ocaml type within a type

Question

If I have a type in a module State

type state = {x: int; y: int}

and I have another type in module Game

type game = State.state

how can I access the record values in a object with type game?

For example if I have a game "g", g.x gives me an "Unbound record field label x" error.

Answer 1

The names of the fields are in the State module namespace. You can say g.State.x, or you can open the State module.

let f g = g.State.x

Or:

open State

let f g = g.x

If you want the fields to appear in the Game module namespace, you can repeat them:

type game = State.state = {x: int; y: int}

You can also use the include facility to include the State module.

For example, your Game module could say:

include State
type game = state

In either of these cases, you can refer to Game.x:

let f g = g.Game.x

Or:

open Game
let f g = g.x

There are also two notations for opening a module for just a single expression:

let f g = Game.(g.x)

Or:

let f g = let open Game in g.x

Edit: Here's a Unix command-line session that shows the first (simplest) solution:

$ cat state.ml
type state = { x: int; y : int }
$ cat game.ml
type game = State.state
$ cat test.ml
let f (g: Game.game) = g.State.x

let () = Printf.printf "%d\n" (f { State.x = 3; y = 4})
$ ocamlc -o test state.ml game.ml test.ml
$ ./test
3