Beta Reduction, undefined vs Type Classes

Consider:

class Thing t where thing :: t

instance Thing Int where thing = 0

instance Thing Char where thing = 'a'

Can someone please explain why

fst (1,thing)

...gets an 'ambiguous type variable' error, but

fst (1,undefined)

...doesn't? And is there anything I can change to make the former work as well? Even modifying fst doesn't work:

fst' :: Thing b => (a,b) -> a fst' (~a,~b) = a

-- ~ Jared Warren Computing Science, Queen's University