[Caml-list] Mutually recursive types and classes

[Caml-list] Mutually recursive types and classes

[Nick Name]

Hi all,

I wish to declare a type like

type a = A of (b -> unit) | ...

where b is a class like

class b = 
object (this)
	method f : a -> unit = 
		function 
		  A f -> f this
                | _ -> unit
end

or, alternatively:

class b =
object 
   inherit [a] otherclass
   ...
end

How can I do such a thing, considering that I can't write an "and"
keyword between a type declaration and a class (or class type)
declaration?

Thanks for help

Vincenzo

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Re: [Caml-list] Mutually recursive types and classes

[Olivier Andrieu]

 Nick Name [Thursday 19 June 2003] :
 > Hi all,
 > 
 > I wish to declare a type like
 > 
 > type a = A of (b -> unit) | ...
 > 
 > where b is a class like
 > 
 > class b = 
 > object (this)
 > 	method f : a -> unit = 
 > 		function 
 > 		  A f -> f this
 >                 | _ -> unit
 > end
 > 
 > or, alternatively:
 > 
 > class b =
 > object 
 >    inherit [a] otherclass
 >    ...
 > end
 > 
 > How can I do such a thing, considering that I can't write an "and"
 > keyword between a type declaration and a class (or class type)
 > declaration?

You could add a type parameter in your definition of a :

  type 'a a = A of ('a -> unit) | ...

and then

  class b = object (this : 'a) method f : 'a a -> unit = ... end

or

  class b = object method f : b a -> unit = ... end

and eventually redefine a :

type a = b a

-- 
   Olivier

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Re: [Caml-list] Mutually recursive types and classes

[brogoff]

You can use the parameterization trick and introduce an extra type variable, 
as has been discussed on the list before. This is the big hammer for solving 
all sorts of forbidden recurrences in OCaml, like the functor instance/type 
recurrence. Thankfully, that particular problem now appears to be fixed in the 
CVS snapshot. 

You can also have recursion between object types and types, and use coercion 
to make sure everything matches, like so 

type a = A of (b -> unit) | B
and b = < f : a -> unit > 

class c = 
  object(this) method f = function A foo -> foo (this :> b) | _ -> () end 

-- Brian


On Thu, 19 Jun 2003, Nick Name wrote:

> Hi all,
> 
> I wish to declare a type like
> 
> type a = A of (b -> unit) | ...
> 
> where b is a class like
> 
> class b = 
> object (this)
> 	method f : a -> unit = 
> 		function 
> 		  A f -> f this
>                 | _ -> unit
> end
> 
> or, alternatively:
> 
> class b =
> object 
>    inherit [a] otherclass
>    ...
> end
> 
> How can I do such a thing, considering that I can't write an "and"
> keyword between a type declaration and a class (or class type)
> declaration?
> 
> Thanks for help
> 
> Vincenzo
> 
> -------------------
> To unsubscribe, mail caml-list-request@inria.fr Archives: http://caml.inria.fr
> Bug reports: http://caml.inria.fr/bin/caml-bugs FAQ: http://caml.inria.fr/FAQ/
> Beginner's list: http://groups.yahoo.com/group/ocaml_beginners
> 

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Re: [Caml-list] Mutually recursive types and classes

[John Max Skaller]

Nick Name wrote:

> Hi all,
> 
> I wish to declare a type like
> 
> type a = A of (b -> unit) | ...
> 
> where b is a class like
> 
> class b = 
> object (this)
> 	method f : a -> unit = 
> 		function 
> 		  A f -> f this
>                 | _ -> unit
> end

Parameterise the class:


class ['a] b' = ...

then say

type a = A of (b-> unit) ..
and b = a b'




-- 
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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Re: [Caml-list] Mutually recursive types and classes

[Nick Name]

Thanks to all, I report another solution for completeness:

  class b = ...

  type a = A of b -> unit | ..

  class c = object (this)
  inherit b 
    method f : a -> unit = ...
  end

wich is similar in spirit to Brian's but allows to write the "inherit"
part.

Vincenzo

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