# Re: Looking for a nail

From: John Prevost (prevost@maya.com)
Date: Fri Jan 29 1999 - 08:20:59 MET

Date: Fri, 29 Jan 1999 02:20:59 -0500
Message-Id: <199901290720.CAA28558@zarya.maya.com>
From: John Prevost <prevost@maya.com>
To: caml-list@inria.fr
Subject: Interesting behavior of "include"

I started using the "include" directive in signatures recently, then
realized that it's not actually documented anywhere (although it's
quite useful at times.)

I also discovered a little "problem" with it.

module type test =
sig
type t
val a : t
end

module type test2 =
sig
include test
val b : t
end

module type test3 =
sig
include test
val c : t
end

sig
include test2
include test3 with type t = t
end

(* This would seem to imply that the new type t is the same as the old
when the above expands to:
sig
type t
val a : t
val b : t
type t = t (* just "type t" without the "with" part above. *)
val a : t
val c : t
end
So that a, b, and c would all have the same type.

If you try it without the with, you'll see that they do actually end up
all with the same type, but no matter how hard you try to constrain the
signature using "with", the second "type t" causes t to be abstract.

If you use the with above to try to constrain the second type t
in the signature to be the same as the first (you can't name
which t you mean, since include doesn't let you refer to the
first t as Test2.t), you get an even worse problem:

sig type t val a : t val b : t type t = t val a : t val c : t end
# module Testbad : testbad with type t = int = struct
# type t = int
# let a = 1
# let b = 1
# let c = 1
# end;;
sig type t = int val a : t val b : t type t = t val a : t
val c : t end
- : Testbad.t = Uncaught exception: Stack overflow

This happens with or without the with in the module definition.

Pretty neat, eh? The with definition is allowing you to define a
cyclic type abbreviation. (This is correctly caught in all other
places you can use with as being an unbound type constructor. But
in the include case, it must see that there is, in fact, a type t,
without being able to express in the form of a signature that in
the definition "type t = t", the second t refers to the previously
defined t.

This leaves two problems of course: the first is "include ... with"
needs to check for cyclic type abbreviations, and the second is that
there's no way to constrain includes to be more "merging" than they