loetzsch09understanding.pdf

UnderstandingtheDynamicsofComplexLispPrograms

MartinLoetzsch

(AI-Lab,VrijeUniversiteitBrussel,Belgium

martin.loetzsch@gmail.com)

JorisBleys

(AI-Lab,VrijeUniversiteitBrussel,Belgium

jorisb@arti.vub.ac.be)

PieterWellens

(AI-Lab,VrijeUniversiteitBrussel,Belgium

pieter@arti.vub.ac.be)

Abstract:RecentadvancesinwebtechnologiesandtheavailabilityofrobustLisp

librariessupportingthemhavemadeitpossibletothinkofnewwaysofunderstanding

anddebugginglargeapplications.Inthispaper,wewilldiscusstwobasicideasfor

assessingandverifyingthebehaviourofLispprograms.First,weproposetouseaweb

browserforgraphicallydisplayingdebugoutputinasimilarbutmoreversatileway

astheLisplistenerisnormallyusedtoprintoutputtraces.Andsecond,wepropose

amethodforcreatingHTMLvisualisationsofcomplexdataandcontrolstructures

thatdon’ttradeinlevelofdetailforreadability.WewillintroduceGTFL(aGraphical

TerminalForLisp),whichwehaveimplementedbasedonthesetwoideas,anddiscuss

itsapplications.

1Introduction

Whenthebehaviourofasystemisdrivenbyunpredictableexternaldataorwhen

itemergesfromtheinterplayofmanydierentprocessesandrepresentations,

itisveryhardtokeeptrackoftheunderlyingcomplexdynamicsexhibitedby

thesystem.Evenwhenthebehaviourisasexpecteditmightbefarfromtrivial

tounderstandorshowwhythisisthecase,orforexamplehowitcouldbe

furtherreﬁned.Evidently,whenthebehaviourisnotthedesiredone,itisusually

veryhardtotrackdownthepartsofthesystemsthatrequiremodiﬁcationto

yieldmoredesireddynamics.Addingtothechallengeisthefactthatoftensome

subsystemscan’tbetestedinisolationbecausetheyareembeddedinarecurrent

networkofotherprocessesandconsequentlythefunctioningofthesystemcan

onlybeinvestigatedasawhole.Thesesortsofdicultiesinunderstandingthe

underlyingdynamicsofcomplexcodeareacommonprobleminmanyareasof

computersciencesuchasrobotics,artiﬁcialintelligenceanddistributedsystems.

Althougheverybodyhastheirownfavouritestyleofdebuggingwehaveob-

servedthatmostLisphackersuseacombinationoffourmajortechniquesfor

analysingthebehaviouroftheirprograms.Theﬁrstone,whichwecallthetrac-

ingapproach,consistsofprintingtracestotheLisplistenereitherbydirectly

addingprintstatementstothecodeorbyusingbuilt-intracefacilitiesorother

custommechanisms.Theadvantagesofthistechniquearethat(1)itisvery

easytodo(andinfactthisiswhatalmosteverybodywholearnstoprogram

startswith,forareviewsee[McCauleyetal.2008])and(2)itallowstomonitor

programexecutionatalmostanylevelofdetail(fromveryhigh-leveldynamics

onlytonear-completeinformation).Whenthelevelofdetailishighhowever,

theoutputrapidlybecomesunmanageable.Furthermore,plaintextinaLisp

listenerisnoteasytoreadandcanonlybepresentedlinearlywhichmakesit

impracticalforgettinganoverviewof,ora‘feeling’for,thedynamicsofthe

system.Andﬁnally,changingthelevelofdetailrequirestocommentoutprint

statementsinthecode(badstyle)ortoswitchonandotracingfacilities.

Asecondmethodistoretrospectivelyanalysethestateofasystembyin-

spectingLispobjects–eitherusinginspectortoolsofLispenvironmentsorby

directlycallingchainsofaccessorfunctionsonanobject.Inspectionhasthead-

vantagethatitdoesnotrequiretochangeorwriteanycodeandthecomplete

stateoftheobjectcanbeanalysed.Butinspectionisonlyapplicabletotheﬁnal

resultsofaparticularalgorithmorprogram(exceptwhenusingbreakpoints).

