Yet more on functional dependencies
Tue, 9 Jan 2001 09:15:10 +1300
George Russell writes:
> I am finding functional dependencies confusing. (I suspect I am not alone.)
> Should the following code work?
> class HasConverter a b | a -> b where
> convert :: a -> b
> instance (HasConverter a b,Show b) => Show a where
> show value = show (convert value)
Let's assume that multi-parameter classes and overlapping instances
Without the functional dependency, the instance decl would be rejected
because the type variable b occurs in the context (HasConverter a b,
Show b) but not in the head (Show a). The compiler needs to generate
some code for `show value', but it's not allowed to make an arbitrary
substitution for b.
The functional dependency provides a way to determine b uniquely once
a is matched. So, the compiler can generate the code for `show value'
without being arbitrary.