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Date: Tue, 03 Jul 2012 14:18:33 +0200
From: Jean-Louis Giavitto <jean-louis.giavitto@ircam.fr>
Reply-To: jean-louis.giavitto@ircam.fr
Organization: UMR STMS 9912 - IRCAM & CNRS & UPMC & Inria
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Subject: [Caml-list] Initialization of a polymorphic field in a record

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.