[Caml-list] Re: OCAML Downcasting?

[Damien Pous <Damien.Pous@ens-lyon.fr>]

Hello,

Jacques GARRIGUE <garrigue@kurims.kyoto-u.ac.jp> wrote:
> Just for fun, here is an example of the second approach:
> 
> class virtual ['a] widget = object
>   method virtual raw : 'a
> end 
> class ['a] button = object (self)
>   inherit ['a] widget
>   method raw = `Button self
>   method press = prerr_endline "pressed"
> end

How to write a signature for this code ?

even "ocamlc -i" fails ! 

<<
#ocamlc -i error.ml
class virtual ['a] widget : object method virtual raw : 'a end
class ['a] button :
  object ('b)
    constraint 'a = [> `Button of 'b ]
    method press : unit
    method raw : 'a
  end
>>

then 
<<
#ocamlc -c error.ml
The implementation error.ml does not match the interface error.cmi:
Class declarations do not match:
  class ['a] button :
    object ('b)
      constraint 'a = [> `Button of 'b ]
      method press : unit
      method raw : 'a
    end
does not match
  class ['a] button :
    object
      constraint 'a = [> `Button of 'a button ]
      method press : unit
      method raw : 'a
    end
One type parameter has type
  [> `Button of < press : unit; raw : 'a; .. > as 'b ] as 'a
but is expected to have type [> `Button of 'c button ] as 'c
Type 'b is not compatible with type 'c button = < press : unit; raw : 'c >
>>

thanks,
Damien

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