class subtyping

class subtyping

[nakata keiko]

I have a question about subtyping between classes.

Why an instance of class a can not coerced into type b?

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) 

I got an error for the last statement.

I am reading the work on extension problem found at 
  http://pauillac.inria.fr/~remy/work/expr/
then, 
I am curious to know how ocaml compiler judges subtyping relation 
involving self types.

Regards,
Keiko NAKATA.

Re: class subtyping

[Martin Sandin]

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

Re: class subtyping

[Martin Sandin]

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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