Hermes: A Tutorial and Reference Manual - Researcher - IBM

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iswrittenoutinfullinsection1.5. sowecansaythatprogramEchoandprogramHelloWorldhavethesame initializationinterface.Thisinterfaceisshownpictoriallyingure1.3.It andthataninitializedresultispassedbackonreturn.PutLine,onthe ever,GetLineassumesthatitsargumentisuninitializedpriortothecall, GetLineandPutLinebothtakeasinglestring-valuedargument.How- 1.IntroductiontoHermes11 theseassumptionsarerecordedintheinterfacedenition.Bothclientsand serversarecheckedatcompile-timetomakesurethatthecodeagreeswith theinterfacewithrespecttobothtypeandtypestate.Typecheckingof interfacesmaybefamiliarfromotherlanguages,suchasAdaandPascal. Typestatecheckingisnew.Everycallinterfacehasapre-callandpost-call typestate.Thecompilerchecksthatacallerputseachargumentinthe correctpre-calltypestate.Thecompilerchecksthatareceiverputseach otherhand,assumesthatitsargumentisinitializedpriortothecall.All withtheEndStreamexception.Foreachexception,theremaybeadierent parameterincorrectpost-calltypestatepriortoreturn.Inthisexample, post-calltypestatespeciedintheinterface.Inthisexample,theinterface GetLine'sinterfacedenitionspeciesthatthecallargumentisuninitializedbeforecallandinitializedafterreturn.Ifthecompilercan'tdeduce willspecifythattheargumenttoGetLinewillnotbeinitializedinthe endofthestreamhasbeenreached,theserverwillreturnthecallmessage theserveriscalledtogetalineandtherearenomorelinesbecausethe thataprogramreceivingacallwillalwaysinitializeitsparameter,itwill generateanerrormessageandrefusetocompiletheprogram. eventofanEndStreamexception. GetLine'sinterfacealsodenesanexceptionnamedEndStream.When empty,sotheblocksimplyterminates,andthereturnfollows. statement.Aclauseissimplyasetofstatements|inthiscase,thesetis tothenearestmatchingexceptionhandlerclauseinanenclosingblock obtainastringandstoreitintovariableLine,andthencallingPutLineto putoutthestring.WhentheEndStreamexceptionisraised,controljumps tivestatementsmayraiseexceptions;user-denedoperationsimplemented viacallsmayraiseuser-denedexceptions.Insomelanguages,arun-time ExceptionhandlingisveryimportantinalanguagelikeHermes.Primi- ThestructureofprogramEchoisaninniteloop,callingGetlineto errorsuchasanoverowordivisionbyzerocausestheentireprogram bereturneduninitializedwhentheEndStreamexceptionisreturned.This tohaltandanerrormessagetobeprinted.InHermesthisisobviously thatyoumightfailtotestthem.AHermesexceptioncan'tbeignored afteranexceptionreturn.ThecompilerwillknowthatLineisuninitializedbecauseGetLine'sinterfacedenitionspeciesthatitsargumentwill becausecontroljumpstoanewlocation. unacceptable|remembertheprogramisn'tjustyourapplication,butthe wholesystem.ReturncodessuchasareusedinChavethedisadvantage Typestatecheckingguaranteesthatyoucan'tuseavariablelikeLine
121.4.PuttingProcessesTogether thecallmayberewrittenusingthefamiliarfunctionnotation.Likeother availablelanguages(asof1989)oer. degreeofcheckinggoesbeyondwhateventhemostmoderncommercially Incidentally,thecalltoGetLinemayalsobewritten: Line:=Parms.GetLine(); Wheneveracallhasnarguments,andthelastargumentisareturnvalue, FIGURE1.3.Interfacetoourtypicalclient expressions,functionsreturnanonymousvalueswhichmaythemselvesbe Client usedasoperands.Inthiscase,theresultissimplyassignedtothevariable seenexamplesofthefollowingHermesstatements:assignment,function Line,sothereisnoparticularadvantagetothefunctionnotation. call,while,block. terface,typestatechecking,exception,exceptionhandlerclause.Youhave aboutexceptions.YouhavelearnedthefollowingHermesterminology:in- denitionsdenewhichparameterspassedascallargumentsfromthecaller andwhichparametersreturnedtothecallerareexpectedtobeinitialized. Theseexpectationsareenforcedbytypestatechecking.Youhavelearned callmessage.(Thisiscalledpassingparametersbyvalue-result.)Interface Summary:Youhavelearnedthatcallsareprocessedbyexchanginga intheinitializationcallmessage.Nowyou'regoingtolearntodoyourown otherprocessesandbindtheoutputportofonetotheinputportofthe sumedthatallportswereboundbytheparentandpassedasparameters Sofar,wehavewrittensimpleone-processprograms.Theseprogramsas- second. systemsprogramming.Wewillwriteaprocessthatwillinstantiatetwo 1.4PuttingProcessesTogether PutLine,GetLine,andGetProgram,sotheyaredepictedasboxeswith threearrowsrepresentingthreeconnectionstotheenvironment. isdepictedingure1.3.Theseprogramsareinitiallypassedthreeports: willconsistofourclientprogram(eitherprogramHelloWorldorprogram Echo|itdoesn'tmatter),connectedtoalterprogram.Thelterhasone TheinterfacewehavebeenusingforprogramsHelloWorldandEcho We'regoingtobuildacompositeprogramtothesameinterface.It GetProgram GetLine PutLine
Page 1 and 2: DavidF.BaconArthurP.GoldbergAndyLow
Page 3 and 4: Preface Thisdocumentcontainsatutori
Page 5 and 6: iv3TypeandTypestateChecking 5Resear
Page 8: Tutorial PartI 1
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Page 39 and 40: 322.1.Requirements PutLine PutLine
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624.3.Variants thevariantcomponents
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thetypestate(i.e.addattributes),but
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accesscontrol{anyonecangetit. 664.4
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685.ResearchDirectionsinHermes
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6Introduction Thereferencemanualpre
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7LexicalandSyntacticRules lectionso
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8Resolution name,attributename,exce
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8.1.2ComponentNames eachhaveacompon
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onthehandlerandusedontheexitstateme
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theelementwillalsobeknownbyapplying
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documentationpurposesevenwheretheya
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ments.Therstargumenttocasemustbeava
