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8.22 Generalized open statements

(Introduced in 4.08)

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/4502 introduced by this open appears in the signature File "exten.etex", line 3, characters 6-7: The value x has no valid type if t/4502 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 8.7.2) can be used instead.

Beware that this extension is not available inside class definitions:

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

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