE MODIFICATIONS CEPAGE is completely language independent the syntax concrete and abstract is a parameter which can be modified as required In the present version descrip tion and modification of the language is carried out by inputting a grammar It is expected that this operation will eventually by implemented as an interface to the system itself i e that one of the languages that CEPAGE will be defined for will be a syntax description language it is entirely in keeping with the general principles of the design of CEPAGE that the user need not have any knowledge of the concrete syntax of this language Language modifications may seem fairly useless in practice as long as CEPAGE is delivered with descrip tions of the main languages However the possibility of easily adapting the language description to local condi tions se
9. Statement Loop Uscleration A m Boulet Statement B x ies va x 1 Concatenated node List node Alternative node Text node Fig 4 Abstract syntax tree 64 INTERFACES AND EDITING I 4 Text nodes correspond to the terminal elements defined by the user using the text editor included in CEPAGE 3 2 OTHER DATA STRUCTURES Other data structures complement the first two Apart from abstract syntax trees the following three represen tations are necessary for documents 1 A form that can be displayed as a set of elements to be transmitted to the display package GESCRAN for display on the terminal screen at each stage of the session 2 A storable form for storage and later retrieval of the current state of documents 3 Text form the ultimate aim of the editing process The abstract syntax forest is also associated with a dictionary containing the different text elements neces sary identifiers etc The leaves of the syntax trees contain references to the dictionary 3 3 ALGORITHMS Note that the objectives defined above imply the absence of syntax analysis parsing in CEPAGE Con struction of a text takes place by successive choices corresponding to the abstract syntax the concrete syntax s constructed by the system which actually does the nverse of syntax analysis sometimes known as un parsing The freedom the user has in describing the language permits a good compromise to b
10. Wilander 80 Donzeau Gouge 81 Teitelbaum 81 Habermann 82 Allison 83 Donzeau Gouge 84 Structural editors as opposed to normal text editors permit documents to be manipu lated not as simple sequences of lines or characters but as structured objects by applying operations to them Work carried out initially at Electricit de France Direction des Etudes et Recherches 1 avenue du G n ral de Gaulle 92141 Clamart France defined in terms of their structure Among the main advantages of this method are 1 The guarantee that only syntactically correct docu ments will be produced 2 The possibility of carrying out transformations that may be complex but are guaranteed correct e g trans formations to improve portability or efficiency of pro grams 3 The possibility of relieving the user of some of the routine tasks associated with the necessity in a normal text editor of supplying all the details of the concrete syntax of the documents 4 The possibility of automatically translating docu ments from a syntactic framework into another frame work e g in the case of conversions between program ming languages 5 The use of standardized data structure generally the abstract syntax tree which can serve as a support for other software tools e g Schroeder 83 or even complete programming environments Habermann 82 CEPAGE A FULL SCREEN STRUCTURAL EDITOR 61 In spite of these qualities structural
11. amming Environ ments D R Barstow H E SHROBE and E SANDEWALL Eds 288 299 1984 McGraw Hill New York Boehm 82 B W Bormm Software Engineering Environ ments 1982 Prentice Hall Englewood Cliffs NJ Brisson 82 G Brisson B MEYER and F VApNE FICHEUX Ensorcel Entr es et Sorties Sans Format 2 partie Atelier Logiciel 6 Electricit de France Dec 1982 Card 83 S K Carp T P Moran and A NEWELL The Psychology of Human Computer Interaction 1983 Lawrence Erlbaum Associates Hillsdale NJ Donzeau Gouge 81 V DonzEau GoucE G Huet G KAHN and B Lang Environment de programmation Men tor pr sent et avenir In Actes des Troisi mes Journ es Francophones sur l Informatique 1981 Geneva Donzeau Gouge 84 V Donzeau Gouce G Huet G KAHN and B Lanc Programming environments based on structured editors the MENTOR experience In Inter active Programming Environments D R BARSTOW H E SHrose and E SANDEWALL Eds 128 140 1984 McGraw Hill New York Habermann 82 N HABERMANN et al The Second Compen dium of Gandalf Documentation 1982 Carnegie Mellon University Pittsburgh PA Hansen 71 W J Hansen Creation of hierarchic text with a computer display ANL 7818 Argonne National Lab oratory Argonne IL 1971 Also as dissertation Computer Science Department Stanford University June 1971 Lewis 81 J W Lewts Beyond ALBE P language and neutral form In
12. ccount as easily as in say GANDALF The grammar graph is a data structure representing the grammar of the language Abstract syntax is used as a basis for language description it is described by a collection of syntax types and production rules Each syntax type appears on the left hand side of at most one production rule any that do not appear to the left of a production rule are said to be terminal There are three types of production rules known as aggregate choice and list illustrated respectively by the following examples conditional c boolean st 1 st 2 statement statement assignment conditional compound compound statement Concrete syntax is obtained by decoration of abstract syntax productions every list type production rule is associated with a header delimiter and end e g begin the semicolon and end in the case of compound The grammar graph contains all this information The abstract syntax forest is a set of abstract syntax trees each associated with a document or an element of a document in the course of elaboration There are four types of internal nodes on an abstract syntax tree Fig 4 corresponding to the four types of production rules 1 Aggregate nodes have a fixed arity 2 Choice nodes simply represent a choice in a production rule of the choice type 3 List nodes can have any number of sons Program Br gis ae Composite oo adeeb
