Hermes: A Tutorial and Reference Manual - Researcher - IBM

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type).Ifthecheckfails,theexceptionDefinitionErrorisraised. ischeckedforresolutionerrorsandotherrestrictions(e.g.eachvariant denitionincludesacaseidforeachpossiblevalueoftheenumeration operandisafullyinitializeddenitionsmodule.Thedenitionsmodule Thisoperationcompletestheinitializationofadenitionsmodule.The Ifthechecksucceeds,theidcomponentofthedenitionsmoduleisre- 11.HermesOperations131 attributebehaveslikeaconstraint:anyattempttomodifythedenitions initionsmodulearereplacedbytheuniqueidentier.Thedenitionsmod- uleacquirestheattributecheckeddefinitions.Thecheckeddefinitions placedbyauniqueidentier.Allreferencestothatidentierwithinthedef- modulewilldiscardtheattributecheckeddefinitions.Unlikeaconstraint attribute,checkeddefinitionscannotbeusedinanassertstatement; anteedthatwhentwotypenamesorattributenamesareequal,theyalways mescanenforce\nameequivalence"oftypes.Noothercheckeddenitions modulecanpossiblyhavethesamemoduleidentier.Thereforeitisguar- denotethesametypeorattributedenition. theattributecanonlybeassertedbyexecutingthecheckdefinitions statement. 11.11Constraints Becausecheckeddenitionsaregivenauniquemoduleidentier,Her- attribute{construction argumentbenon-negative.Aprogramlibrarymightbedenedasatable ::=constraint( ofcheckedprograms. Forinstance,theinterfacetoasquare-rootfunctionmightrequirethatthe Constraintsareprogrammer-denedpredicatesonthevaluesofvariables.It isoftendesirabletoincludeconstraintsininterfacesorintypedenitions. )isconstraint{typestateconstraint{expression [declaration[,declaration]...] betrackedastypestateattributes.Currently,Hermesisnotsophisticated enoughtounderstandthesemanticsofconstraintpredicateswellenough totrackhowtheyarepreservedunderalloperations.Forexample,the compilerwillnotdeterminethataddingonetoanon-negativenumberwill yieldanothernon-negativenumber.Whatitwilldoisthefollowing: InHermes,theprogrammercanspecifythatcertainconstraintsareto addthetypestateattributeNonNegative(a)iftheoperationassert associateanamewithapredicate{e.g.NonNegative(x)withthe whichwilltestwhethera>=0andraiseanexceptionifitisnot. predicatex>=0.ThisdenesanoperationassertNonNegative(a)
13211.11.Constraints ConstraintDenitions Thissimpletrackingofconstraintsisenoughformanypracticalpurposes. tracktheattributeNonNegative(a)throughsuccessorstatements NonNegative(a)isnotinthetypestate untileitheraismodied,orcontrolmergeswithapathinwhich NonNegative(a)succeeds. Aconstraintdenitionincludes:adeningoccurrenceofanattributename, asetofformalparameters,thetypeandtypestateoftheseformalparametersportedexactlyasaretypedenitions Example: Assert NonNegative:constraint(x:integer)is Interval:constraint(x:integer,y:integer)is Constraintattributedenitionsoccurindenitionsmodulesandareim- finit(x)gx>=0; simple{statement finit(x),init(y)gx
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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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402.5.Front-endProcess paringavalue
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ationtable[expr].Wehaveencounteredt
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ishiddenandinnowayaectsthesemantics
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462.7.TheWindowManager callCurrentW
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name.Ifwewaitedtobuildtheapplicatio
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502.8.CreatingaWindowApplication re
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522.8.CreatingaWindowApplication be
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examplesillustratedhere. errorhandl
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563.TypeandTypestateChecking anerro
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583.TypeandTypestateChecking Therei
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604.1.ExpressionBlocks anexpression
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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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Page 100 and 101: 11 HermesOperations Throughoutthere
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Page 142 and 143: A.1LexicalRules Webeginwiththerules
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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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