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From: Jacques Garrigue <garrigue@math.nagoya-u.ac.jp>
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Date: Fri, 8 Jun 2012 12:22:17 +0900
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To: Hongbo Zhang <bobzhang1988@gmail.com>
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Subject: Re: [Caml-list] First Class modules -A bug in 4.00?

On 2012/06/08, at 11:35, Hongbo Zhang wrote:

> Hi, List
>  I am not sure this is a bug or not?
> 
>  Below is a contrived example:
> ------------------
>  module type S = sig
>    type t=int
>    module  X : sig type u   end
>  end
> 
>  let f ( module X : S)  (y:X.X.u) =
>  3
> --------------------
> Error: This pattern matches values of type X.X.u
>      but a pattern was expected which matches values of type X.X.u
>      The type constructor X.X.u would escape its scope
> -- Thanks

Definitely, this is not a bug.
Type X.X.u is abstract, and showing it outside (as by taking an argument
of that type) would be meaningless.

What you might have meant is:

 let f (type a) (module X : S with type X.u = a) (y : X.X.u) = 3

This is typable (but I'm not sure it means anything...)

Jacques Garrigue