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Date: Thu, 10 Apr 2014 21:14:07 +0100
From: Neel Krishnaswami <krishnan@cs.bham.ac.uk>
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Subject: [Caml-list] First class modules question

Hello,

I'm experimenting with Ocaml's first-class modules feature, and
have run into a problem. I have a record type

   type 'a monoid = {unit : 'a; join : 'a -> 'a -> 'a}

and a signature

   module type APPLICATIVE = sig
     type 'a t
     val map : ('a -> 'b) -> 'a t -> 'b t
     val pure : 'a -> 'a t
     val ( $ ) : ('a -> 'b) t -> 'a t -> 'b t
   end

and I would like to write a function which turns a monoid into
an applicative module, as follows:

   let monoid_to_app (type s) (m : s monoid) =
     let module I = struct
       type 'a t = s
       let map _ x = x
       let pure _ = m.unit
       let ($) x y = m.join x y
     end
     in (module I : APPLICATIVE with type 'a t = s)

Here, I want to constrain the type of APPLICATIVE to be the monoid
element type s. However, Ocaml complains with a syntax error, and
looking at the grammar for type constraints, the annotation on I
is a package-type, which only permits constraints using the
package-constraint production

   package-type ::= modtype-path
                 |  modtype-path with package-constraint
                                 {and package-constraint}

   package-constraint ::= type typeconstr =  typexpr

This does not permit a constraint of the form "type 'a t = s",
unlike the module-constraint production of the grammar.

Is this a fundamental limitation on first-class modules, or is it
just a restriction for ease of implementation?

-- 
Neel Krishnaswami
N.Krishnaswami@cs.bham.ac.uk