[Caml-list] Recursive type and class

[Caml-list] Recursive type and class

[David Frese]

Hello folks,

I have a problem with mixing a class and a type recursively. Written
down directly, I want to have something like this:

type atype = Obj of aclass | I of int

class aclass = object
  val mutable field0 : atype = I 5
end

But this does not work, of course. Can anyone tell me, how this can be
done?

Thanks, David.

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Re: [Caml-list] Recursive type and class

[John Max Skaller]

David Frese wrote:

> Hello folks,
> 
> I have a problem with mixing a class and a type recursively. Written
> down directly, I want to have something like this:
> 
> type atype = Obj of aclass | I of int
> 
> class aclass = object
>   val mutable field0 : atype = I 5
> end
> 
> But this does not work, of course. Can anyone tell me, how this can be
> done?


You first declare a class *type* with a type parameter:

class type ['t] aclass_t' = object val mutable field0: 't end

Now you declare the recursion:

type atype = Obj of atype aclass_t' | I of int

and finally you make a typedef for the class:


type aclass_t = atype aclass

Well that's got the typing right, so now you can
provide an implementation:

class ['t] aclass init = object val mutable field0: atype = init end

Note that the init value had to be lifted out, so you need a monomorphic

function to instantiate the instance class:

let mkclass () = new aclass (I 5)

Finally, to check it works:

# Obj (mkclass ());;
- : atype = Obj <obj>

[You might be able to make this simpler using a constraint,
I've never quite figured them out :]

-- 
John Max Skaller, mailto:skaller@ozemail.com.au
snail:10/1 Toxteth Rd, Glebe, NSW 2037, Australia.
voice:61-2-9660-0850


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