storing object in record

storing object in record

[Michael]

it seems that I'm not able to figure out how to do  this:

class baseclass = object(this)
  method asBase = (this :> baseclass)
(* ... *)	
end;;

class ex = object inherit baseclass method name = "ex" end

type state_rec = { mutable state:  'a. #baseclass as 'a };;


this gives an error:

let x = new ex;;

{ state = x };;
Error: This field value has type ex which is less general than
         'a. #baseclass as 'a

or:

{ state = x#asBase };;
Error: This field value has type baseclass which is less general than
         'a. #baseclass as 'a

or:

type state_rec = { mutable state:  'a. < asBase: baseclass;..> as 'a };;
gives:
Error: Unbound type parameter ..


what I'm missing here?

cheers
 Michael

Re: [Caml-list] storing object in record

[Jérémie Dimino]

Michael wrote:
> it seems that I'm not able to figure out how to do  this:
> 
> class baseclass = object(this)
>   method asBase = (this :> baseclass)
> (* ... *)	
> end;;
> 
> class ex = object inherit baseclass method name = "ex" end
> 
> type state_rec = { mutable state:  'a. #baseclass as 'a };;

There is no value of type: forall 'a. #baseclass as 'a.

I think what you want to do is:

  type state_rec = { mutable state: exists 'a. #baseclass as 'a };;

You can actually do this with objects:

  type state_rec = { mutable state : baseclass }

  { state = (x :> baseclass) }

cheers,
Jérémie

Re: [Caml-list] storing object in record

[Michael]

Am Wednesday 08 April 2009 09:54:48 schrieben Sie:

> 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? )


 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?


 Michael

Re: [Caml-list] storing object in record

[Jérémie Dimino]

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