CODEWITHSUNDEEP
typesmoduleocaml
user1044618
user1044618

Reputation: 51

OCaml types/modules

If I have two modules A and B, and define a type in module A,

type t1 = THIS of int | THAT of char

and then want to do some pattern matching or constructing objects of that type in module B, I have to use something like

A.THIS(3)

This is what it forces me to do, even after I have some statement like

type t2 = A.t1

in the signature and implementation of the module. What if I wanted to be less verbose and not use 'A.' in module B? I could make a whole new type and a function to convert one to the other, but this seems clunky. Is there anything I'm missing?

Upvotes: 3

Views: 575

Answers (2)

dkim
dkim

Reputation: 3970

You can use the open keyword, which is similar to Java's import:

module A = struct
  type t1 = THIS of int | THAT of char
end

module B = struct
  open A
  let b = THIS 3
end

If you want to avoid polluting the global namespace of module B, it is possible to use the let open ... in ... expression, which was introduced in OCaml 3.12:

module A = struct
  type t1 = THIS of int | THAT of char
end

module B = struct
  let b = let open A in THIS 3
end

In some special (and relatively rare) cases, you may want to or have to use the include keyword instead of open. You can find good explanation for their difference in the Opening a module path and Including the components of another structure sections of the OCaml manual.

Upvotes: 8

Jeffrey Scofield
Jeffrey Scofield

Reputation: 66823

I'm not a module expert, but you can do this (I just tested it):

(In b.ml):

type t1 = A.t1 = THIS of int | THAT of char

You can also open the A module, of course. Often this is too drastic for my taste. To make it less drastic, you could define the type in a third module and open it in both A and B.

Upvotes: 5

Related Questions