Re: [Caml-list] Feature request.

[Alain.Frisch@ens.fr]

On Tue, 8 Apr 2003, Lukasz Lew wrote:

>   I found that it would be usefull if some of the language constructions
>   could be made localy. For example "open ... in" or "type ... in".
>   Why "open in" isn't in standard parser?
>
>   Why some constructions are restricted to global (module) use?

Local modules can help you !

If X is a structure item (open, type, exception,...), you can
encode  (X in e) as:

  let module Foo = struct
    X
    let result = e
  end in
  Foo.result

I wrote a Camlp4 syntax extension to support this:

http://www.eleves.ens.fr/home/frisch/soft#openin

It defines the following construction:
    open M in e
and the generalization:
    struct ... end in e


There is a small run-time overhead, because of the creation of the local
structure. Also, note that type variables defined outside a local
structure are not visible within the structure; the following does not
work:

 let f (x : 'a) =
   let module Foo = struct
     exception E of 'a;;
     raise (E x)
   end in
   ()


-- Alain

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