Chapter 12 Language extensions

22 Generalized open statements

(Introduced in 4.08)

definition	::=	...
	∣	open module-expr
	∣	open! module-expr

specification	::=	...
	∣	open extended-module-path
	∣	open! extended-module-path

expr	::=	...
	∣	let open module-expr in expr
	∣	let open! module-expr in expr

This extension makes it possible to open any module expression in module structures and expressions. A similar mechanism is also available inside module types, but only for extended module paths (e.g. F(X).G(Y)).

For instance, a module can be constrained when opened with

module M = struct let x = 0 let hidden = 1 end open (M:sig val x: int end) let y = hidden

Error: Unbound value hidden

Another possibility is to immediately open the result of a functor application

let sort (type x) (x:x list) = let open Set.Make(struct type t = x let compare=compare end) in elements (of_list x)

val sort : 'x list -> 'x list = <fun>

Going further, this construction can introduce local components inside a structure,

module M = struct let x = 0 open! struct let x = 0 let y = 1 end let w = x + y end

module M : sig val x : int val w : int end

One important restriction is that types introduced by open struct ... end cannot appear in the signature of the enclosing structure, unless they are defined equal to some non-local type. So:

module M = struct open struct type 'a t = 'a option = None | Some of 'a end let x : int t = Some 1 end

module M : sig val x : int option end

is OK, but:

module M = struct open struct type t = A end let x = A end

Error: The type t introduced by this open appears in the signature. The value x has no valid type if t is hidden.

is not because x cannot be given any type other than t, which only exists locally. Although the above would be OK if x too was local:

module M: sig end = struct open struct type t = A end … open struct let x = A end … end

module M : sig end

Inside signatures, extended opens are limited to extended module paths,

module type S = sig module F: sig end -> sig type t end module X: sig end open F(X) val f: t end

module type S = sig module F : sig end -> sig type t end module X : sig end val f : F(X).t end

and not

  open struct type t = int end

In those situations, local substitutions(see 12.7.2) can be used instead.

Beware that this extension is not available inside class definitions:

class c =
  let open Set.Make(Int) in
  ...