Ingeneral,changesofdatastructurescan’tbeeasilyobservedusinginspection.

Additionally,itrequirestohaveaccesstotheproblematicinstanceinquestion

andoftenitisunclearwhichoneofthethousandsofobjectstoinspect.Finally,

inordertoquerydetailsofdeeplyhierarchicaldatastructures,aprogrammer

needstoperformahighnumberofmanualsteps,whichpreventsinspectionto

beusefulinmanycases.

Third,manyLispimplementationsanddevelopmentenvironmentsprovide

mechanismsformanuallysteppingthroughcodeeitherbyusingthebuiltinstep

facilitiesorbyinvokingthestepperdirectlyfromthecode.Controlstructures

canbetraversedandfunctionparameters,returnvaluesandlocalvariablescan

usuallybefurtherinvestigatedwithinspectors.Thiscertainlyisveryhelpfulfor

ﬁndinglogicalmistakesinsmallpiecesofcodesuchassignerrors,wrongloop

exitconditionsandsoforth,butinordertomakeuseofsteppingonehasto

knowwhichpartofthecodetolookat.Steppingthroughacompleteapplication

justforexploringitsdynamicswouldtakehoursandisthusimpracticable.

Alasttechniqueistocreatevisualisations,eitherwithinacustomGUIor

byusingspecialisedgraphicslibraries(e.g.gnuplot).Visualisationsaregreat

togetanintuitionofanalgorithm’sdynamicsbecauseourmindunderstands

graphicalrepresentationsmuchbetterthantextoutput,especiallyfornumerical

data[Kosslyn2006].That’swhyinareassuchasroboticsitisexceptionalto

evenwriteprogramswithouthavinggraphicalmeanstoverifythatthesystem

behavesasexpected(ideallyevenforeachintermediatestep).Visualisationsare

costly,becauseittakestimetoimplementthem,buttheyoftenpayointhelong

runwhentheycanbecomeaninvaluableeyeintothesystem’sinternals.Aclear

disadvantageisthatvisualrepresentationsonlyallowforalowtomediumlevelof

detail:complexdatastructureshavetobetransformedintotwo-dimensional(or

sometimes3D)representations,involvingconstraintssuchasavailablewindow

sizeandarequiredvisualclaritysothattherepresentationremainsreadable.

Furthermore,despitemanyrecentdevelopmentsincross-platformGUIAPIs,

thereisnonethatreallylooksgoodandthateasilyworksinallmajorCLs.And

unlesstheLispprograminquestionalreadyhasagraphicaluserinterface,there

isalotofoverheadindealingwithwindows,menusandotherUIelements,event

handlingandtheinteractionwiththeactualcodetomonitor.Thisisusually

toomuchoverheadwhenthegoalisto‘justdrawsomething’.Additionally,Lisp

isoftenusedforsymbolicprogramming,whichinvolvestextualandhierarchical

datastructures,whichinturnarehardtovisualise.Althoughvirtuallyevery

graphiclibraryhasmeanstodisplaytextonthescreen,theresponsibilityfor

arrangingtextblocks(estimatingwidthsanheightsoftextareasdependingon

availablespace,avoidingoverlap,reﬂowingmulti-linetext)isusuallyinthehand

oftheprogrammer.

Weproposeanoveltechniqueforanalysinganddebuggingcodeonthesystem

level,whichcombinesmanyofthemainadvantagesofthepreviouslyintroduced

techniques,whileatthesametimeremovingsomeoftheirrespectivedisadvan-

tages.

2GTFL-AGraphicalTerminalforLisp

WeproposetheGTFL(aGraphicalTerminalforLisp)libraryasatoolaimed

atLispprogrammersthatwanttounderstandthedynamicsoftheircode,es-

peciallywhentheprogramistoocomplextodebugbyprintingtexttracesto

thelistenerorbyusinginspectionandsteppingtools.GTFLrunsonallmajor

CLimplementations,isfree(BSD-Stylelicense)anditsdocumentationtogether

withthesourcecodecanbefoundat[Loetzsch2009].

