Hermes: A Tutorial and Reference Manual - Researcher - IBM

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incompatiblewiththeinputporttypeoftheinitializationportofthenew iscreatedisnotaprocessrunningthedesignatedprogram,butinsteadis ageneratorprocess. Theeectofprocedureisidenticaltothatofcreateexceptthatwhat Eachtimethegeneratorprocessiscalled,itconstructsanewinstanceof 11.HermesOperations125 exactlylikethesemanticsofproceduresinAlgol-likelanguages. iscalledprocedureofbecausethesemanticsofgeneratorprocessesis aprocessrunningthedesignatedprogram.Itthensendsthecallmessageit receivedtotheinitializationportofthenewprocess. ofthedesignatedprogram.Theoperationtocreateageneratorprocess callaninputportinthesameprocesswillproduceadeadlock.Acalltoa generatorprocesswillnotdeadlocksinceeachcallgeneratesanewinstance Ordinaryprocessesarenotrecursive,sinceanattemptbyaprocessto 11.9PolymorphOperations Apolymorphisavalueconsistingoftwoparts:(1)awrappedvalue,which PolymorphTypeFamily formaltypestateofthewrappedvalue. thepurposesofassignmentandcomparison.Forexample: canbeofanytypeandtypestate,(2)awrapperdescribingthetypeand Resource:polymorph; Apolymorphtypedenitiondenesadistinctdomainofpolymorphsfor type{construction MailItem:polymorph; ::=polymorph wrappedvaluesofanytype;however,avariableoftypeResourcecannot ifandonlyifbothwrappersandbothwrappedvaluesareequal. Wrap beassignedwithorcomparedtoavariableoftypeMailItem. VariablesoftypeResourceandvariablesoftypeMailItemcancontain Twopolymorphscanbecomparedforequality.Thepolymorphsareequal containingthesourcevariable'stypeandformaltypestate,andmovesthe resultantpolymorphvalueintothepolymorphvariable.Thepolymorph ofthetypestateofthewrappedvalue.Thesourcevariable'stypestatewill Thewrapstatementremovesavaluefromasourcevariable,addsawrapper variableitselfwillhaveatypestateofinitaftertheoperation,regardless simple{statement ::=wrapsource{expressionaspolymorph{variable
12611.9.PolymorphOperations positive(x.id),gt(x.id,y)g.Thenafterexecutingthestatement tateattributesincludingxare:finit(x),init(x.name),init(x.id), andavariablethatdoesnot. constraintwhichincludebothavariablethatoverlapsthesourcevariable thedestinationvariablewillbedropped. beuninitialized.Beforethewrapstatementtypestateanalysiswilldropany nordestinationcanbeconstant,andattributesotherthaninitinvolving Othertypestaterestrictionsapplyasforamovestatement:neithersource inalvalueofx,andwhosewrappercontainsatypeofpersonanda formaltypestateoffinit(*),init(name),init(id),positive(id)g. Theconstraintfgt(x.id,y)gwillbedropped. wrapxasy; thevalueofywillbeapolymorphwhosewrappedvalueistheorig- Forexample,supposethatvariablexisoftypeperson,andthetypes- Unwrap Thenewtypestateforywillbesimplyfinit(y)g. expectedbythedestinationvariable.Thesource(polymorph)variablemust thatthetypeandtypestateonthewrappermatchthetypeandtypestate andmovesthevalueintoadestinationvariable.Aruntimecheckinsures Theunwrapstatementremovesawrappedvaluefromapolymorphvariable simple{statement ::=unwrapresult{variable expectedtypestateisspeciedbycodingaformaltypestatewithinthe beinitialized. Theexpectedtypeissimplythetypeofthedestinationvariable.The frompolymorph{expressionunwrapped{typestate unwrapstatement. type,orifthetypestateonthewrapperisnotatleastashighastheformal typestatespeciedontheunwrapstatement,thenaPolymorphMismatch exceptionisraised. Ifthetypeonthewrapperdoesnotmatchthedestinationvariable's attributesofthedestinationvariableotherthanthosedenedbytheformal typestatearedropped. above,wecodethestatement: nordestinationcanbeconstant,thesourcevariablebecomesuninitialized, movingthevalue.Thismayentaildiscardingsomeofthewrappedvalue. theunwrapstatement,thentheappropriateattributesaredroppedbefore Othertypestaterestrictionsapplyasforamovestatement:neithersource Forexample,assumethatimmediatelyafterthewrapstatementshown Ifthetypestateonthewrapperishigherthanthetypestatespeciedon unwrapxfromyfinit(*),init(id),positive(id)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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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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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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