Hermes: A Tutorial and Reference Manual - Researcher - IBM

11611.7.VariantOperations sexpression:variantoflisptype( );'pair'->pair:conscellffullg conscell:record( 'atom'->atom:idfinitg, 'nil'->null:emptyfg,
sexpressioncontaincomponentsoftypeconscell,whichinturncontaincomponentsoftypesexpression.
beenrevealedtobeincase'pair',thenthetypestatewillbe );cdr:sexpression Notethatthesetypedenitionsaremutuallyrecursive|variablesoftype Inthisexample,ifVisavariableoftypesexpression,andithasjust car:sexpression,
Unite untiltheyarerevealedinturn.ItisnotlegaltodiscardV.Pair.Car,leavingapartiallyinitializedpair.Itis,however,legaltoreplacethevalueof
init(V),case(V,V.Pair),init(V.Pair),init(V.Pair.Car),init(V.Pair.Cdr) Innercomponents,suchasV.Pair.Car.Pair.Cdr,willnotbeaccessible
Theunitestatementinitializesavarianttoaparticularoneofitscases. Itstwooperandsareavariablenamedesignatingavariantcomponent, component.Thetypestateoftheexpressionmustbeatleastashighasthe andanexpressionevaluatingtoavalueofthesametypeasthatvariant casetypestateofthevariantcomponent.Ifitishigher,itiscoerceduntil itisexactlyequaltothecasetypestate. simple{statement
Ifthevarianthadavalue,thatvalueisdiscarded.Thevalueofthe ::=unitevariant{componentfromsource{expression
thevariantwillberevealedandthevariantcomponentwillbeaccessible. Thedissolvestatementreversestheeectoftheunitestatement.The Dissolve expressionisthenmovedintothevariantcomponent.Aftertheoperation,
rstvariablenamedesignatesavariantcomponent.Thesecondvariable namedesignatesavariableofthesametypewhichwillreceivethevalueof thatvariantcomponent. destinationvariableisdiscarded.Thevalueofthevariantcomponentis Thevariantcomponentmustberevealed.Anypreviousvalueofthe simple{statement ::=dissolvevariant{componentintoresult{variable