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storing object in record
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Date: -- (:)
From: Jérémie Dimino <jeremie@d...>
Subject: Re: [Caml-list] storing object in record
Le mercredi 08 avril 2009 à 12:29 +0200, Michael a écrit :
> > There is no value of type: forall 'a. #baseclass as 'a.
> 
> is this similar? :
> 
>  type 'a xx = 'a constraint 'a = < asBase: baseObject; .. > ;;
> 
> (why does: type 'a xx = < asBase: baseObject; ..>  as 'a  not work instead? )

No, [type 'a xx = 'a constraint 'a = < asBase: baseObject; .. >] just
means that ['a] must match [< asBase : baseObject; .. >]. Actually
constraints are just inlined:

  # type 'a t = 'a constraint 'a = _ * _;;
  type 'a t = 'a constraint 'a = 'b * 'c
  # let f (x : 'a t) = ();;
  val f : ('a * 'b) t -> unit = <fun>

If you write [type 'a xx = < asBase: baseObject; ..>] (with or without
[as 'a]), then you have an unbound variable in the type definition (the
[..]).

>  type 'a r = { state: 'a xx };;
> 
> gives:
> type 'a r = { state : 'a xx; } constraint 'a = < asBase : baseObject; .. >
> 
> 
> so the record holds objects with an asBase method and more, isn't this the 
> same as #baseObject?

[#baseObject] is an abbreviation for [< asBase : baseObject; .. >].

Jérémie