Graphical Program Development with PECAN Program Development Systemst

Transcription

1 Graphcal Program Development wth PECAN Program Development Systemst Steven P. Ress Department of Computer Scence Brown Unversty Provdence, RI ABSTRACT Ths paper descrbes the user's vew of the PECAN famly of program development systems. PECAN s a program development system generator for algebrac programmng languages. The program development systems t produces support multple vews of the user's program, ts semantcs, and ts executon. The program vews nclude a syntax-drected edtor, a declaraton edtor, and a structured flow graph edtor. The semantc vews nclude expresson trees, data type dagrams, flow graphs, and the symbol table. Executon vews show the program n acton and the stack contents as the program executes. PECAN s desgned to make effectve use of powerful personal machnes wth hgh-resoluton graphcs dsplays and s currently mplemented on APOLLO workstatons. 1. Introducton The avalablty of powerful personal computers and the desre for ncreased programmer productvty have led to the recent development of nteractve programmng envronments for algebrac languages. In ths paper we descrbe the t Ths research was supported n part by Natonal Scence Foundaton grants MCS , MCS , SER and MCS , by the Offce of Naval Research under contract nos. N C-0396 and N K-0146 and by DARPA order n Permsson to copy wthout fee all or part of ths materal s granted provded that the copes are not made or dstrbuted for drect commercal advantage, the ACM copyrght notce and the ttle of the publcaton and ts date appear, and notce s gven that copyng s by permsson of the Assocaton for Computng Machnery. To copy otherwse, or to republsh, requres a fee and/or specfc permsson ACM / user's vew of the PECAN famly of envronments developed at Brown Unversty. PECAN s a generator of program development systems for algebrac programmng languages, l, 2 It dffers from other systems by generatng complete envronments from smple specfcatons. The generated envronments dffer n ther use of the graphcal facltes of personal workstatons and n ther support for multple concurrent vews. PECAN envronments provde vews of the program, ts semantcs, and ts executon Objectves One objectve n developng PECAN s to nvestgate ways of makng full use of the computng power and graphcs avalable on the new generaton of personal machnes for program development. PECAN s a prototype system that llustrates some of these ways. PECAN envronments are desgned for both the experenced and the novce programmer. They are easy and fast to use, offer mmedate feedback to the user, and allow the user to vsualze hs program. A smple example of the use of PECAN for program development s presented n secton 3 of ths paper. PECAN envronments combne many of the best features of smlar program development systems. These nclude: * Immedate feedback of semantc and syntactc errors whle the user s edtng. * An undo faclty whereby the user can undo (and redo) any acton back to the begnnng of hs sesson. * Structured templates for buldng the program that are avalable as commands. * The flexblty to type text at any tme nstead of usng templates. 30

2 The use of menus as alternatves to typng for most commands. A multple wndow dsplay to make effectve use of the screen. Incremental semantcs that allow the program to be compled as t s edted. A framework that handles a varety of (algebrac) programmng languages wth the same commands. Forward and backward executon, wth user control of speed and the ablty to step one statement at a tme. The ablty to edt durng executon Vews n PECAN PECAN envronments dffer from other program development systems n ther use of multple vews of shared data structures. The program s represented nternally as an abstract syntax tree. The user does not see ths tree drectly, but nstead sees vews or concrete representatons of t. One such vew s a syntax-drected edtor. Another vew of the program s a Nass- Sehnederman structured flowchart. A thrd vew s a declaraton edtor. A fourth vew would be a module nterconnecton dagram showng how the program s organzed. Each of these vews may be read-only or edtng. They each dsplay the abstract syntax tree. Ths dsplay s updated automatcally as the tree changes. Edtng vews support modfcatons to ths tree usng the dsplayed representaton. The current vews are dscussed n secton 4. PECAN provdes more than program vews. In addton to the data structure representng the syntax of programs, the ncremental compler supports semantc data structures for the symbol table, the set of data types, expresson trees, and control-flow graphs. PECAN supports ncremental vews of these data structures. These are descrbed n secton 5. PECAN supports nterpretve program executon. It provdes a controlled envronment that lets the user execute hs program forwards and backwards, set breakponts, and sngle step. It uses the program and semantc vews to show where the program s executng at a gven tme. It provdes addtonal vews to dsplay the program's data. Ths currently ncludes a stack and data vew that shows the values on the program stack as they are allocated, freed