GTFLisaterminalinthesensethatcontentispushedbyaLispprogram

toaclientandappearsthereinthesameorderasitwassent.Whatmightbe

surprising–butaswillbeshownhasgreatconsequences–theclientisaHTML

pagerunninginawebbrowser(seeFigure1).GTFLusestheHunchentootweb

server[Weitz2006]todelivertheclientpageandHT-AJAX[Marshak2007]to

exchangerequestsandcontentbetweentheclientandtheLispenvironment.

AfterinstallingGTFL(eitherbydownloadingitfrom[Loetzsch2009]orvia

asdf-install)andloadingthelibrarywithasdf,theinternalwebserverneedsto

bestartedwith:

(start-gtfl)

Figure1:GTFLinaction.ALispenvironment(left)hasGTFLloadedandsends

outputtotheclient.TheclientHTMLpage(right)showstheoutputasitis

sent.

TheclientHTMLpageisnowaccessibleat http://localhost:8000 (adierent

addressandportcanbespeciﬁedaswell).SinceGTFLusesquiteadvancedweb

technologies,werecommendtousearecentversionofSafari,FirefoxorChrome.

Thepageisinitiallyemptyexceptforasmall“reset”button.Sendingoutputto

theclientisquitesimple(seealsoexamplesinFigure1):

(gtfl-out(:p"hello"(:span:style"color:red;""world!"))

(:p"a"(:b"second")"paragraph"))

Theoutputimmediatelyappearsontheclientpageandlookslikethis:

hello world!

asecondparagraph

Themacro gtfl-out takesnumbersofs-expressionsasargumentsandrenders

themintoactualHTMLcodeusingCL-WHO[Weitz2003].Therearemany

otherLisplibrariesprovidingthisfunctionalityandingeneralwebrelatedLisp

toolsgiveusersthefreedomtouseanyofthem,butwehadtoselectonebecause

GTFLalsoprovidestoolsforcreatingcomplexHTMLconstructs.CL-WHOwas

ourﬁrstchoicebecauseitseemstobeoneofthewiderknownonesandbecause

itdoesn’tcreateintermediatestructuresatrun-time,makingitveryecient.

Behindthescenes,GTFLpushestherenderedHTMLcodesentby gtfl-out

onaninternal‘requests’list.Intheclientpage,aJavascriptbasedeventloop

usesAjaxcallstoquerythatlistontheLispsideevery200ms.Wheneverthere

issomethingnewtodisplay,itissenttotheclientpageasaresponsetosuch

anAjaxcall.

Withthisbasic‘terminal’functionality,GTFLcanalreadybeusedforde-

buggingwiththetracingtechniquedescribedabove.Theuserisonlyrequiredto

changetherelevantprintstatementsinthecodeto gtfl-out calls.Themajor

advantageisthattextcanbedisplayedindierentcolors,amultitudeoffont

stylesandsizesandwithbackgroundsorborders,whichallhelpinmakingthe

outputmuchmorereadablethanplaintext.

Furthermore,GTFLisgraphicalbecauseHTMLis.Inadditiontothepossi-

bilitiesofbasicfontformattingmentionedabove,HTMLtablesandotherblock

elementscanbeusedtostructuredataandtolayoutrepresentations.Contem-

poraryHTMLrenderingenginesareextremelygoodindistributingtheavailable

browserwindowwidthamongrecursivelynestedchildelements(andinreﬂow-

ingthelayoutwhenthewindowsizechangesorwhenmoredataisaddedtothe

page).Atrivialexampleforthisis:

(defparameter*text*"contentthatautomaticallyadaptsitswidth

accordingtotheavailablespace")

(gtfl-out

(:table:cellspacing"5":cellpadding"5"

(:tr(:td:style"background-color:yellow"(princ*text*))

(:td:style"border:1pxsolidblack"(princ*text*)))))

HTMLstyleattributesareusedtodisplay *text* indierentgraphicalstyles

twotimesnexttoeachother.Moreimportantly,theHTMLrendererremoves

theburdenofdecidinghowwidetodrawthetextblocksfromtheprogrammer:

Exploitingthisversatility,GTFLcontainsfunctionalityfordrawingtreesin

HTML.Thisisagainatrivialexample(forexamplesof‘real’treesseeFigures

1and2):

(gtfl-out

(draw-node-with-children

(who-lambda(:div:style"border:1pxsolid#000"

(:b(princ*text*))))

(list(who-lambda(:div:style"border:1pxdottedblue"

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