0001607: polymorphic instance variable in object ?

Notes
(0000150) administrator (administrator) 2003-03-20 14:30	From: lvibert@irisa.fr > first, a feature which : could it be possible to have polymorphic > instance variable ? Not trivial: one can redefine instance variables with {< v = ... >}, so this can only be done explicitly, the same way as record fields. But why not. > trying to pass this restriction, I fall in the 2 following bug > (CVS version) : > > class type ct = object ('s) > method fold : ('b -> 's -> 'b) -> 'b -> 'b > end > > type t = {f : 'a 'b. ('b -> (#ct as 'a) -> 'b) -> 'b} > (* type checker crash ) Looks like an infinite loop. I shall investigate it. > class type ct = object ('s) > method fold : ('b -> ct -> 'b) -> 'b -> 'b > end > > type t = { mutable f : 'b. ('b -> ct -> 'b) -> 'b -> 'b } > > class c = object (s: #ct) > method fold = failwith "undefined" > ( OK, thanks to the relaxed value restriction ) > end There is no relation with the relaxed value restriction: a method body is a value, this is a function implicitely taking self as parameter. > class c2 = object (self: #ct) > val v = {f = failwith "undefined"} ( bug ? ) > method fold = v.f > ( initializer v.f <- ...some code using self... *) > end > > This field value has type ('a -> ct -> 'a) -> 'a -> 'a > which is less general than 'b. ('b -> ct -> 'b) -> 'b -> 'b Interesting interaction of features: with the relaxed value restriction, [failwith "undefined"] can indeed be given the type \forall 'a. 'a; however, in order to catch errors earlier, the type checker first unifies the type of [failwith "unified"] with a monomorphic instance of the expected type, before attempting generalization. And since this type contains contravariant variables, it is not generalizable anymore. This can indeed be seen a bug, but I'm afraid you cannot write any meaningful program using such a feature: [failwith "undefined"] will be called immediately, and you will not be able to construct any object of this class anyway. If typing were the only problem, there is an easy workaround: class c2 = let dummy = failwith "undefined" in object (self : #ct) val v = { f = dummy } method fold = v.f end;; It is typable, but generates an exception at class definition time. The correct way to do it would be class c2 = object (self: #ct) val v = {f = fun _ -> failwith "undefined"} method fold = v.f end;; which already types correctly. Jacques

(0000151) administrator (administrator) 2003-03-26 16:29	From: lvibert@irisa.fr > first, a feature which : could it be possible to have polymorphic > instance variable ? (Aux developpeurs) Quelqu'un a une opinion sur le sujet? Les variables d'instance ressemblant beaucoup a` des champs d'enregistrement, on peut les traiter exactement de la meme facon pour le polymorphisme. Est-ce que ca pose un probleme? > trying to pass this restriction, I fall in the 2 following bug > (CVS version) : > > class type ct = object ('s) > method fold : ('b -> 's -> 'b) -> 'b -> 'b > end > > type t = {f : 'a 'b. ('b -> (#ct as 'a) -> 'b) -> 'b} > > (* type checker crash *) C'est maintenant corrige' en CVS. Ca m'a aide' a` corriger un autre bug, merci. Jacques

(0000152) administrator (administrator) 2004-07-04 17:11	Crash fixed by JG (2003-03-26). There is a wish too (polymorphic instance variables)

Issue History
Date Modified	Username	Field	Change
2005-11-18 10:13	administrator	New Issue

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