Chapter 10Language extensions

22Generalized 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/556 introduced by this open appears in the signature File "extensions/generalizedopens.etex", line 3, characters 6-7: The value x has no valid type if t/556 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 endopen struct let x = A endend
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 10.7.2) can be used instead.

Beware that this extension is not available inside class definitions:

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