Hermes: A Tutorial and Reference Manual - Researcher - IBM

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1.2GettingStarted|ASimpleHermesProgram another.Thereissomecost:youhavetowritemoredeclarations. thoughsomeprogramsmayhavebugsandtheusersdon'ttrustone 1.IntroductiontoHermes5 Let'sstartwithourrstHermesprogram. siblytypedenitions.We'llshowyouinalatersectionhowtowritethem. receiveParmsfromInit; Pascal Let'sassumefornowthatyouhavealreadywrittenthemorthattheyhave alreadybeensuppliedforyou. doesitdierfromthecorrespondingprograminC,Ada([SYW85]),or Beforeyoucompilethisprogram,youhavetoadddeclarationsandpos- ThisprogramcanbeusedtodisplaythemessageHello,World!.How callParms.PutLine("Hello,World!"); returnParms; PutLine.InHermes,PutLineisaparameter;itscounterpartsintheother languagesareconstants. mesversionofthissimpleprogramandalltheothers|thelatebindingof syntax,thereisneverthelessanimportantdistinctionbetweentheHer- Althoughatrstglancetheprogramsappeartodieronlyinsupercial write("Hello,World!")fAPascalprogramg printf("Hello,World!");/*aCprogramtodotheabove*/ IntheCprogram,printfhasaxedbinding|itdenotesalibraryprogramwhichwritesthecharacterstringtothestandardoutput.Theonly Ifyoudothis,youwillalsochangethebindingofprintfforallthemod- ([KR78]),functionsarenotvariables.Ifyouwanttocontrolthebindingof ulesofyourprogram.TousetheterminologyoftheCreferencemanual afunctionwithaCprogram,youmustsimulatefunctionvariablesusing pointerstofunctions.(SimilarlyinPascal([Coo83])andAda,functionsare notvariables.Intheselanguagesthereisawaytoachievealimiteddegree ofdynamicbinding.) ofHermes:(1)nothingisglobal,and(2)everyfunctionorprocedurename referstoavariablewhosevalueisdeterminedatrun-time,notcompiletime. asParms.PutLine.Hermesisdesignedthiswaybecauseitassumesyou receivetheparameterlistParmsandbecausewemustrefertothefunction SoeveninthistinyHermesprogram,youcanseetwoimportantfeatures PUT("Hello,World!")--AnADAprogram waytochangethebindingistolinkyourCprogramtoadierentlibrary. WhydowedothisAfterall,theprogramislongerbecausewemust
61.2.GettingStarted|ASimpleHermesProgram User address spaces arewritingamulti-userormulti-applicationsystemratherthanasingle application. FIGURE1.1.Atraditionaloperatingsystemwithmanyapplications Kernel theaddressspace.Joe'sapplicationandJane'sapplicationcaneachhave dierentbindingstotheirrespectiveprintffunctions. plicationtohaveasingleglobalmeaning,because\global"meanswithin kernelmaintainsmultipleaddressspacesinwhichyourunyourapplicationprograms.YourCapplicationisaprogramrunninginasingleaddress space.Resourcesoutsidetheaddressspace(e.g.terminalsandprinters) Lookatgure1.1,whichdepictsatraditionalsystemsuchasUNIX.A aremanagedbytheoperatingsystem,andassignedtotheaddressspace asawhole.Itmakessenseforalltheprintfstatementswithinanap- application,Jane'sapplication,modulesavailabletobothusers,andmoduleswrittenbyoneuserbutaccessibletosomemodulesofanother.(See, forexample,gure1.2.)Inthissituation,itisclearthatnofunctionname canbeglobaltothewholesystem.AlthoughJoe'sapplicationandJane's applicationmaybothbepartofasinglelarge\program",itdoesnotmake Butsupposemyapplicationisanentiresystem,whichmayincludeJoe's
Page 1 and 2: DavidF.BaconArthurP.GoldbergAndyLow
Page 3 and 4: Preface Thisdocumentcontainsatutori
Page 5 and 6: iv3TypeandTypestateChecking 5Resear
Page 8: Tutorial PartI 1
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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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theelementwillalsobeknownbyapplying
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documentationpurposesevenwheretheya
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ments.Therstargumenttocasemustbeava
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Ifinit(CM)ispresentandCMisacallmess
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6.Determinethepostconditionsforeach
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droppedattributesafternormalcomplet
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11 HermesOperations Throughoutthere
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Thediscardstatementremovesthevaluef
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Ifnoneoftheclausescanexitsnormally,
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Ifthereisanoptionalselectexpression
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stmemberisanatomwithprintnamematchi
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types.Additionally,theoperations\an
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Less,Less-equal,Greater,Greater-equ
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RecordTypeFamily 11.5RecordOperatio
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valueofthetable,ifany,isdiscarded.
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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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