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Re: [Caml-list] Mutually recursive classes
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Date: -- (:)
From: Damien <Damien.Pous@e...>
Subject: Re: [Caml-list] Mutually recursive classes
I could solve my problem by first defining two "open class type", and
then unifying 'self with the parameter in two other class types :

<<
class type ['a, 'b] a_ot = object
	constraint 'b = ('a, 'b) #b_ot
	method	 coerce: 'a
	method	b: 'b
end and ['a, 'b] b_ot = object
	constraint 'a = ('a, 'b) #a_ot
	method	 coerce: 'b
	method	a: 'a
end

class type ['b] a_t = object ('s) inherit ['s, 'b] a_ot end
class type ['a] b_t = object ('s) inherit ['a, 's] b_ot end
>>

It was the same kind of problem as the one discussed in
<http://caml.inria.fr/archives/200303/msg00349.html>

the parameters of a class are generalized after its type has been
inferred.
thus, with

<<
class type ['b] a = object ('sa)
  method coerce: 'sa
  method b: 'b
  constraint 'b = 'sa #b
end and ['a] b = object ('sb)
  method coerce: 'sb
  method a: 'a
  constraint 'a = 'sb #a
end
>>

ocaml tries to unify 'sa (resp. 'sb) and 'a (resp. 'b) which is
impossible since this would close the self types

I lack some OO theory knowledges to understand the Jacques Garrigue's
distinction between "structural definition" and "definition by name"
(could someone explain me or send me some useful links ?)

but I think that recursion with distinct type parameters should be
allowed in the case above (recursive class _types_),
disallowing it seems to me like a boring syntactic restriction, since
one can always :
 - duplicate and separate the class type definitions ; (a_ot, b_ot)
 - add parameters for forward type definitions ('a, 'b)
 - glue all these types together to find back the first ones (a_t, b_t)

Does anyone know a case where this is not true ? (with class types)
What kind of "fully-recursive" classes are meaningless (just curious)

thanks,
damien

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