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Ifinit(CM)ispresentandCMisacallmess
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6.Determinethepostconditionsforeach
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droppedattributesafternormalcomplet
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11 HermesOperations Throughoutthere
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Thediscardstatementremovesthevaluef
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Ifnoneoftheclausescanexitsnormally,
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Ifthereisanoptionalselectexpression
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stmemberisanatomwithprintnamematchi
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types.Additionally,theoperations\an
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Less,Less-equal,Greater,Greater-equ
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RecordTypeFamily 11.5RecordOperatio
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valueofthetable,ifany,isdiscarded.
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thetableisordered,theelementisinser
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notallowedonconstantcopies.Avariabl
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thereisnoselectedelement,aNotFounde
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willbeuninitialized,andthedestinati
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Itistheabilityforprogramstosendandr
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Connect Theconnectstatementhastwoop
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possible,andstoresitinadestinationv
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incompatiblewiththeinputporttypeoft
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Inspect-polymorph willincludefinit(
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theprogramwhichembedsit.Thereforeth
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type).Ifthecheckfails,theexceptionD
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isusuallygeneratedasacoercionwhenme
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A.1LexicalRules Webeginwiththerules
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accuracy against andy assert drop e
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Thesimplestatements: simple{stateme
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oolean{guard event{guard::=eventinp
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::=(expression)AppendixA.HermesConc
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user{exception constant{parameters
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formal{variable module{name::=ident
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Typestatesappearinginparticularcont
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destination,source,andposition.Some
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operandwillnolongerbeinitialized. B
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Followingaretheinferencefunctionsap
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lowestentrycondition(message,port):
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copy(source,destination):Foreachatt
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assert(continued) Description:Evalu
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case(continued) Description:Copythe
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copy(continued) Preconditions: unco
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drop(continued) SpecialRules:Thenum
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exists(continued) SpecialRules:Thes
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forall(continued) Description:Ifeve
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greater(continued) Description:Ifso
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insert{at(table,element,position) T
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integer{literal(result) TypeRules:
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merge{at(continued) Description:Rem
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not(result,source) TypeRules: Appen
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print(variable) TypeRules:None Prec
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emove(element,table) TypeRules: App
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select(continued) SpecialRules:Allt
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subtract(result,source1,source2) Ty
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unary{minus(result,source) TypeRule
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wrap(continued) Preconditions: var(
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definitions predefined:using() Appe
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);'named'->typename:typenameffullg
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{{Specicinformationneededtocomplete
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attrdefinition:record( attributeid:
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statement:record( id:statementid, o
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{{Ablockstatementqualieridentiesthe
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{{expressionisfalse. ifqualifier:re
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{{expression.The'element'rootvariab
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[ASU88]AlfredV.Aho,RaviSethi,andJer
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[SYB87a]RobertE.Strom,ShaulaAlexand
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AppendixC.PredenedModule215 coercio
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AppendixC.PredenedModule217 dening,
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variablename discarded,29 variant,6
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