O'Haskell OOP Polymorphic Functions

[Tom Pledger]

Ashley Yakeley writes:
 [...]
 > Subtyping-overlapping is quite different from type-substitution
 > overlapping.

Different, but with some similarities.

 > Consider:
 > 
 > struct B
 > 
 > struct D1 < Base = 
 >      a1 :: Int
 > 
 > struct D2 < Base = 
 >      a2 :: Int
 > 
 > class TheValue a where theValue :: a -> Int
 > instance TheValue B  where theValue _ = 0
 > instance TheValue D1 where theValue _ = 1
 > instance TheValue D2 where theValue _ = 2
 > 
 > struct M < D1,D2
 > 
 > m = struct
 >  a1 = 0
 >  a2 = 0
 > 
 > f = theValue m
 > 
 > What's the value of f?

Undefined, because neither of the overlapping instances is strictly
more specific than the other.  I hope that would cause a static error,
anywhere that 'instance TheValue D1', 'instance TheValue D2', and
'struct M' are all in scope together.

Here's a similar example using type-substitution overlapping:

    instance               TheValue Char      where ...
    instance Monad m    => TheValue (m Char)  where ...
    instance TheValue a => TheValue (Maybe a) where ...

    trouble = theValue (Just 'b')

Regards,
Tom