generalization and let module

generalization and let module

[Julien SIGNOLES]

Hello all,

Just a single question: why is 'f' not generalizable in the following
example? I'm not able to find an example using any "let module" and
breaking the type safety.

=====
module M = struct end
let f = let module N = M in fun x -> x
=====

--
Julien

Re: [Caml-list] generalization and let module

[Jacques Garrigue]

From: Julien SIGNOLES <julien.signoles@cea.fr>

> Just a single question: why is 'f' not generalizable in the following
> example? I'm not able to find an example using any "let module" and
> breaking the type safety.
> 
> =====
> module M = struct end
> let f = let module N = M in fun x -> x
> =====

What about

let f =
  let module M = struct let x = ref [] end in
  fun x -> let y = List.hd !M.x in M.x := [x]; y ;;

Making this function polymorphic would clearly be unsound.
However, you're right on one account: this can only happen with
structures, but not with module aliasing, or even functor application.
So the non-expansiveness check could be refined.

Jacques Garrigue