13. e established between ease of use and the degree of detail that the system permits e g expression can be treated as a terminal Another technique for this type of syntax entity not used in the present version of CEPAGE is that of Kaiser 82 which is halfway between parsing and unparsing Though there is no syntax analysis we still had one difficult algorithm to handle for constructing the dis played form of documents The issue here is to provide at any given moment a representation of the state of the document which is as clear as possible while respecting the limits imposed by the physical size of the terminal Using a text editor whether full screen or not we can usually only provide an extract of the document giving a few contiguous lines some editors have an option of excluding groups of lines from the part displayed in order to concentrate on the most important elements at any given moment A structural editor must be capable of giving a global view of the document or part of it even if it cannot represent all its details on the screen The solution is elision certain elements of the document can be replaced by an abbreviation more exactly by a simple indication of their type In this way a 2000 line procedure could be represented by the simple indication procedure we call this type of abbreviation an abstrac tion The second type of abbreviation carried out by CEPAGE is collapsing which consists of
14. editors have not made much headway in industry One of the main reasons for this is in our opinion their external interface which in most cases is of the line by line type i e dialogue with the user takes place by way of exchanges of commands and responses Today s programming en vironments however increasingly frequently offer full screen text editors such as SPF on IBM EMACS on MULTICS and VAX UNIX or VI on VAX UNIX which take full advantage of the potential provided by modern terminals The characteristics of these systems can include the following Meyer 83a 1 Use of the whole screen instead of a line as a unit of communication between the system and the user giving the user a much wider view of the document during construction and consequently permitting him to exer cise better control on the whole editing process 2 The possibility of providing more easily than in a line by line system personalized dialogue by storing individual user profiles 3 Providing users with several simultaneous views of the document being handled 4 Finally and more generally the application of the direct manipulation principle Schneiderman 83 according to which the user has a better command of the system if he is provided at any given time with a clear representation of the current state of the objects at hand The advantage of these different characteristics is such that it is almost impossible to persuade
15. ems highly desirable to us In particular it permits programming standards to be set up more conveniently and in a way that makes them more easily acceptable than by using a posteriori validation tools Language subsets conventions relating to comments and to the structure of programs etc can also be enforced in this way 3 CEPAGE technical choices 3 1 BASIC DATA STRUCTURES In the course of a session CEPAGE works Fig 3 on two main data structures 1 The internal description of the language or grammar graph 2 The internal description of a set of documents or abstract syntax forest It is important to note that these two data structures are treated in the same way This is what makes CEPAGE a Abstract syntax forest Display algorithms Fig 3 Data structures completely parameterized system with respect to lan guage the description of the language is interpreted repetitively by the system This distinguishes CEPAGE clearly from a system such as GANDALF which is parametrizable but uses a compiled description of the language i e a kernel version of GANDALF and a description of a language X or Z or C is used as a basis to obtain a specific tool GANDALF X or GANDALF C adapted to the chosen language The approach adopted by CEPAGE offers greater flexibility and makes it possible to modify a language easily On the other hand it does not permit semantic actions to be taken into a
16. have in a linear way with respect to the number of nodes in the syntax tree This is one of the fields in which we have had to resort to invention 4 The future of CEPAGE As was indicated at the beginning of this article the December 1983 version is a prototype which however covers the essential functions of the system We foresee the following developments 1 It will be necessary to study users reactions The design of CEPAGE rests on what we think is a good ergonomic basis for interactive systems an opinion supported by recent studies resting on solid scientific bases Card 83 but which naturally needs experimental confirmation 2 We also plan to port the system to other environ ments CEPAGE was designed to be portable the decision to use PASCAL in preference to an object oriented language such as SIMULA 67 used previously with success by the same group to implement high quality interactive tools was deliberately taken with just this in mind In the short term we intend to adapt CEPAGE toa UNIX environment both on a VAX and on a SUN workstation at the University of California the SUN is a programmer s workstation based on a 68000 with a high resolution bit mapped screen and a mouse This project is particularly important for us as it is only in hardware environments of this quality that tools such as CEPAGE can in our opinion fulfil all their promise We hope that CEPAGE will also be adapted to other si
