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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