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Date: Wed, 30 Mar 2011 05:50:09 +0300
From: Lauri Alanko <la@iki.fi>
To: caml-list@inria.fr
Message-ID: <20110330025009.GA5064@melkinpaasi.cs.helsinki.fi>
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Subject: [Caml-list] Functor applications cannot be opened

I have a signature that uses types from a functor applied to a member
of the signature:

module type T = sig
  type t
end

module F(M : T) = struct
  type l = M.t list
end

module type S = sig
  module M : T
  val x : F(M).l
end

I would like to get the types in F(M) into scope to avoid putting the
functor application everywhere:

module type S = sig
  module M : T
  open F(M)
  val x : l
end

However, this does not work, since [open] takes only a module-path,
not an extended-module-path. There are workarounds:

module type S2 = sig
  module M : T
  module FM : module type of F(M)
  open FM
  val x : l
end

This, however, requires that an implementation of S2 contains [module
FM = F(M)], and we may need to add that constraint elsewhere, too.

Alternatively:

module SF(M : T) = struct
  module FM = F(M)
  open FM
  module type S = sig
    val x : l
  end
end

And then use [SF(X).S] instead of [S with module M = X]. But this is
ugly, and precludes use with unknown M.

Is there some fundamental reason why opening of functor applications
is forbidden, or is this just an oversight and I should submit a
feature ticket?


Lauri