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1、InitializingMutuallyReferentialObjectsChallengesandAlternativesDonSymeMicrosoftResearch,CambridgeUKRestrictionsinCoreMLOnlyrecursivefunctions:"letrec"canonlybindlambdaexpressionsalsorecursivedatainOCamlNopolymorphicrecursion"letrec"bindingsmustberecursivelyusedatu

2、niformpolymorphicinstantiationsValuerestrictionlimitsonthegeneralizationofpolymorphicbindingsthatinvolvecomputationThistalkisabout...TheproblemofinitializingmutuallyreferentialcomputationalstructuresEspeciallyinthepresenceofabstraction+effectsAnalternativewaytoadd

3、ressthisproblemButonethatfitsnicelywithCoreMLRelatedtheoryandpracticeaka“ValueRecursion”Pleasenote!RecursivedefinitionsinMLletrecfx=ifx>0thenx*f(x)else1CoreMLletrecones=1::onesOCamlRecursivefunctionRecursivedataletconsxy=x::yletrecones=cons1onesImmediatedependency

4、typewidgetletrecwidget=MkWidget(fun...->widget)PossiblydelayeddependencyExample1:TypicalGUItoolkitsform=Form(menu)menu=Menu(menuItemA,menuItemB)menuItemA=MenuItem(“A”,{menuItemB.Activate})menuItemB=MenuItem(“B”,{menuItemA.Activate})Aspecification:menumenuItemB

5、menuItemAformtypeForm,Menu,MenuItemvalMkForm:unit->FormvalMkMenu:unit->MenuvalMkMenuItem:string*(unit->unit)->MenuItemvalactivate:MenuItem->unit…Assume:AssumethisabstractAPIEvolvingbehaviourWidgetsExample1:TheObviousIsNotAllowedletrecform=MkForm()andmenu=MkMenu()a

6、ndmenuItemA=MkMenuItem(“A”,(fun()->activatemenuItemB)andmenuItemB=MkMenuItem(“B”,(fun()->activatemenuItemA)…Theobviouscodeisn'tallowed:Constructioncomputationsonr.h.sofletrecDelayedself-referencesletform=MkForm()letmenu=MkMenu()letmenuItemB=refNoneletmenuItemA=M

7、kMenuItem(“A”,(fun()->activate(the(!menuItemB))menuItemB:=Some(MkMenuItem(“B”,(fun()->activatemenuItemA))…Example1:ExplicitInitializationHolesinMLSoweendupwriting:MLprogrammersunderstandref,Some,None.Theuseofexplicitmutationisdeeplydisturbing.VRMitigationTechniq

8、ue1Manuallybuild“initialization-holes”andfillinlaterMostprogrammershatethis.WhybotherusingMLifyouendupdoingthis?Example1:ImperativeWiringinML//Createle