and as they change. The data part of ths vew s used for dsplayng structures, arrays, and the values ponted to by ponters. Future dsplays wll be obtaned by mergng PECAN wth the BALSA system.3, 4 BALSA provdes multple dynamc vews of a fxed program's data structures whle the program s executng. The current executon vews are dscussed n secton 6. PECAN supports multple vews concurrently wth automatc updatng. There can be several vews of the same abstract syntax tree. If any one of them modfes the tree, PECAN sends messages to nform each vew that t should be updated. The ncremental compler s treated as an nvsble vew and s called whenever a tree changes. In turn, as the compler updates the semantc representaton, vews of the semantcs are notfed of the change so that they can update ther dsplays. The same phlosophy s used for executon vews. As a varable or the stack or the locaton counter changes, any vews that are dsplayng these tems are notfed to update ther dsplay. PECAN s currently beng developed at Brown Unversty on APOLLO workstatons. The frst verson s desgned wth expermentaton n mnd and wth the goal of supportng student programmng. The system desgn s flexble so that,new vews may be easly tred and so that a reasonable combnaton of vews can be found. The system desgn s hghly modular. Components such as the ncremental compler can be changed wthout affectng other vews or modules. 2. Related Work There are several efforts amed at buldng nteractve programmng envronments for personal machnes. Whle PECAN borrows from many of these systems, t s dfferentated by ts use of multple vews, ts retargetablty, and ts relance on graphcs. In ths secton we brefly relate the more well-known systems. The Cornell Program Syntheszer s provdes a full system for PL/C ncludng a syntaxdrected edtor and an nterpreter. Its edtor s template-based, but provdes text edtng for fxed constructs such as expressons. It has recently been mplemented as a generator so that t s possble to create syntheszers for dfferent languages usng attrbute grammers to descrbe the output and semantcs for each producton of the abstract syntax. The COPE system 6 also developed at Cornell provdes another approach based on an 31

3 ntellgent parser. The edtor n ths system s a text edtor ted to an error correctng parser. The parser s able to nsert mssng keywords and tokens to get about the same effect as templates do n the Syntheszer. Ths scheme has an advantage n that the user can type hs program at any tme. It has the dsadvantage that the user s not shown what templates are currently vald. The current PECAN edtor s a compromse between these two schemes. It provdes templates at every poston, but allows the user to type at any pont. Moreover, the parser has some error correctng capabltes. COPE also ncludes the mportant concept of beng able to undo and redo both edtng and executon. PECAN provdes a smlar ablty. The GANDALF effort at Carnege-Mellon Unversty s an ncremental programmng envronment generator currently workng for several algebrac languages. 7 It ncludes the ALOE syntax-drected edtor generator. 8 The abstract syntax descrptons and prnt specfcatons used by PECAN are derved from those defned for ALOE. ALOE produces general structure edtors not geared toward programmng languages. Thus, the semantcs for the language n queston have to be descrbed procedurally. ALOE also s a template based edtor n that t provdes templates for all productons n the tree and does not automatcally allow parsng. It s possble, however, for the user to wrte hs own parser and to use t for lmted text edtng. Much effort n GANDALF has been amed toward programmng n the large, an ssue not addressed by PECAN. GANDALF untl recently has not been targeted for personal machnes wth graphcs capabltes. Several programmng envronments have been developed at the Xerox Palo Alto Research Center. The Smalltalk system 9 s an nterpreter for an object-orented language based on message passng. It makes heavy use of wndows and the capabltes of the graphcal dsplay, but s language-dependent. The Interlsp envronment 1 provdes a good example of what can be done wth an nterpreted language and a hghresoluton dsplay. The Mesa 11 envronment, and more recently CEDAR, 12 apply many of the deas from Smalltalk and Interlsp to an algebrac, compled language. Many of the deas proposed for CEDAR can be found n PECAN. CEDAR s desgned as a producton programmng envronment. PECAN dffers n ts emphass on programmng n the small, nteracton, the use of graphcs, and on showng the user multple vews of hs program. PECAN s desgned to be nteractve and to eventually support graphcal programmng. 