Hermes: A Tutorial and Reference Manual - Researcher - IBM

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6Introduction ThereferencemanualpresentstherulesofHermes.Mostoftherulesare programs,compilation,BNF,etc. denedinmachine-readabletables.Weexplainhowtoreadthetables. Weexplaintheotherrulesinformallybutrigorously.Themanualcontains minglanguage.Therefore,wewillnotexplainbasicconceptssuchasstored examplesofhowtoapplytherules,buttheseexamplesarenotnecessarily typicalorpreferredusesofHermes. torial.Wealsoassumethatyouhaveexperiencewithsomeotherprogram- HerearethestagesofwritingandexecutingaHermesprogram. WeassumethatyouhavealreadyreadandunderstoodtheHermestu- Youwritethecodeusingyourfavoriteeditor/programgenerator, TherststageofthecompilerconvertsyoursourceleintoaHermes producingasourcele. Youstorethecheckedprogramintoaprogramlibrary. Youretrievethecheckedprogram,instantiateitasaprocess,and TheHermescompilercheckstheHermesprogram,producingachecked gramwillincludeitstranslationtomachinecode.Likeallinternal programvalue.TheinternalrepresentationofacheckedHermespro- representations,thiswillbeinvisibletotheprogrammer. program.Thisbypassesthersttwostepsmentionedabove. producethesourcele.Theremainingstepsarebrokendownasfollows: YoumayalsogenerateaHermesprogramvaluedirectlywithinaHermes Inthismanual,weignoretherststepbecausewedonotcarehowyou Lexicalanalysis:Thecharactersintheprogramaredividedintotokens.Spaceandcommenttokensarethrownaway,andtheother writteninaformalgrammar.Lexicalanalysisisexplainedinchapter 7. tureoftheprogram.Thesyntaxrulesarewritteninaformalgrammar inAppendixA. executeit. Syntaxanalysis:Thestreamoftokensisparsed,revealingthestruc- tokensarepassedasinputtosyntaxanalysis.Thelexicalrulesare
726.Introduction Conversionandresolution:Theparsedprogramisconvertedinto Typecheckingandinference:Thetypeofeachvariablemustsatisfy deningoccurrence.Theresolutionrulesaredescribedinformallyin chapter8. rences.Resolutionmeansassociatingeachappliedoccurrencewitha adeningoccurrencewheretheidentierisassociatedwithitsdenition.Otherappearancesofoftheidentierarecalledappliedoccur- names.Namesaremadeupofidentiers.Foreachidentierthereis aHermesprogramvalue.Thisrequiresthecompilertoresolveall Typestatechecking:Thetypestateateachpointintheprogramis typecheckingandinferencealgorithmisexplainedinchapter9 operand.Forvariableswithoutdeclarations(expressiontemporaries), thecompilerappliestypeinferencetogenerateadeclaration.Itis checkedthatexactlyonetypecanbeinferredandthatthistype satisesthetyperules.Thetyperulesaregiveninanappendix.The thetyperulesfortheoperationwherethevariableappearsasan Otherstaticrestrictions:Theseareconstraintsonthevalueofaprogramotherthanthosecapturedinthetable-drivenrules.Example: appendix.Thetypestatecheckingalgorithmisexplainedinchapter Thenumberofargumentstocallmustmatchthenumberofcom- aregeneratedwherenecessary.Thetypestaterulesaregiveninan sistentwiththetypestaterequirementsofthatoperation.Coercions computed.Itischeckedthatthetypestateateachoperationiscon- 10. Execution:Wedescribethesemanticsofeachoperationrigorously butinformallyinchapter11. formallynexttotheconstructstheyrestrict. ponentsinthecallmessagetype.Theserestrictionsaredescribedin-
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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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101.2.GettingStarted|ASimpleHermesP
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121.4.PuttingProcessesTogether thec
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141.4.PuttingProcessesTogether newI
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fullycheckedfortypeandtypestateerro
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constructs:thenewinput-port,connect
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Page 80 and 81: 7LexicalandSyntacticRules lectionso
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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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