Re: [Caml-list] Initialization of a polymorphic field in a record

[Philippe Veber <philippe.veber@gmail.com>]

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Hi,

how about that:

# type t = { check : 'a. 'a -> bool
};;
type t = { check : 'a. 'a -> bool;
}

# let return_true : 'a. 'a -> bool = fun _ ->
true;;
val return_true : 'a -> bool =
<fun>

# let make1 () = { check = return_true;
};;
val make1 : unit -> t =
<fun>

cheers,
  Philippe.


2012/7/3 Jean-Louis Giavitto <jean-louis.giavitto@ircam.fr>

> Hello.
>
> I am trying to build a record with a polymorphic field and I am unable to
> initialize this field. The problem can be summarized as follow. The
> following definitions works well:
>
>    type t = { check : 'a. 'a -> bool }
>
>    let return_true _ = true
>
>    let make1 () = { check = return_true; }
>
> But this definition raises an error:
>
>    let make2 f = { check = f; }
>
> with the message:
>
>   Error: This field value has type 'a -> bool which is less general than
>          'b. 'b -> bool
>
> Note that
>
>    let return_false _ = true
>
>    let make3 c = { check = if c then return_false else return_true; }
>
> is working but that
>
>    let g c = if c then return_false else return_true
>    let make4 c = { check = g c; }
>
> raises the same error message. Making explicit the argument of make does
> not helps:
>
>   let make5 f = { check = f; }
>   in make5 return_true
>
> (same error message). And making explicit the type of make does not help
> neither:
>
>   let make6 : 'a. ('a -> bool) -> t
>   = function f -> { check = f; }
>
> (same error message).
>
>
>
> Do you have an idea how I can specify a function similar to make to buid a
> record of type t?
>
> In the real life, the argument f will be the result of a computation and
> instead of a simple signature 'a -> bool, I must deal with a signature
>
>    'a 'b. (('b) #SomeClass as 'a) * 'b -> bool
>
>
> Thanks for your advice,
> Jean-Louis Giavitto.
>
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