Hermes: A Tutorial and Reference Manual - Researcher - IBM
Hermes: A Tutorial and Reference Manual - Researcher - IBM
Hermes: A Tutorial and Reference Manual - Researcher - IBM
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type).Ifthecheckfails,theexceptionDefinitionErrorisraised. ischeckedforresolutionerrors<strong>and</strong>otherrestrictions(e.g.eachvariant denitionincludesacaseidforeachpossiblevalueoftheenumeration oper<strong>and</strong>isafullyinitializeddenitionsmodule.Thedenitionsmodule Thisoperationcompletestheinitializationofadenitionsmodule.The Ifthechecksucceeds,theidcomponentofthedenitionsmoduleisre-<br />
11.<strong>Hermes</strong>Operations131<br />
attributebehaveslikeaconstraint:anyattempttomodifythedenitions initionsmodulearereplacedbytheuniqueidentier.Thedenitionsmod-<br />
uleacquirestheattributecheckeddefinitions.Thecheckeddefinitions placedbyauniqueidentier.Allreferencestothatidentierwithinthedef-<br />
modulewilldiscardtheattributecheckeddefinitions.Unlikeaconstraint attribute,checkeddefinitionscannotbeusedinanassertstatement; anteedthatwhentwotypenamesorattributenamesareequal,theyalways mescanenforce\nameequivalence"oftypes.Noothercheckeddenitions modulecanpossiblyhavethesamemoduleidentier.Thereforeitisguar-<br />
denotethesametypeorattributedenition. theattributecanonlybeassertedbyexecutingthecheckdefinitions statement.<br />
11.11Constraints Becausecheckeddenitionsaregivenauniquemoduleidentier,Her-<br />
attribute{construction<br />
argumentbenon-negative.Aprogramlibrarymightbedenedasatable ::=constraint(<br />
ofcheckedprograms. Forinstance,theinterfacetoasquare-rootfunctionmightrequirethatthe Constraintsareprogrammer-denedpredicatesonthevaluesofvariables.It isoftendesirabletoincludeconstraintsininterfacesorintypedenitions. )isconstraint{typestateconstraint{expression [declaration[,declaration]...]<br />
betrackedastypestateattributes.Currently,<strong>Hermes</strong>isnotsophisticated enoughtounderst<strong>and</strong>thesemanticsofconstraintpredicateswellenough totrackhowtheyarepreservedunderalloperations.Forexample,the compilerwillnotdeterminethataddingonetoanon-negativenumberwill yieldanothernon-negativenumber.Whatitwilldoisthefollowing: In<strong>Hermes</strong>,theprogrammercanspecifythatcertainconstraintsareto<br />
addthetypestateattributeNonNegative(a)iftheoperationassert associateanamewithapredicate{e.g.NonNegative(x)withthe whichwilltestwhethera>=0<strong>and</strong>raiseanexceptionifitisnot. predicatex>=0.ThisdenesanoperationassertNonNegative(a)
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