F#: Mutually recursive functions

Question

Possible Duplicate:
[F#] How to have two methods calling each other?

Hello all,

I Have a scenario where I have two functions that would benefit from being mutually recursive but I'm not really sure how to do this in F#

My scenario is not as simple as the following code, but I'd like to get something similar to compile:

let rec f x =
  if x>0 then
    g (x-1)
  else
    x

let rec g x =
  if x>0 then
    f (x-1)
  else
    x

See also http://stackoverflow.com/questions/1378575/f-forward-type-declarations — Brian, Sep 01 '10 at 20:46
I hesitate to mark this one as the duplicate, because the title is probably better... — Benjol, Sep 02 '10 at 05:03
@Benjol: Generally we don not *delete* duplicates with substantially different titles in order to improve searchability, but we still close them. — dmckee --- ex-moderator kitten, Sep 03 '10 at 17:31

score 29 · Accepted Answer · answered Sep 01 '10 at 18:53

29

You can also use let rec ... and form:

let rec f x =
  if x>0 then
    g (x-1)
  else
    x

and g x =
  if x>0 then
    f (x-1)
  else
    x

answered Sep 01 '10 at 18:53

elmattic

1

Beat me to it by 42 seconds... :-) – J D Sep 01 '10 at 18:56
1

+1, Nice, didn't realize you could use `and` with let bindings. I thought it's usage was restricted to `type` declarations. – JaredPar Sep 01 '10 at 19:02
It's specially useful (necessary) if you have mutually recursive types (like two DUs) and two functions that take each as an input argument. – elmattic Sep 01 '10 at 19:11

score 2 · Answer 2 · answered Sep 01 '10 at 18:44

2

To get mutually recursive functions simply pass one to the other as a parameter

let rec f g x =
  if x>0 then
    g (x-1)
  else
    x

let rec g x =
  if x>0 then
    f g (x-1)
  else
    x

answered Sep 01 '10 at 18:44

JaredPar

score 2 · Answer 3 · answered Sep 01 '10 at 18:54

2

Use the let rec ... and ... construct:

let rec f x =
  if x>0 then
    g (x-1)
  else
    x

and g x =
  if x>0 then
    f (x-1)
  else
    x

answered Sep 01 '10 at 18:54

J D

3 Answers3