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Date: Mon, 9 May 2011 10:27:02 -0400
From: Yaron Minsky <yminsky@janestreet.com>
To: caml-list@inria.fr
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Subject: [Caml-list] "with module" surprises

I've gotten bitten recently by the semantics of "with module", and after
getting an explanation about how this seems to work in OCaml, I'm now
deeply confused.  Here's the example I was shown:

  module M = struct
    let x = 13
  end

  module type S = sig
    module M' : sig end
  end
  with module M' = M

The inferred types for this will be:

  module M : sig val x : int end
  module type S = sig module M' : sig val x : int end end

Whereas I would have expected this:

  module M : sig val x : int end
  module type S = sig module M' : sig end end

In other words, the "with module" constraint has added new structure to
the signature S, rather than just adding constraints.  This strikes me
as deeply strange, and indeed, has caused a bunch of head-scratching
here when using "with module".  Is this a bug?  Or is this really the
desired semantics.  My understanding is that in SML, "with module"
simply adds in a bunch of type-level sharing constraints.  From that
point of view, this behavior is pretty surprising.

Not only that, it's what the OCaml manual says.  From section 6.10.4

  The constraint [module module-path = extended-module-path] adds type
  equations to all type components of the sub-structure denoted by
  [module-path], making them equivalent to the corresponding type
  components of the structure denoted by [extended-module-path].
      
y

-- 
Yaron Minsky