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Mutually recursive functions in different modules
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Date: -- (:)
From: Julien Signoles <Julien.Signoles@l...>
Subject: Re: [Caml-list] Mutually recursive functions in different modules
Hello,

> Is it possible to have mutually recursive functions in separate modules?

I know (at least) 4 solutions to your problem: one use recursive modules
as suggested by Jacques Garrigue, one use higher-order functions as
suggested by Jean-Christophe Filliatre, one use functors and one use
references on functions.

For example, if you want something (stupid) like

   module A = struct let f x = if x <= 0 then 0 else B.f (x - 2) end
   module B = struct let f x = if x = 1 then 1 else A.f (x - 2) end

you can write:

(* 1- using recursive modules *)
module rec A : sig val f : int -> int end = struct
   let f x = if x <= 0 then 0 else B.f (x - 2)
end and B : sig val f : int -> int end = struct
   let f x = if x = 1 then 1 else A.f (x - 2)
end

(* 2- using higher-order functions *)
module A' = struct let f g x = if x <= 0 then 0 else g (x - 2) end
module B = struct let rec f x = if x = 1 then 1 else A'.f f (x - 2) end
module A = struct let f = A'.f B.f end

(* 3- using functors *)
module FA(X:sig val f : int -> int end) = struct
   let f x = if x <= 0 then 0 else X.f (x - 2)
end
module B = struct
   let rec f x =
     let module A = FA(struct let f = f end) in
     if x = 1 then 1 else A.f (x - 2)
end
module A = FA(struct let f = B.f end)

(* 4- using references on functions *)
module A' = struct let f = ref (fun _ -> assert false) end
module B = struct let f x = if x = 1 then 1 else !A'.f (x - 2) end
module A = struct
   let () = A'.f := fun x -> if x <= 0 then 0 else B.f (x - 2)
   let f = !A'.f
end

In my opinion, solution 1 is the more natural when A and B are in the 
same file.

Hope this helps,
Julien
-- 
mailto:Julien.Signoles@lri.fr ; http://www.lri.fr/~signoles
"In theory, practice and theory are the same,
but in practice they are different" (Larry McVoy)