3. An Example In ths secton we examne a sesson wth a PECAN envronment n whch we create and execute a smple program. The language used here s an extended verson of Pascal. The system starts wth an ntal dsplay that can be specfed by the user. For the purposes of demonstraton and debuggng we use the dsplay shown n fgure 1. Ths conssts of a large number of the possble vews all dsplayed at once. Ths would be much tob cluttered for the average user of the system, and a typcal user mght restrct hmself to an edtor, the executon montor, and a data vew. The large vew n the upper rght s a syntax-drected edtor for the syntax tree labeled ezample. Below t are semantc vews of the flow graph and the current data type. Below the data type wndow s a transcrpt wndow where the system shows the user all of hs commands. Ths wndow can be used for controllng the undo and redo capabltes of the system. The column of wndows on the rght contans the control vew from the nterpreter and semantc vews of the current expresson and the symbol table. Fnally, the two vews n the lower-left show what the four buttons on the locator devce do and the tme respectvely. PECAN systems are bult on top of the Brown workstaton envronment. 13 Ths envronment contans a varety of tools. Four of these tools are used extensvely by PECAN. MAPLE s the nput management package. It s used to provde the varous menus wthn wndows and to drect nputs to partcular parts of the program dependng on whch wndow the user s currently n. ASH s the graphcs package that s used throughout. It provdes a varety of raster graphcs operatons through the abstracton of a wndow and ts vew on the screen. ASH provdes a herarchy of wndows. It allows wndows to overlap and to be made nvsble. It takes care of mantanng and dsplayng all the wndows, ncludng those that are partally or totally obscured. VT s a package that sts on top of ASH and provdes extensve text support n an ASH wndow. It allows multple fonts along wth edtng operatons lke those provded by an ntellgent termnal. It stores a sem-nfnte pad and manages the dsplay for any partcular part of the pad. Fnally, WILLOW provdes a user- 32

4 confgurable wndow manager. WILLOW provdes the utlty buttons at the bottom of the screen and the cons above them that are used for creatng new vews. It also provdes the move and resze buttons n the upper-left and upperrght of each of the wndows. Other wllow buttons are those n the nterpretor control vew, ncludng the scroll bar for controllng the speed of executon, and buttons for pushng, poppng and removng wndows that are hdden under the ttle banners of each of the wndows. WILLOW allows the user to confgure the system to hs lkng. It lets hm choose the ntal screen layout. It lets hm choose the fonts to be used n each of the wndows. It even lets hm choose the usernterface to be used for manpulatng wndows. In fgure 2 we have started the edtng sesson. We have moved the cursor nto the syntaxdrected edtor wndow and have ndcated that we want to create a new PROGRAM. The system flled n the name of the program from the tree name. Then we moved to the declaraton and, usng the menu on the rght of the edtor vew as a gude, ndcated that we wanted a VAR declaraton. Ths could have been done by usng the locator devce to pck the VAR button on the menu. Instead, we chose to type n the word var and have the system realze that ths meant to use the button. Fnally, we entered the varable defnton by typng t all n nstead of selectng off the menu. These actons llustrate the three means that the edtor provdes for program entry. The user can ether select templates usng the menu, can select templates by typng n a prefx of the correspondng button name on the menu, or can type n the full text for the current construct. Fgure 2 also shows that PECAN comples as t goes. Ths s seen n the semantc vews that are actve. The symbol table vew n the lowerleft shows the symbol table as t exsts up to ths pont. Smlarly, the data flow vew n the lowerrght shows the complete flow graph. The hexagonal flow nodes ndcate that the correspondng varable should be allocated at ths pont. Both of these vews are concrete representatons of the ntermedate representaton that s generated by the ncremental compler. The varous semantc vews are more fully llustrated n fgure 3. Here we have contnued to use the edtor to enter the functon #cd. The system augments our edtng wth automatc comments such as that on the BEGIN statements and the names of the parameters on the functon call. After enterng the body of the functon we used the locator to pont and move to the parameter b n the recursve call. The semantc vews are keyed on the current edtng locaton. Thus the expresson vew contans the expresson tree correspondng to the call, wth the node correspondng to the current parameter hghlghted. The symbol table vew smlarly hghlghts the current varable. The data type vew shows the data type of ths varable. Fnally, the flow graph vewhghlght sthe correspondng node of the flow graph, although ths node s not currently vsble. We next fnsh typng n the program that reads two values, calls the functon gcd wth them, and then prnts the result. Then we are ready to try out the program. We frst rearrange the screen, elmnatng most of the semantc vews, enlargng the flow graph vew so that t shows the whole program, and creatng a stack vew to show the data. We use the GO button n the nterpreter control vew to start executon, enter two values when requested, and then stop the program durng executon by clckng on the locator. The result s shown n fgure 4. Several elements of PECAN can be seen n fgure 4. The current