Hermes: A Tutorial and Reference Manual - Researcher - IBM

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compile-timeistooconstraining.Hermessupportsvariantsandpolymorphs Sometimestherequirementthatthetypeofeveryvaluebeknownat 4.3Variants toallowthisrequirementtobeweakened.Howeverwedonotgiveupthe fundamentalprinciplesthatmachine-levelimplementationdetailmustbe 4.AdditionalHermesConstructs61 forpurposesofthisexample,we'llassumetobejustacharacterstring ustalkaboutLISPobjects: hidden,andthatthelanguagemustbesecure. whohasprogrammedinLISP.ALISPobjectiseitheranatom(which printname),apair,ornothing(nil).HerearetheHermestypeswhichlet LispPair:record(Car:LispObject,Cdr:LispObject); LispKind:enumeration('atom','pair','nil'); Weillustratevariantswithanexamplewhichwillbefamiliartoanyone LispObject:variantofLispKind( nents:L.PrintNameoftypeCharstring,L.PairoftypeLispPair,and varianttype. whichisusedconventionallytorepresentanullvalue.We'llexplainthe languages.Emptyisapredenedtype|anenumerationwithnovalues| SupposeLisavariableoftypeLispObject.ThenLhasthreecompo- Theenumerationandrecordtypesarejustlikesimilartypesinother 'pair'->Pair:LispPairffullg, 'nil'->Nil:Emptyfg); 'atom'->PrintName:Charstringffullg, L.NiloftypeEmpty.Inthisrespect,avariantisjustlikearecord.But whereasaninitializedrecordmayhaveseveralcomponents,onlyonecomponentofavariantcanexistatanyonetime.Whichcomponentitisdependsuponthecaseofthevariant.Thecaseisanenumerationvalue|in thisexample,either'atom','pair',or'nil'.Eachcomponentisassociatedwithoneofthecasesofthevariant.Sothevalueofaninitialized whereintheprogramthevariantishidden,andwhereitisrevealed.When itisrevealed,thecompilertracksitscase.SoifLisaLispObject,the variantisacasevalueplusexactlyoneofthevariantcomponents. isatypestateproperty.Thatmeansthatthecompilerwillkeeptrackof possibletypestatesofLare: thatwedon'tknowthecaseatcompiletime.Revealedmeanswedo.This Aninitializedvariantcanbeeitherhiddenorrevealed.Hiddenmeans initializedandrevealedincase'pair' uninitialized initializedandhidden initializedandrevealedincase'atom'
624.3.Variants thevariantcomponents.Theunitestatementsimultaneously:(1)setsthe assignmentstatementtocopythevalueofanothervariableofthesame youbuildanewvariant,youmustdecidewhichcasetoputthevariant in.Youmaynotdirectlyassigntoacomponentofanuninitializedvariant. Youmustinsteadusetheunitestatementtomoveavalueintooneof typewhichisalreadyinitialized.Oryoucanbuildanewvariant.When Therearetwowaystoinitializeanuninitializedvariant.Youcanusean initializedandrevealedincase'nil' caseofthevariant,(2)movesthevalueofitsoperandintothevariant component,(3)tellsthecompilerthatthevariantisnowinitializedand revealed. 1.HereisanexampleofastatementwhichinitializesLintocase'atom': 2.ThisstatementputsLintocase'nil': 3.ThisstatementputsLintocase'pair': (EmptyvalisavariableoftypeEmpty.Itdoesn'tneedtobeinitialized, uniteL.NilfromEmptyval; uniteL.Printnamefrom"Foo"; sinceinthedenition,componentNilisexpectedtobeuninitialized.) example,whenvariantLisrevealedincase'atom',theattributecase(L, newPair; unitePair.Car.Printnamefrom"Foo"; atypestateforeachvariantcomponent.Thisisthetypestatethevariant L.PrintName)ispresent.ThismeansthatL.PrintNameisthecomponent whichisknowntoexist. Thetypestateattributecaseispresentwhenthevariantisrevealed.For Whenyouwritethedenitionofthevarianttype,youhavetosupply uniteL.PairfromPair; unitePair.Cdr.NilfromEmptyval; timeitisrevealed|thatis,itmaybemademoreinitializedbutnotless. allwhenLisincase'nil'.` initializedwhenLisincase'pair',butL.Nilneednotbeinitializedat tively: isrevealed.Itisalsotheminimumtypestatethecomponentcanhaveany ThedenitionofLispObjectillustratedaboveshowsthatL.PrintName componentwillhave(1)justbeforeitbecomeshidden,and(2)justafterit mustbefullyinitializedwhenLisincase'atom',L.Pairmustbefully Thetypestateaftereachofthethreeunitestatementsaboveisrespec- 1.finit(L),case(L,L.PrintName),init(L.PrintName)g
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DavidF.BaconArthurP.GoldbergAndyLow
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Preface Thisdocumentcontainsatutori
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iv3TypeandTypestateChecking 5Resear
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Tutorial PartI 1
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ortransactions.AHermesmoduleisastra
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61.2.GettingStarted|ASimpleHermesPr
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81.2.GettingStarted|ASimpleHermesPr
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Page 41 and 42: 342.3.Interfaces WMInterface:callme
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Page 78 and 79: 6Introduction Thereferencemanualpre
Page 80 and 81: 7LexicalandSyntacticRules lectionso
Page 82 and 83: 8Resolution name,attributename,exce
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Page 100 and 101: 11 HermesOperations Throughoutthere
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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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