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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Ifinit(CM)ispresent<strong>and</strong>CMisacallmessage,thentheattributes theformaltypestatedenedforcomponentAinthetypedenitionfor aninitializedbuthiddenvariant. typestateofarevealedvariantmustbehigherthanthetypestateof variantV.Thisfollowsfromthefactthatwerequiretheseattributes tobepresentwhenwehidethevariant,<strong>and</strong>werequirethatthe 10.TypestateChecking87<br />
intheminimumtypestateofCMmustbepresent.Theseattributes<br />
isitselfuninitialized. typestateareillegal|e.g.anassignmenttorecordcomponentR.AwhenR Otherwiselegaloperationswhoseeectwouldbetoproduceaninvalid areobtainedbysubstitutingCMintotheformaltypestatedenedor impliedastheminimumtypestate.(Seediscussionofcallmessages.) discardedbytheprocessreceivingthecall. thatcertaincallparametersmustremaininitialized<strong>and</strong>cannotbe Thisisbecausetheprogrammerisallowedtostateintheinterface<br />
typestatewhenthesetofattributesoftherstisasubsetofthesetofattributesofthesecond.Themeetoftwotypestatesissimplytheintersection<br />
Thesemilatticestructureissimple.Atypestateislowerthanasecond oftheattributesets. 10.4OrderingofTypestates<br />
10.5Coercions Coercionsaretypestate-loweringoperationswhichareinsertedautomaticallybythecompiler.Therearethreecoercions:<br />
hideVremovestheattributecase(V,V.X),alongwithanyother dropconstraint(P1,...,Pn)removestheattributeconstraint(P1, discardXremovestheattributeinit(X),aswellasanyotherat-<br />
...,Pn). attributesofV.X.<br />
init(V.B)gisfinit(V)g. finit(V),case(V,V.A),init(V.A)g<strong>and</strong>finit(V),case(V,V.B), Example:finit(A)gislowerthanfinit(A),even(A)g.Themeetof<br />
tributesofXorX'scomponents.
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