17. milar systems PERQ APOLLO SM90 etc 3 We also need to add the major functions missing from the prototype particularly the language modifica tion tool and to prepare CEPAGE grammars for the main languages currently in use the prototype was tested with a grammar of a language similar to PASCAL We expect to be able to answer some questions that have had to be left hanging in the light of experience of the system in operation syntax analysis is one such issue and we have to decide whether to add a parser to later versions allowing existing programs obtained by other means to be manipulated by CEPAGE CEPAGE A FULL SCREEN STRUCTURAL EDITOR 65 We hope that the use of the first versions will confirm our view of the great potential importance of a powerful and ergonomic programming environment which could be derived from it REFERENCES Abrial 80 J R ABRIAL S A SCHUMAN and B MEYER A specification language In On the Construction of Programs C A R Hoare and R Perrot Eds 1980 Cambridge University Press Cambridge U K Allison 83 R ALLISON Syntax directed program editing Software Practice and Experience 13 453 465 April 1983 Audin 80 E Aunin G Brisson B MEYER and F VAPN FICHEUX Gescran Manuel de R f rence Atelier Logiciel 22 Electricit de France 1980 Fourth Edition 1984 Barstow 84 D R Barstow A display oriented editor for INTERLISP In Interactive Progr
18. of PASCAL in addition it uses the Gescran package for handling the screen interface Audin 80 produced by the same group and made up of 4000 lines of FORTRAN 77 Gescran is a collection of subprograms permitting full screen inter action to be described easily by manipulating only objects belonging to four abstract types called screen window zone and terminal and accessible only via primitives specific to the package Meyer 82 it uses the ENSORCELE I O package Meyer 81 Brisson 82 The rather special conditions in which this project was carried out no doubt explain the fact that all this hardly corresponds to good practice as defined by the most eminent authorities Boehm 82 It may be of interest to note that partial use of formal specifications based on the language Z Abrial 80 then on the M method Meyer 84a were of some assistance 2 Using CEPAGE 2 1 THE SCREEN The screen allocated to a CEPAGE session is divided into a certain number of windows Fig 1 Each window fulfils a specific function 1 A Document window contains a representation of the current state of the document being constructed or modified some elements of this representation dis played between chevrons e g lt statement gt corre spond to elements of the document that have not yet been polished and are said to be non terminal 2 A Text window is intended to receive non structured texts which it may be necessary
19. ord and their inclusion is a result only of the limited number of function keys available there are twelve CEPAGE therefore has no command language in the normal sense of the word all interactions within the system are made by pointing and touching In particular the user writing a program text with CEPAGE e g in PASCAL never has to strike a key for concrete syntax elements such as key words like if 62 INTERFACES AND EDITING I a document menu FUNCTION KEY procedure p in x integer type t lt record gt DOWN t1 array lt index gt of UP lt type gt FORWARDS var BACK Vit REFINE v lt type gt MODIFY begin INSERT while lt condition gt do DELETE p x 1 lt statement gt repeat type lt statement gt Rect nal untilx 0 ana a message 7 b ON aG A w n a menu INSTRUC KEY document procedure p in x integer type t lt record gt tl array lt index gt of lt type gt var vit v lt type gt ASSIGN 1 CALL 2 WHILE LOOP 3 UNTIL LOOP 4 CONDITIONAL BLOCK 6 READ 7 WRITE 8 ABORT 0 begin while lt condition gt do p x 1 e lt statement gt repeat type lt statement gt until x 0 message end c document procedure p in x integer type t lt record gt t1 array lt index gt of lt type gt var FORWARDS vit BACK v lt type gt REFINE begin MODIFY while lt condition gt do INSERT p
20. sly saved 5 generation production of the final form of a fully defined document 6 session management document selection change of document selection library definition etc a library is a collection of documents in the course of a session one can work on several documents but only one of them is active at a given moment one can move freely from one to another 2 4 DELIMITING ELEMENTS OF TEXT Delimiting text Fig 2 is an essential operation for functions that require the user to define a syntactic subsection of the document copying or writing for example make it necessary to define the part of the document to which the operation applies The mechan ism provided applies direct manipulation principles document menu procedure p in x integer type t lt record gt t1 array lt index gt of lt type gt var vit v lt type gt FUNCTION KEY l while lt condition gt do 4 L IPAXZ lt statement gt repeat type lt statement gt lt statement gt until x 0 end C nee caret Fig 2 Delimiting In order to delimit an element the cursor is placed anywhere within it and the extent of the element is given by a series of commmnds issued using function keys as indicated by the appropriate menu at each stage the system highlights the element by means of special display attributes e g colour or reverse video Commands provided for delimitation are as follows 1
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