Hermes: A Tutorial and Reference Manual - Researcher - IBM

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Thediscardstatementremovesthevaluefromthevariable.Discardingan initializedcallmessagereturnsthecallmessage;thecallerwillreceivethe simple{statement ::=discardvariable{name11.HermesOperations95 rstbeloweredtotheirminimumtypestate|thisconceptisdenedlater, callmessagetype.Discardedexception.Thecallmessagecomponentswill inthesectiononcallmessages.Discardingavaluecontainingcallmessages| forexampleaninputportholdingenqueuedcallmessages|discardsthe callmessages.Discardinganyothertypeofvaluejustthrowsthevalueaway. limitedresources.TheDepletionexceptionisraisedwheneverduetoaresourcelimitationoranimplementationrestriction,thecorrectresultcannoitylimits,integeroverow.Implementationsareencouragedtoavoidgiv- AnidealHermesmachinehasunlimitedresources.Realmachineshave 11.2TheDepletionException Mostoperationscanraisethisexception,soitisdescribedhereratherthan repeatedlyundereachoperation. becomputed. don'trequireallimplementationstobethissophisticated,sowereserve ingDepletionexceptionsratherthantorequireprogrammerstodevelop theirowncircumventions.Forexample,integerswhichdon'ttinahardwareregistercouldberepresentedbydynamic-precisionintegers.Butwe Examplesinclude:runningoutofaddressspace,exceedingdiskcapac- theDepletionexceptiontoinformtheuserthattheimplementationis inadequatetorunthisparticularexecution. Hermesimplementationsmustguaranteethatstoragedepletioncannotoccurwhilediscardingorhidingavalue. Coercionoperations|e.g.discard|cannotfailwithanyexception.
9611.2.TheDepletionException 11.3ControlFlowOperations Block compound{statement constant{section ::=block endblock [handler]... begin [declaration{section] [constant{section] declaration{section ::=constant([base{variable[,base{variable]...]) ::=declare[declaration;]... [statement;]... declarenewvariables,youcanspecifythatthevaluesofexistingvariables Ablockstatementintroducesanewscope.Withinthisscope,youcan handler::=on([exception{name[,exception{name]...]) ::=onexit([exit{name[,exit{name]...]) bythenamesoftheexitsorexceptionsithandles.Theabbreviationothers Theotherclausesarethehandlerclauses.Eachhandlerclauseispreceded mustremainconstant,andyoucanprovidehandlersforexceptionsorexits withinthemainbodyoftheblock. Thestatementseriesafterthewordbeginisthemainclauseoftheblock. [statement;]... withinahandlerclausecannotcausecontroltotransfertoanotherhandler clauseofthesameblock. tothehandleroftheinnermostblock.Rememberthatexceptionsraised Twoclausesofthesameblockcannothandlethesameexitorexception. containinghandlersforthesameexceptionorexit,controlwilltransfer standsforallexceptionswhicharenotexplicitlyhandled.Restriction: issuinganexitstatement,thentheclauseissaidtoexitnormally..Ifany Ifastatementiswithinthemainclausesoftwoormorenestedblocks controltotransfertotheclauseheadedbythatexceptionorexitname. Ifallstatementsofaclausecompletewithoutraisinganexceptionor Anexceptionorexitraisedwithinthemainclauseofablockcauses wordsblockandendblockdonotappear.Asecondblockwithanempty clauseofablockexitsnormally,thentheblockitselfexitsnormally. on(others)handlerisimpliedaroundthisblock.Thisisnecessarysothat Thebodyofaprocessstatementisactuallyablock,althoughthe
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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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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 96 and 97: 6.Determinethepostconditionsforeach
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Page 100 and 101: 11 HermesOperations Throughoutthere
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Page 106 and 107: Ifthereisanoptionalselectexpression
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Page 126 and 127: Itistheabilityforprogramstosendandr
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Page 142 and 143: A.1LexicalRules Webeginwiththerules
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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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