Hello,
Le mar 01 jui 2008 17:53:34 CEST,
SerP <[EMAIL PROTECTED]> a écrit :
> class type session_t =
> object
> method id:int;
> method location: #location_t;
> end;
>
...
>
> Error: This expression has type session = < id : int; location :
> location > but is here used with type
> #session_t as 'a = < id : int; location : 'b.
> #location_t; .. >
My guess is that session_t is a bit too polymorphic: location is
a polymorphic method (indicated by the 'b. in the error message). Thus,
a class implementing session_t should have a location method able to
return any subtype of location_t, which might be a bit complicated to
say the least. A more sensible class type would require that the
location method returns a subtype of location_t, which may differ from
one implementation to the other, but is fixed for a given
implementation. By the way, this matches more or less your second
solution.
At the class/class type level, this can be done by adding a type
parameter to the class type and using a constraint as shown below
(warning, this is the regular ocaml syntax, not the revised one):
class type ['a]session_t =
object
method id:int
method location: 'a
constraint 'a = #location_t
end
let check_session (session:'a #session_t) = begin
print_int session#id;
print_int session#location#a;
print_string session#location#b;
end
--
E tutto per oggi, a la prossima volta.
Virgile
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