A function returning itself

A function returning itself

[Paolo Donadeo]

Hi everybody, I need your help again.

In a particular application I need (or at least I think so...) to
create a function that, besides it's own computation, returns itself.
Something like:

$ cat test.ml
let rec f a b =
  let computation = a + b in
    (computation, f)
;;

This file doesn't compile; the error is:

$ ocamlc -c test.ml
File "test.ml", line 3, characters 4-20:
This expression has type int * (int -> int -> int * 'a)
but is here used with type int * 'a

If I use the -rectypes option, it seems to work, even though the
inferred type is quite cryptic (to me):

$ ocamlc -c -rectypes -i test.ml
val f : int -> int -> int * 'a as 'a

Since I don't completely understand the inner implications of
-rectypes and a quick check in the mailing list archives revealed that
"rectypes is evil", the question I pose is whether is it possible to
avoid the -rectypes option in this particular case, possibly with the
definition of an auxiliary type.

My idea is that the function makes a computation and then decides if
the next computation of the same type will be carried out by "f"
itself, or by another function with identical signature.


-- 
Paolo
~
~
:wq

Re: [Caml-list] A function returning itself

[Mathias Kende]

Le lundi 24 novembre 2008 à 23:37 +0100, Paolo Donadeo a écrit :
> Since I don't completely understand the inner implications of
> -rectypes and a quick check in the mailing list archives revealed that
> "rectypes is evil", the question I pose is whether is it possible to
> avoid the -rectypes option in this particular case, possibly with the
> definition of an auxiliary type.

Usually, using an intermediate type with a constructor, you can achieve
such thing. Here, something along the line of this should work :

type r = {res: int; f: int -> int -> r}
let rec fct a b = {res = a + b; f = fct}

Mathias

Re: [Caml-list] A function returning itself

[Paolo Donadeo]

> Usually, using an intermediate type with a constructor, you can achieve
> such thing. Here, something along the line of this should work :

Thanks, this is exactly what I need.


-- 
Paolo
~
~
:wq