[Caml-list] Problem with forall and method

[Caml-list] Problem with forall and method

[Polux Moon]

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(Sorry for duplicate mail, I forgot to change the subject in the first one)

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 *)

Re: [Caml-list] Problem with forall and method

[Jacques Garrigue]

This is a bug.
Please report to http://caml.inria.fr/mantis

Jacques Garrigue

On 2011/07/26, at 21:17, Polux Moon wrote:

> (Sorry for duplicate mail, I forgot to change the subject in the first one)
> 
> 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 ?
> 
> <forallclass.ml>