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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thereisnoselectedelement,aNotFoundexceptionisraised. VariantTypeFamily 11.7VariantOperations 11.<strong>Hermes</strong>Operations115<br />
Avariantisavaluewhichwillhaveoneofaxed,pre-speciedsetof type{construction ::=variantofenumeration{type(<br />
adierentcomponentnametodesignateeachdierenttypeofvalue.A types.Sincein<strong>Hermes</strong>,everyvariablenamehasexactlyonetype,weuse case{declaration ::=named{literal!component{declaration component{typestate )[case{declaration[,case{declaration]...]<br />
variantVcanbeeither initialized<strong>and</strong>revealed:exactlyonecomponenthasavalue.Thepro-<br />
initialized<strong>and</strong>hidden:exactlyonecomponentexists.Theprogram uninitialized:ithasnovalue<strong>and</strong>therearenotypestateattributes<br />
ponent<strong>and</strong>noother.Thetypestateisinit(V),case(V,V.X).There gramknowswhichcomponenthasavalue<strong>and</strong>canaccessthatcom-<br />
ponentisaccessible.Thetypestateisinit(V). doesn'tknowwhichcomponentexists<strong>and</strong>whichdonot,sonocom-<br />
involvingvariableV<br />
componentsareequal. thecasesofthevariant,<strong>and</strong>theenumerationtypeisthecasetype. thenamedliteralsofanenumerationtype.Thesenamedliteralsarecalled literalofthecasetype,acomponentname,componenttype,<strong>and</strong>aformal Twovariantsareequaliftheyhavethesamecase<strong>and</strong>iftheirexisting Thecomponentsofavarianttypeareinone-to-onecorrespondencewith Avarianttypedenitionspecies:(1)thecasetype,(2)foreachnamed maybeothertypestateattributesinvolvingvariableV.X.<br />
lower. casetypestate.Thecasetypestateisthetypestatethecomponentwillhave revealed,thetypestatemaybecomehigherthanthistypestate,butnever justbeforeitishidden<strong>and</strong>justafteritisrevealed.Whilethevariantis Thefollowingisanexampleofavarianttypedenition. id:charstring; lisptype:enumeration('nil','atom','pair');
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