From: Andreas Rossberg <rossberg@mpi-sws.mpg.de>
To: Alan Falloon <alan.falloon@marseda.com>
Cc: caml-list@inria.fr
Subject: Re: [Caml-list] How do apply functors with module constraints?
Date: Fri, 19 Oct 2007 10:21:43 +0200 [thread overview]
Message-ID: <188637FD-0995-4FA4-A26B-4782F8398182@mpi-sws.mpg.de> (raw)
In-Reply-To: <ff8pmk$eik$1@ger.gmane.org>
On Oct 19, 2007, at 01.19h, Alan Falloon wrote:
> 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 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)
>
As you noted, abstraction prevents the types in the Pie submodules
from being equivalent. And there is no way to retract that
abstraction later - it wouldn't be abstraction otherwise. So you will
not be able to apply Joe as you tried. This is not a question of syntax.
The solution is to avoid the over-abstraction and keep the Pie
submodules transparent. Try:
> module Bob : (BAKER with module Pie = Apple) = struct
> module Pie = Apple
> let dozen () = Array.init 13 (fun _ -> Pie.bake ())
> end
>
> module Daisy : (PIG with module Pie = Apple) = struct
> module Pie = Apple
> let feed p = Pie.eat p; print_endline "OINK!"
> end
- Andreas
next prev parent reply other threads:[~2007-10-19 8:22 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2007-10-18 23:19 Alan Falloon
2007-10-19 0:32 ` [Caml-list] " Julien Moutinho
2007-10-19 8:20 ` Nicolas Pouillard
2007-10-19 8:21 ` Andreas Rossberg [this message]
2007-10-22 14:54 ` Alan Falloon
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