statement beng executed s hghlghted n both the flow graph vew and n the syntax-drected edtor. When these vews are dsplayed, ths hghlghtng s done automatcally when executon stops. The MONITOR button n the nterpreter control wndow causes the hghlghtng to be done contnuously as the proglam executes. The nterpreter control vew shows the user the state of executon as well as any nput and output of the executng program. The stack dsplay shows the current state of all the program's varables. It shows the recurson nherent n the program and the dfferent varable values at each level. In the next sectons we consder the current and planned vews. Secton 4 looks at the program vews. Secton 5 looks at the semantc vews. Secton 6 consders the executon vews. 4. Program Vews Program vews are vsual representatons of abstract syntax trees. They can be ether readonly vews or they can be edtng vews. In ether case they are automatcally updated as the trees they represent are changed, whether they ntated the change or not. 33

5 4.1. The Syntax-Drected Edtor The syntax-drected edtor vew provdes a textual representaton of the program. It offers a cross between a tradtonal text edtor and a structure edtor. It allows the user to move to any pont n the program and do whatever text edtng operatons are approprate. The edtor parses the result, puts t nto ts nternal representaton, and reformats the dsplay. The freedom to treat a textual representaton of the program as text s mportant snce many of the changes a programmer makes are textual ones, such as correctng typng errors. Moreover, when there are many correctons to a program, the programmer s lkely to want to treat the program as a large textual object, make the correctons, and not want to worry about ts structure. Several systems, ncludng COPE 6 and POE 14 use a textual approach to syntax-drected edtng. At the same tme the syntax-drected edtor vew provdes a complete set of templates that allow the programmer to make full use of the structure of the underlyng syntax tree. The current locaton s not a sngle character, but s a tree node, and the text for ths node s placed n a box on the dsplay. The user has avalable smple commands to clmb around the syntax tree and to pck and put nodes of the tree. Whenever the current node s a meta varable or a leaf node, the edtor provdes a specal menu that lsts the possble expansons or contents. For meta nodes these are the templates assocated wth such syntaxdrected edtors as ALOE s or the Cornel Program Syntheszer. ~ For leaf nodes, they are a lst of the approprate canddate names from the symbol table. Edtng wth templates s often easer for novce programmers and can save consderable typng on program entry. Whle tryng to compromse between structured edtng and text edtng, the syntax-drected edtor also makes heavy use of the facltes of the graphcal dsplay and the pontng devce. The edtor uses multple fonts to dstngush between keywords, meta symbols that have yet to be expanded, text for dentfers, and text that contans semantc errors. The user s free to choose any of the avalable fonts for these purposes. The edtor uses lne drawng to place a box of whatever shape s necessary around the text correspondng to the current node. The nterface to the edtor uses the pontng devce and the keyboard. The pontng devce can select any of the menu buttons that are seen n the example. These provde the basc edtng commands, and, for meta nodes and approprate leaf nodes, the relevant templates and name alternatves. The pontng devce s also used wth the dsplayed tree. The user can change the current node by pontng at the desred locaton. Other buttons on the puck allow the user to pck, put, delete and nsert nto the tree. Multple clcks at the same locaton allow the user to move about the tree structure. The keyboard s used prmarly for text edtng, but also can be used for many of the edtor commands. The user s free to type text or text-edtng commands. When the user s done text edtng, he ssues a command not nvolvng text-edtng or an explct done command, the edted text s parsed, and the result s put nto the tree. Control and functon keys on the keyboard provde most of the tree-edtng commands (delete, nsert, pck, put), as well as tree movement and vew scrollng commands. Ths makes t possble for the user to edt wthout contnually havng to move hs hands from the keyboard to the pontng devce and back agan Nassl-Sehnederman Vew A second program vew s a graphcal one based on Nass-Schnederman flow charts. 15 These are a form of structured flow charts that use dfferent types of blocks for dfferent program constructs and use the nestng of blocks to represent the nestng of the program. Wth ths vew, the user sees the complete structured form of hs program drawn as a structured flow graph. The vew s based on a pretty-prnt notaton that descrbes how each abstract syntax construct should be represented graphcally usng a smple prnt rule. The rule shows the nestng and the creaton of new blocks n the flow graph. These blocks are ether rectangular blocks that ndcate smple processng, decson blocks wth a test on top and the alternatve paths n sub-boxes beneath, teraton boxes that contan the loop body and the tests on the outsde, and a separate box type to ndcate scopng. The vew s ted nto the system and s automatcally updated as the user changes hs program. We are currently workng on addng basc edtng operatons to ths vew to allow the user to edt hs program graphcally Declaraton Vew Another edtng vew s provded exclusvely for declaratons. The user can select a partcular 34

