Re: [Caml-list] does class polymorphism need to be so complicated?

[Issac Trotts <ijtrotts@ucdavis.edu>]

Brian Hurt wrote:

>On Wed, 20 Aug 2003, Benjamin Geer wrote:
>
>  
>
>>class printer =
>>   object
>>     method print (obj : printable) = obj#print()
>>   end ;;
>>    
>>
>
>Instead of declaring obj to be printable, why not just declare that it has 
>a function print?  Like:
>
>class printer = 
>    object
>        method print (obj: <print: unit->unit>) = obj#print ();
>    end;;
>  
>
You still have to up-cast objects having more methods than just 'print'.

# class printer =
 object
  method print (o:<print:unit>) = o#print
 end;;

# class foo =
   object
    method print = print_string "[foo]"
   end;;

# class bar =
   object
    method print = print_string "[bar]"
    method frob = ()
   end;;

# let f = new foo;;
val f : foo = <obj>
# let p = new printer;;
val p : printer = <obj>
# let b = new bar;;
val b : bar = <obj>
# p#print f;;
[foo]- : unit = ()
# p#print b;;
Characters 8-9:
  p#print b;;
          ^
This expression has type bar = < frob : unit; print : unit >
but is here used with type foo = < print : unit >
Only the first object type has a method frob
# p#print (b :> <print:unit>);;
[bar]- : unit = ()

Issac

>This removes the need for a coercion, as it gets around the need to 
>upcast.
>
>Brian
>
>
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>
>  
>


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