GHC exits with error

[William Lee Irwin III]

After some naive toying around, I discovered the following program
caused a runtime error in the GHC compiler itself:

> module Set (
>        Set,
> )
> where
> import List

> data Set a = Set [a]
>        deriving (Eq, Ord, Read, Show)

> instance Functor Set where
>         f `fmap` (Set xs) = Set $ f `fmap` xs

> instance Monad (forall a. Eq a => Set a) where
>         (Set xs) >>= k = Set $ foldr (union . (\(Set ys) -> ys) . k) [] xs
>         return x = Set [x]

The error this caused was the following:
rename/RnEnv.lhs:238: Non-exhaustive patterns in function get_tycon_key

Cheers,
Bill