O'Haskell OOP Polymorphic Functions

[Ashley Yakeley]

At 2001-01-17 16:07, Johan Nordlander wrote:

>Ashley Yakeley wrote:
>> 
>> OK, I've figured it out. In this O'Haskell statement,
>> 
>> > struct Derived < Base =
>> >     value :: Int
>> 
>> ...Derived is not, in fact, a subtype of Base. Derived and Base are
>> disjoint types, but an implicit map of type "Derived -> Base" has been
>> defined.
>> 
>> --
>> Ashley Yakeley, Seattle WA
>
>Well, they are actually subtypes, as far as no implicit mapping needs to be
>defined.  But since Derived and Base also are two distinct type constructors,
>the overloading system treats them as completely unrelated types (which is 
>fine, in general).  

All O'Haskell treats them as completely unrelated types. In fact, this 
O'Haskell...

> struct Base =
>       b1 :: Int
>       b2 :: Char
> 
> struct Derived < Base =
>       d1 :: Int

...is a kind of syntactic sugar for this Haskell...

> data Base = Base (Int,Char)
> dotb1 (Base (x,_)) = x
> dotb2 (Base (_,x)) = x
> 
> data Derived = Derived (Int,Char,Int)
> dotd1 (Derived (_,_,x)) = x
> 
> implicitMap (Derived (b1,b2,d1)) = Base (b1,b2)

This seems to be stretching the concept of 'subtype'.

Sorry if I sound so bitter and disappointed. I was hoping for a Haskell 
extended with real dynamic subtyping...

-- 
Ashley Yakeley, Seattle WA