[Caml-list] Recursion between class and value?

[Caml-list] Recursion between class and value?

[Tim Freeman]

I'd like to be able to have a class and a function be mutually
recursive, kind of like this:

   class foo = object
     method bar x =
       if x > 0 then baz (x - 1)
       else ()
   end

   let baz z =
     if z > 0 then
       (new foo)#bar (z - 1)
     else ()

The best I know how to do is transform baz into a method on foo,
yielding this:

   class foo = object (self)
     method bar x =
       if x > 0 then self#baz (x - 1)
       else ()
     method baz z =
       (* Except for the type recursion, this doesn't have to be a 
          method on foo. *)
       if z > 0 then
         (new foo)#bar (z - 1)
       else ()
   end

   let baz z = (new foo)#baz z

The comment points at the ugly part.  Is there a better way?
-- 
Tim Freeman       
tim@fungible.com
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Re: [Caml-list] Recursion between class and value?

[Jacques Garrigue]

From: tim@fungible.com (Tim Freeman)
> I'd like to be able to have a class and a function be mutually
> recursive, kind of like this:
> 
>    class foo = object
>      method bar x =
>        if x > 0 then baz (x - 1)
>        else ()
>    end
> 
>    let baz z =
>      if z > 0 then
>        (new foo)#bar (z - 1)
>      else ()

The recursion is between baz and the constructor of foo (not the
type). So this is only a value level problem.

While your solution is ok:
> The best I know how to do is transform baz into a method on foo,
> yielding this:

A simpler one is to make new foo a parameter of baz:

  let baz ~new_foo z = 
    if z > 0 then new_foo#bar (z - 1)

  class foo = object
    method bar x = if x > 0 then baz ~new_foo:(new foo) (x - 1)
  end

  let baz n = baz ~new_foo:(new foo) n

A little heavy, but pretty classical.
An even heavier approach, but maybe more intuitive, is to put baz in
another class, do that you can access foo's constructor.

Jacques Garrigue
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