class type param constraint

class type param constraint

[Radzevich Belevich]

class [ 'c ] a = 
  object(self)                  
    constraint 'c = < .. >
    method as_c =  (self :> 'c)
    method id = Oo.id self
  end          ;;
class ['a] a :
  object ('a)
    constraint 'a = < as_c : 'a; id : int; .. >
    method as_c : 'a
    method id : int
  end
================

Why type 'c must have methods "as_c" and "id" ? 

Re: [Caml-list] class type param constraint

[Radzevich Belevich]

In previous message poorly worded question.
Why 'c should include "a". ":>" cast, 'c should be smaller type, but not
great

Re: [Caml-list] class type param constraint

[Jacques Garrigue]

From: Radzevich Belevich <radzevich.belevich@gmail.com>
> class [ 'c ] a = 
>   object(self)                  
>     constraint 'c = < .. >
>     method as_c =  (self :> 'c)
>     method id = Oo.id self
>   end          ;;
> class ['a] a :
>   object ('a)
>     constraint 'a = < as_c : 'a; id : int; .. >
>     method as_c : 'a
>     method id : int
>   end
> ================
> 
> Why type 'c must have methods "as_c" and "id" ? 

This may be a bit confusing, but the .. in your constraint on 'c means
that no actual subtyping will occur when you use this type as target.
Indeed, if the constraint was left unchanged, it would mean that by
choosing an arbitrary instance, you could use the as_c method to
convert an object of type "t a" to any "t".
Since no subtyping occurs, (self :> 'c) is just equivalent to
(self : 'c) (i.e. unify 'c with the type of self).
As a result, you get
  object ('a)
    ...
    method as_c : 'a
    ...
  end

Jacques

Re: [Caml-list] class type param constraint

[Radzevich Belevich]

On Thu, 2009-04-09 at 19:13 +0900, Jacques Garrigue wrote:
> This may be a bit confusing, but the .. in your constraint on 'c means
> that no actual subtyping will occur when you use this type as target.
> Indeed, if the constraint was left unchanged, it would mean that by
> choosing an arbitrary instance, you could use the as_c method to
> convert an object of type "t a" to any "t".
Why it can't be converted to any valid "t"?