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type aliases and recursive modules
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Date: -- (:)
From: Josh Berdine <berdine@d...>
Subject: RE: [Caml-list] type aliases and recursive modules
Hi,

 

Josh Berdine wrote:

> 

> Hi,

> 

> There's something I'm not understanding about recursive modules. 

> Consider the following distilled example:

> 

 

      [snip buggy simplification]

 

> 

> Why doesn't the typechecker know that ASet.t and C.t are the same 

> type? Anyone know a workaround?

> 

> This is with the 3.10.0 beta version, but seems to be the same back to 

> 3.07.

> 

> Cheers, Josh

> 

 

Hi,

 

There is an incoherency in your definition of ASet.get_its_elements.

 

In the signature of ASet, get_its_elements take an argument of type 

ASet.t, but then in its definition, it takes an argument of type A.t (as 

A.get takes an argument of type A.t, and you give ASet.get_its_elements' 

argument to A.get)

 

--

Philippe Wang

mail@philippewang.info

 

 

Thanks Philippe and Christopher, yes, mea culpa, I forgot, e.g., a call to
choose.  But my confusion remains:

 

 

# 

module rec A : sig

  type t = It of ASet.t

  val compare : t -> t -> int

  val get : t -> ASet.t

end = struct

  type t = It of ASet.t

  let compare = compare

  let get = function It(x) -> x

end

 

and ASet : sig

  type t

  val get_its_elements : t -> A.t list

end = struct

  module C = Set.Make(A)

  type t = C.t

  let get_its_elements x = C.elements (A.get (C.choose x))

end

;;

                                      Characters 350-370:

    let get_its_elements x = C.elements (A.get (C.choose x))

                                        ^^^^^^^^^^^^^^^^^^^^

This expression has type ASet.t but is here used with type

  C.t = Set.Make(A).t

#

 

 

Am I still doing something stupid?

 

Cheers,  Josh