6 declaraton ether by pontng to the declaraton or by pontng to the name of the declaraton somewhere n the program text. If no declaraton already exsts for the name, then a new one s created automatcally n the current scope. Once a declaraton s chosen, the user can use ths vew to add, delete or change the assocated names, and to locally edt felds of the declaraton such as the data type of a varable. He can change the class of the declaraton, such as local varable to global varable or varable to constant. He can move the declaraton to a dfferent scope by pontng to the new scope and selectng the SCOPE button. Ths vew serves two purposes. Frst, t allows the user to create programs nteractvely, declarng varables as he uses them. Second, t supplements the Nass-Schnederman and future graphcal program vews that do not drectly support declaratons Other program vews There are other program vews that could be supported by PECAN and that we hope to see mplemented some tme n the future. In the short term, we are plannng to do a generalpurpose structured flowgraph edtng vew. The ntal mplementaton of ths wll support Rothon dagrams and Nass-Schnede.rman dagrams. Future program vews wll support programmng n the large through a procedure-level connecton dagram and through pctures of the top-level data flow. Other possble vews nclude one that supports data-flow programmng and a verfcaton vew that lsts the predcates known at each pont n the program and allows the user to prove correctness whle programmng. 5. Semantc Vews Whle abstract syntax trees representng user programs are the most logcal data structure to vew, t s often useful to provde the user wth specalzed vews of the nternal forms supported by PECAN. Durng program edtng, the relevant forms are the semantc representatons of the program: the symbol table, data type defntons, expresson trees, and control flow graphs The Symbol Table Vew The compler bulds and ncrementally mantans a scoped symbol table of all bult-n and user-defned symbols. The correspondng vew dsplays the scopes and symbols that are defned for a partcular syntax tree. The boxes dsplay the nestng of the varous scopes. Each box contans a bref descrpton of the scope and a lst of all the names defned by the user program n that scope. Each name s dsplayed wth ts class, such as varable, type, or label, and other pertnent nformaton such as the data type of a varable. If the current node n a program vew s a varable, then ths varable s hghlghted n the symbol dsplay. Pontng at a name or scope n the symbol dsplay causes the approprate program vew to make the defnton node for that scope or name the current locaton The Data Type Vew The compler mantans nformaton on bult-n and user-defned data types. A semantc vew dsplays the current data type based on ths nformaton. If the user s edtng a type defnton, then the type beng edted s dsplayed. If the current edtng node s a varable, then the type of the varable s dsplayed. If the current node s an expresson, then the type of the expresson s dsplayed. The top lne of a type dsplay contans the class of the type such as RECORD, POINTER, ARRAY. The followng lnes show the parameter values for ths class. Where these parameters are also types, the dsplay s recursve. To avod nfnte dsplays, the recurson typcally stops after one level. However, the user s free to choose'an unexpanded type and ask that t be expanded, ether wthn the dsplay or by makng t the only type dsplayed. Pontng at a type defnton causes the approprate edtor to make that defnton ts current locaton The Expresson Vew The thrd component of the compler mantans expresson trees for each expresson n the program. The correspondng vew draws these trees, affordng the user a dfferent perspectve of the expressons. The tree that s drawn s the one that s the current focus for edtng operatons. The node beng edted s hghlghted on the dsplay. The user may pan over the expresson tree when the tree s too large to ft n the wndow assgned for expresson dsplay. He can also pont at the tree to make the correspondng node be the current locaton for edtng The Flow Vew The fnal component of the ncremental compler bulds and mantans graphs that descrbe the flow of control for an abstract syntax 35

