Re: [Caml-list] How do apply functors with module constraints?

["Nicolas Pouillard" <nicolas.pouillard@gmail.com>]

Excerpts from Alan Falloon's message of Fri Oct 19 01:19:05 +0200 2007:
> I am having a really hard time figuring out how to instantiate a functor 
> when its arguments have module constraints. Here is an example:

You  can  also  expose  that Bob is an apple pie baker, and Daisy an apple pie
eater:

module Bob : BAKER with module Pie = Apple = ...
module Daisy : PIG with module Pie = Apple = ...

HTH,

> module type PIE = sig
>    type pie
>    val bake : unit -> pie
>    val eat : pie -> unit
> end
> 
> module type BAKER = sig
>    module Pie : PIE
>    val dozen : unit -> Pie.pie array
> end
> 
> module type PIG = sig
>    module Pie : PIE
>    val feed : Pie.pie -> unit
> end
> 
> (* Need a module constraint to make sure that the baker and pig are 
> talking about the same pie *)
> module Farmer (Baker : BAKER) (Pig : PIG with module Pie = Baker.Pie) = 
> struct
>    let feed_pigs () =
>      let pies = Baker.dozen () in
>        Array.iter Pig.feed pies
> end
> 
> module Apple : PIE = struct
>    type pie = Pie of string
>    let bake () = Pie "apple"
>    let eat (Pie "apple") = ()
> end
> 
> module Bob : BAKER = struct
>    module Pie = Apple
>    let dozen () = Array.init 13 (fun _ -> Pie.bake ())
> end
> 
> module Daisy : PIG = struct
>    module Pie = Apple
>    let feed p = Pie.eat p; print_endline "OINK!"
> end
> 
> module Joe = Farmer(Bob)(Daisy) (*BOOM*)
> let () = Joe.feed_pigs()
> 
> 
> However, this fails to compile because when I try to make module Joe 
> because Bob.Pie and Daisy.Pie aren't the same. Fair enough, thats what 
> abstraction is for, however I can't figure out the syntax to instantiate 
> Joe! I tried this:
> 
> module Joe = Farmer(Bob)(Daisy:PIG with module Pie=Bob.Pie)
> 
> But then I get a syntax error (?)
> File "bar.ml", line 39, characters 30-31:
> Syntax error: ')' expected
> File "bar.ml", line 39, characters 24-25:
> This '(' might be unmatched
> 
> Now I'm fresh out of ideas.
> 
> Thanks
> Alan Falloon
> 

-- 
Nicolas Pouillard aka Ertai