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Module type troubles
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Date: 2006-07-18 (04:31)
From: Chris King <colanderman@g...>
Subject: Module type troubles
(Sorry for the long explaination that follows, it's the most concise
example I can come up with.)

Say I have two polymorphic types with associated constructors:

type 'a foo = { foo: 'a }
let make_foo a = { foo = a }

type 'a bar = { bar: 'a }
let make_bar a = { bar = a }

Using the "combine" function, I am able to define constructors for
compound types:

let combine make_a make_b (a, b) =
    make_a a, make_b b

type 'c foobar = 'a foo * 'b bar constraint 'c = 'a * 'b
let make_foobar c: 'c foobar =
    combine make_foo make_bar c

type 'c foobarfoo = 'a foobar * 'b foo constraint 'c = 'a * 'b
let make_foobarfoo c: 'c foobarfoo =
    combine make_foobar make_foo c

Now I would like to do the same thing with modules:

module Foo = struct
    type 'a t = { foo: 'a }
    let make a = { foo = a }

module Bar = struct
    type 'a t = { bar: 'a }
    let make a = { bar = a }

module type Combinable = sig
    type 'a t
    val make: 'a -> 'a t

module Combine(A: Combinable)(B: Combinable) = struct
    type 'c t = 'a A.t * 'b B.t constraint 'c = 'a * 'b
    let make (a, b) = A.make a, B.make b

module FooBar = Combine(Foo)(Bar)
module FooBarFoo = Combine(FooBar)(Foo)

Everything works fine until the last line.  The compiler complains:

Type declarations do not match:
  type 'a t = 'b Foo.t * 'c Bar.t constraint 'a = 'b * 'c
is not included in
  type 'a t

I realize that this is because type 'a FooBar.t cannot exist without
its constraint.  Is there a way to acheive the module structure I
want, save using Obj?

Thanks in advance,
Chris King