From: Alan Falloon <alan.falloon@marseda.com>
To: caml-list@inria.fr
Subject: How do apply functors with module constraints?
Date: Thu, 18 Oct 2007 19:19:05 -0400 [thread overview]
Message-ID: <ff8pmk$eik$1@ger.gmane.org> (raw)
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
next reply other threads:[~2007-10-18 23:20 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2007-10-18 23:19 Alan Falloon [this message]
2007-10-19 0:32 ` [Caml-list] " Julien Moutinho
2007-10-19 8:20 ` Nicolas Pouillard
2007-10-19 8:21 ` Andreas Rossberg
2007-10-22 14:54 ` Alan Falloon
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