Introduction to the module system

Learning
1

Stephen Diehl has started a series of blog posts about module systems using OCaml as an example. This provides a nice introduction to the finer points.

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2

I believe the last example in the blog aims to demonstrate that generative functors create new (incompatible) types.

a generative functor which is denoted by an extra parameter () which will produce an output module containing non-equal abstract types

But this is not the case here. Maybe I’m misunderstanding the point of the example.

module FA = F(A);;
module type S = sig type t val s : t end
module A : S
module F : functor (M : S) -> sig type a = M.t val b : a end
module FA : sig type a = A.t val b : a end
module GA = G(A)();;
module G : functor (M : S) () -> sig type a = M.t val b : a end
module GA : sig type a = FA.a val b : a end

utop # FA.b = GA.b;;
- : bool = true
3

G.a isn’t abstract.

a generative functor which is denoted by an extra parameter () which will produce an output module containing non-equal abstract types

means “if the functor produces an abstract type t, separate applications will make the resulting ts not equal”

4

Yes. So the example given in the article doesn’t demonstrate what it talks about. Which is a bit unexpected.

5

It is not a matter of abstract types or not but a question of definitions

module F() = struct
   type t = A | B
end
module M = F()
module N = F()
let error = [M.A;N.A]

compared to abbreviations

type original = A | B | C
module F () = struct
  type t = original = A | B | C
end
module M = F()
module N = F()
let ok = [M.A; N.A]
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