# Multi-parameter OOP

**George Russell
**
ger@tzi.de

*Fri, 19 Oct 2001 17:02:54 +0200*

Recently I've been experimenting with a sort of OOP with GHC, using existential types and
(overlapping, undecidable) multi-parameter type classes, but it doesn't seem to work as you
might expect because of the way GHC resolves overloaded functions at compile-time. For example,
given class A a
data WrappedA = forall a . A a => WrappedA a
data A1 = A1
instance A A1
class B b
data WrappedB = forall b . B b => WrappedB b
data B1 = B1
instance B B1
class AB a b where
toBool :: a -> b -> Bool
instance (A a,B b) => AB a b where
toBool _ _ = False
instance AB A1 B1 where
toBool _ _ = True
instance AB WrappedA WrappedB where
toBool (WrappedA a) (WrappedB b) = toBool a b
a naive user (like me a month ago) might expect that this to work, so that
toBool (WrappedA a) (WrappedB b) will return False unless a is an A1, and b a B1,
in which case it returns True. In fact ghc5.02 (rightly) gives an error message
with the second instance declaration:
Could not unambiguously deduce (AB a b) from the context (A a, B b)
The choice of (overlapping) instance declaration
depends on the instantiation of `a, b'
So is there any other way of doing this sort of dynamic lookup at runtime, in
a reasonably neat way?