Hermes: A Tutorial and Reference Manual - Researcher - IBM

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droppedattributesafternormalcompletionoperation. Threetypesofstatementsarehandledspecially: Thecompoundstatementshavemorecomplexpostconditionrules Thecallstatementisdierentbecausethenormalandexception postconditionscomefromtheinterfacedenition. 10.TypestateChecking91 Similarly,everystatementcontainingaselectorisanalyzedasifit owgraph. manualwedescribethetypestatepostconditionresultingfromthis weretwostatements:aselectorfollowedbyaprimitivestatement. Therulesinthetabledenethepreconditionandpostconditionrules booleanteststatementandtheembeddedclauses.Inthereference anifstatementbehavesasifitwerecomposedofamoreprimitive becausetheyareequivalenttominiatureowgraphs.Forinstance, typestatepropagatedfromthestatementperformingtheforwardbranch. withinblock,ifandselectstatementstotheendofthestatementare betweentheprevioustypestateassociatedwiththedestinationandthe forwardbranches.Whenaforwardbranchisencountered,themeetistaken Exceptions,exitstatements,andthebranchesfromtheendofclauses Eachloopconstruct(while,for)performsasinglebackwardbranchfrom selectors. fortheprimitivestatement;theserulesareappliedaftertherulesfor ofthelooparethenre-analyzed. typestatepreviouslyassociatedwiththetopoftheloop.Thestatements theendoftheloopislowerthanorincomparabletothetypestateatthe beginning,themeetistakenbetweenthetwotypestates.Thislowersthe isdoneexcepttointroduceanynecessarycoercion.Ifthetypestateat theendofthelooptothebeginning.Ifthetypestateattheendoftheloop isstrictlygreaterthanorequaltothetypestateatthebeginning,nothing Thepossibletypestateerrormessagesare: tateandnoloopsneedtobere-analyzed.Inpractice,noloopwillneedto beanalyzedmorethantwice. 10.9TypestateErrors Analysiscontinuesuntilallprogrampointshavebeenassignedatypes- Deadcode:Astatementcannotbereached.Rememberthattypestatecheckingignoresthevaluesofvariables,sothatevenastatemenception.Thiserrorariseswhenyouwriteastatementafteranexit beginningif'false'then...doesnotgenerateadeadcodeex- statementorafteracompoundstatementallofwhosealternatives
9210.9.TypestateErrors Cannotcoerce:Theoperationrequiresanattributetobedropped, Attributenotpresent:Arequiredtypestateattributewasnotpresent. terminatewithexitstatements.Italsoarisesifyoucodeahandler whichcanneverbebranchedto. Forexample,youcodedaninsertoperationbutthetablewasnot initializedandthesecondtobeuninitialized.SupposeParmsisa butthecoercionwhichdropstheattributewouldalsodropanother attributewhichisrequired.Example: callService(Parms.A,Parms.B); SupposetheinterfacetoServicerequirestherstargumenttobe initialized. callmessagewhoseminimumtypestaterequiresbothParms.Aand Parms.Btobeinitialized.ThenthecallstatementrequiresParms.B Cannotdrop:Thepostconditionrulemandatesdroppingthisattribute,buttodosowouldproduceaninvalidtypestate.Forexample, Cannotadd:Thepostconditionrulemandatesaddingthisattribute, thismessage. tobemadeuninitialized,butthiscannotbedonewithoutdiscarding Parms,whichwouldthenmakeParms.Auninitialized. assignmenttoParms.AwithoutrstinitializingParmswouldproduce buttodosowouldproduceaninvalidtypestate.Forexample,an dependenterrorswhicharedetectedduringtypestatechecking: Thefollowing,whilenottechnicallytypestateerrors,arestaticcontext- Illegalposition:Youhavecodedthepositionofoperationina Illegalconstant:Youhavecodedastatementwhichmodiesoneof itsoperands,buttheoperandisconstant. droppingacallmessagebelowitsminimumtypestate. contextwhereitisnotlegal.Thisoperatorislegalonlywhenthe tableelementoperandisaconstantcopy.
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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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201.5.DeclarationsandDenitions whil
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221.5.DeclarationsandDenitions Stan
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whichtheoutputportconnects|calledth
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261.6.ASimpleServer begin QuitCM:Qu
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281.6.ASimpleServer towritedataabst
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toaservice,whileretainingtheability
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322.1.Requirements PutLine PutLine
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342.3.Interfaces WMInterface:callme
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asmallchangetotheinterfaceFilter.Re
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382.5.Front-endProcess callParms.Ge
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Page 49 and 50: ationtable[expr].Wehaveencounteredt
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Page 53 and 54: 462.7.TheWindowManager callCurrentW
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Page 57 and 58: 502.8.CreatingaWindowApplication re
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Page 61 and 62: examplesillustratedhere. errorhandl
Page 63 and 64: 563.TypeandTypestateChecking anerro
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Page 67 and 68: 604.1.ExpressionBlocks anexpression
Page 69 and 70: 624.3.Variants thevariantcomponents
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Page 73 and 74: accesscontrol{anyonecangetit. 664.4
Page 75 and 76: 685.ResearchDirectionsinHermes
Page 78 and 79: 6Introduction Thereferencemanualpre
Page 80 and 81: 7LexicalandSyntacticRules lectionso
Page 82 and 83: 8Resolution name,attributename,exce
Page 84 and 85: 8.1.2ComponentNames eachhaveacompon
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Page 92 and 93: ments.Therstargumenttocasemustbeava
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Page 96 and 97: 6.Determinethepostconditionsforeach
Page 100 and 101: 11 HermesOperations Throughoutthere
Page 102 and 103: Thediscardstatementremovesthevaluef
Page 104 and 105: Ifnoneoftheclausescanexitsnormally,
Page 106 and 107: Ifthereisanoptionalselectexpression
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Page 110 and 111: types.Additionally,theoperations\an
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Page 114 and 115: RecordTypeFamily 11.5RecordOperatio
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Page 126 and 127: Itistheabilityforprogramstosendandr
Page 128 and 129: Connect Theconnectstatementhastwoop
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Page 132 and 133: incompatiblewiththeinputporttypeoft
Page 134 and 135: Inspect-polymorph willincludefinit(
Page 136 and 137: theprogramwhichembedsit.Thereforeth
Page 138 and 139: type).Ifthecheckfails,theexceptionD
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Page 142 and 143: A.1LexicalRules Webeginwiththerules
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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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