Re: [Caml-list] ICFP 2011: Call for participation

[Polux Moon <polux.moon@gmail.com>]

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Hi,

I have a type error on code similar to :

-----
type 'par t = 'par

let ident v = v

class alias =
object
  method alias : 'a . 'a t -> 'a = ident (* type error here *)
end
-----

Which gives :

-----
Error: This expression has type 'a. 'a t -> 'a
       but an expression was expected of type 'b. 'b t -> 'b
       Type 'a a = 'a is not compatible with type 'b t = 'b
-----

The problem is related to unification of foralls containing named type in a
method.
The compiler accepts equivalent code when not using the named type "t" or
when not using a method (see attached file).

Is it a [known] limitation ? Is there a way to work around it ?

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type 'par t = 'par

let ident v = v (* here adding : 'a . 'a a -> 'a changes nothing *)

class alias =
object
  method alias : 'a . 'a t -> 'a = ident (* NOT OK *)
end

(* but *)
let alias : 'a . 'a t -> 'a = ident (* OK *)
type r = { alias : 'a . 'a t -> 'a}
let alias = { alias = ident } (* OK *)