Hermes: A Tutorial and Reference Manual - Researcher - IBM

compile-timeistooconstraining.Hermessupportsvariantsandpolymorphs Sometimestherequirementthatthetypeofeveryvaluebeknownat 4.3Variants toallowthisrequirementtobeweakened.Howeverwedonotgiveupthe fundamentalprinciplesthatmachine-levelimplementationdetailmustbe 4.AdditionalHermesConstructs61
forpurposesofthisexample,we'llassumetobejustacharacterstring ustalkaboutLISPobjects: hidden,andthatthelanguagemustbesecure. whohasprogrammedinLISP.ALISPobjectiseitheranatom(which printname),apair,ornothing(nil).HerearetheHermestypeswhichlet LispPair:record(Car:LispObject,Cdr:LispObject); LispKind:enumeration('atom','pair','nil'); Weillustratevariantswithanexamplewhichwillbefamiliartoanyone
LispObject:variantofLispKind(
nents:L.PrintNameoftypeCharstring,L.PairoftypeLispPair,and varianttype. whichisusedconventionallytorepresentanullvalue.We'llexplainthe languages.Emptyisapredenedtype|anenumerationwithnovalues| SupposeLisavariableoftypeLispObject.ThenLhasthreecompo-
Theenumerationandrecordtypesarejustlikesimilartypesinother 'pair'->Pair:LispPairffullg, 'nil'->Nil:Emptyfg); 'atom'->PrintName:Charstringffullg,
L.NiloftypeEmpty.Inthisrespect,avariantisjustlikearecord.But whereasaninitializedrecordmayhaveseveralcomponents,onlyonecomponentofavariantcanexistatanyonetime.Whichcomponentitisdependsuponthecaseofthevariant.Thecaseisanenumerationvalue|in
thisexample,either'atom','pair',or'nil'.Eachcomponentisassociatedwithoneofthecasesofthevariant.Sothevalueofaninitialized
whereintheprogramthevariantishidden,andwhereitisrevealed.When itisrevealed,thecompilertracksitscase.SoifLisaLispObject,the variantisacasevalueplusexactlyoneofthevariantcomponents. isatypestateproperty.Thatmeansthatthecompilerwillkeeptrackof possibletypestatesofLare: thatwedon'tknowthecaseatcompiletime.Revealedmeanswedo.This Aninitializedvariantcanbeeitherhiddenorrevealed.Hiddenmeans
initializedandrevealedincase'pair' uninitialized initializedandhidden initializedandrevealedincase'atom'