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From: Jacques Garrigue <garrigue@math.nagoya-u.ac.jp>
In-Reply-To: <CAMu2m2L4jA3KeXQoTva=8UGYvsPHZ6FWEM=am3GkASGgkGhvzw@mail.gmail.com>
Date: Thu, 5 Nov 2015 11:10:21 +0900
Cc: OCaml Mailing List <caml-list@inria.fr>
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References: <CAMu2m2L4jA3KeXQoTva=8UGYvsPHZ6FWEM=am3GkASGgkGhvzw@mail.gmail.com>
To: Ashish Agarwal <agarwal1975@gmail.com>
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Subject: Re: [Caml-list] substituting recursive types inside a signature

On 2015/11/03 05:01, Ashish Agarwal wrote:
>=20
> $ cat foo.ml
> module M =3D struct
>   type t =3D A | B of u
>   and u =3D C of t
> end
>=20
> module type N =3D sig
>   include module type of M
>     with type t =3D M.t
>     and type u =3D M.u
> end
>=20
> $ ocamlc -c foo.ml
> File "foo.ml", line 8, characters 9-21:
> Error: This variant or record definition does not match that of type M.t
>        The types for field B are not equal.
>=20
> I guess the "and" in the constraint doesn't make the constraints mutually=
 recursive, so what does it do?

with constraints are handled incrementally. I.e., the code you wrote is exa=
ctly equivalent to:

   include (((module type of M) with type t =3D M.t) with type u =3D M.u)

It has always been that way. An interesting question is whether we could ac=
tually delay the checking to
after applying all the constraints. This would be a non-trivial change.

Jacques Garrigue=