nakata keiko wrote:
> Why an instance of class a can not coerced into type
b?

I'll assume you mean the other way around :-)

> class a = object (self : 'a)
>   method f (x : 'a) = x#g
>   method g = 0
> end
> 
> class b = object 
>   inherit a
>   method h = 1
> end
> 
> let x = ((new b) :> a)

Afaik the self type refers to actual type of the
object, eg in object
of class b it refers to type b. Thus, the method f of
class b
requires an object of type b as a parameter, and type
b implies
having a method h. Objects of class a don't have this
method,
and so b#f can't accept all the arguments a#f can.
Google for co/contra variance for discussions of these
issues.

The below class avoids the problem:

class a = object
  method f (x : a) = x#g
  method g = 0
end

/Martin



		
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