Chapter 8  Language extensions

This chapter describes language extensions and convenience features that are implemented in OCaml, but not described in the OCaml reference manual.

23  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 : 'a list -> 'a 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/3742 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/3742 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 8.8.2) 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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