Recursive class+type definitions

Recursive class+type definitions

[Raj B]

Hi all

Can we have mutually recursive definitions mixing types and classes?  
For example, I want to define
a class called mypoint, which contains another data structure (say, a  
custom list) of mypoints.

class virtual mypoint =
	object
		method virtual getx : float
		method virtual getlist : mypointlist
	end

type mypointlist = Empty | Cons of (mypoint * mypointlist)

What is the best way to achieve this kind of mutual recursion?

Thanks
Raj

Re: [Caml-list] Recursive class+type definitions

[Jon Harrop]

On Monday 16 July 2007 02:29:47 Raj B wrote:
> Hi all
>
> Can we have mutually recursive definitions mixing types and classes?
> For example, I want to define
> a class called mypoint, which contains another data structure (say, a
> custom list) of mypoints.
>
> class virtual mypoint =
> 	object
> 		method virtual getx : float
> 		method virtual getlist : mypointlist
> 	end
>
> type mypointlist = Empty | Cons of (mypoint * mypointlist)
>
> What is the best way to achieve this kind of mutual recursion?

Good question. No idea. But you can do this:

class virtual ['a] mypoint = object
  method virtual getx : float
  method virtual getlist : 'a
end;;

type mypointlist = Empty | Cons of (mypointlist mypoint * mypointlist);;

-- 
Dr Jon D Harrop, Flying Frog Consultancy Ltd.
The OCaml Journal
http://www.ffconsultancy.com/products/ocaml_journal/?e

Re: [Caml-list] Recursive class+type definitions

[Chris King]

On 7/15/07, Raj B <rajb@rice.edu> wrote:
> class virtual mypoint =
>         object
>                 method virtual getx : float
>                 method virtual getlist : mypointlist
>         end
>
> type mypointlist = Empty | Cons of (mypoint * mypointlist)
>
> What is the best way to achieve this kind of mutual recursion?

I'd write something like:

type mypointlist = Empty | Cons of (< getx: float; getlist:
mypointlist > * mypointlist)

class virtual ...


or:

type mypointlist = Empty | Cons of (mypoint * mypointlist)
and mypoint = < getx: float; getlist: mypointlist >

in the case that having an inheritable virtual class isn't important to you.

You can get the exact behavior you want using a pair of recursive
modules (see the "Language Extensions" section of the manual), one for
each of mypoint and mypointlist, but it's a lot more verbose (since
recursive modules require you to explicity give their signatures).

HTH,
Chris

Re: [Caml-list] Recursive class+type definitions

[Jacques GARRIGUE]

From: "Chris King" <colanderman@gmail.com>

> You can get the exact behavior you want using a pair of recursive
> modules (see the "Language Extensions" section of the manual), one for
> each of mypoint and mypointlist, but it's a lot more verbose (since
> recursive modules require you to explicity give their signatures).

I would think that one recursive module is sufficient.
Note also that, as this virtual class has no concrete methods, it
could just be defined as a class type (just means you can only use it
in interfaces and type annotations.) Then there is no code
duplication.

module rec Mypoint : sig
  class type p =
     object
       method getx : float
       method getlist : Mypoint.plist
     end
  type plist = Empty | Cons of (p * Mypoint.plist)
end = Mypoint

Jacques Garrigue