7 tree. The correspondng vew provdes a flowchart of the program that s dynamcally updated as the program changes. Nodes of the flowchart represent expresson evaluatons, condtonal branches, contour entrys and exts, varable allocatons, gotos and labels. The user can pan over a complex flowchart to vew the currently relevant porton. He can also pont at a gven block and cause the correspondng code to become the focus for edtng operatons. 6. Executon Vews PECAN supports vews of program executon. These vews are of three dfferent types, control, program, and data. The executon control vew s provded by the nterpreter. Ths vew contans messages that ndcate the current executon state of the program. It s used to dsplay error messages ncurred durng executon. It also dsplays the program nput and output usng dfferent fonts to provde a full transcrpt of the user's debuggng. The control vew provdes user debuggng facltes. These nclude the ablty to reverse program executon, to step through the program one statement at a tme, ether forward or backward, and to nsert breakponts by pontng at the place to stop and then httng the BREAK button. It also provdes a speed control so that the user can slow down the program executon. Executon can be montered n the program vews and n the semantc flow graph vew. If montorng s turned on n the control vew, then each of the program vews currently actve wll be nstructed to hghlght the current statement as t s executed. If the flow graph vew s actve, then t wll be used to show the step-by-step executon of the program. The user s provded wth vews of hs data as the program executes. The stack vew s the only current data vew, although more vews are planned. The stack vew dvdes ts wndow nto two portons. The left sde provdes a dsplay of the current executon stack. It shows where each block on that stack begns. It shows each varable n the block and dsplays ts current value. Undefned values are labeled as such. Smple values such as ntegers, enumeraton constants, reals, and characters are dsplayed drectly. Complex values such as arrays, records, and ponters are not dsplayed, but have an approprate token n ther place. The rght sde of the vew s used to dsplay these complex values. The user ponts at the token for an array or record or ponter and the correspondng value wll be dsplayed on the rght. Ths technque can be used recursvely to dsplay chans of ponters or nested structures. A future goal of PECAN s to provde a wde varety of executon vews based on experences wth the BALSA system. 8 These wll nclude data vews that render a graphcal representaton of a user's data structure such as a tree that s automatcally updated as the program executes. Ths work wll be based on an extensve lbrary of possble graphcal representatons and s the subject of current research. Other data vews would provde the user wth more classcal representatons of the stack or of a selected set of varables. These wll agan be updated as the program executes. Executon vews wll also show the program n acton. Smple program vews wll hghlght each statement as t s executed. Other vews wll provde performance nformaton such as executon counts n varous graphcal forms. 7. Acknowledgements The effort that has gone nto PECAN has been supported by much of the envronment actvty at Brown. Marc Brown s largely responsble for the MAPLE menung package. Joe Pato and Mark Vckers developed the vrtual devce nterface for the Apollos. Marc Brown and Mke Strckman dd the core of the BALSA mplementaton. Rob Rubn and Jeanette Hung dd the Nass-Schnederman vew. 8. References 1. Steven P. Ress, "PECAN: Program development systems that support multple vews," Proceedngs of the Seventh Internatonal Conference on Software Engneerng, (March 1984). 2. Steven P. Ress, "An approach to ncremental complaton," Proceedngs of the SIG- PLAN '84 Symposum on Compler Constructon, (June 1984}. 3. Marc H. Brown, Norman Meyrowtz, and Andres van Dam, "Personal computer networks and graphcal anmaton: ratonale and practce for educaton," ACM SIGCSE Bulletn 15(1) pp (February 1983). 4. Marc H. Brown and Steven P. Ress, "Toward a Computer Scence Envronment for Powerful Personal Machnes," Proceedngs of the 17th Hawa System Scences Conference, (January 1984). 36

8 Tm Tetelbaum and Thomas Reps, "The Cornell program syntheszer: a syntax- drected programmng envronment," Cornell Unversty Techncal Report TR (May 1980). James Archer, Jr. and Rchard Conway, "COPE: A cooperatve programmng envronment," Cornell TR (June 1981). A. N. Habermann, "The Gandalf Research Project," Computer Scence Research Revew, Carnege-Mellon Unversty 1979 (1979). Raul Medna-Mora and Davd S. Notkn, "ALOE users' and mplementors' gude," Carnege-Mellon Computer Scence Department Research Report CS (November 1981). Adele Goldberg, "The nfluence of an object-orented language on the programmng envronment," Proc. ACM Computer Scence Conference, (February 1983). Warren Tetelman, Interlsp Reference Manual, XEROX (1974). James G. Mtchell, Wllam Maybury, and Rchard Sweet, "Mesa language manual," Xeroz CSL-7g-8, (Aprl 1979). L. Peter Deutsch and Edward A. Taft, "Requrements for an expermental programmng envronment," Xerox CSL (June 1980). Joseph N. Pato, Steven P. Ress, and Marc H. Brown, "The Brown workstaton envronment," Brown Unversty CS (October 1983). Charles N. Fscher, Greg Johnson, and Jon Mauney, "An Introducton to Edtor Allen Poe," Unv Wsconsn-Madson TR 451 (October 1981). I. Nass and B. Schnederman, "Flowchart technques for structured programmng," SIGPLAN Notces 8(8)(August 1973 I. 37

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