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How do apply functors with module constraints?
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Date: -- (:)
From: Alan Falloon <alan.falloon@m...>
Subject: How do apply functors with module constraints?
I am having a really hard time figuring out how to instantiate a functor 
when its arguments have module constraints. Here is an example:

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