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Subject: [Caml-list] polymorphic methods

Dear all,

I am attempting to store polymorphic methods in classes (i.e. keep the
type variables out of the class-type by putting them directly into the
method signatures).

However, I have some trouble when it comes to convince the compiler that
my code is typecorrect:

type ('a, 'b) f_arg = < x : 'b; .. > as 'a ;;
let f : 'a 'b . (('a,'b) f_arg) -> 'b  = fun a -> a#x ;;
class foo = object method f : 'a 'b . ('a, 'b) f_arg -> 'b = f end  ;;

yields:

Error: This expression has type
'a 'b. (< x : 'b; .. > as 'a) -> 'b
but an expression was expected of type
'c 'b. (< x : 'b; .. > as 'c) -> 'b
The type variable 'd occurs inside 'd

What is 'd? Why do these types not unify?

-- 
Christoph Höger

Technische Universität Berlin
Fakultät IV - Elektrotechnik und Informatik
Übersetzerbau und Programmiersprachen

Sekr. TEL12-2, Ernst-Reuter-Platz 7, 10587 Berlin

Tel.: +49 (30) 314-24890
E-Mail: christoph.hoeger@tu-berlin.de