| bar :: (C T) => T | *Main> :t bar | | <interactive>:1:0: | No instance for (C T) | arising from a use of `bar' at <interactive>:1:0-2 | Possible fix: add an instance declaration for (C T) | In the expression: bar I'm not sure where that comes from, but it does seem to be an artifact of GHC's type inference, which seems unwilling to infer a flexible context even if flexible contexts are enabled: *Main> :show languages active language flags: -XImplicitPrelude -XFlexibleContexts *Main> let f _ = negate [] *Main> :t f f :: (Num [a]) => t -> [a] *Main> let f _ = negate [()] <interactive>:1:10: No instance for (Num [()]) arising from a use of `negate' at <interactive>:1:10-20 Possible fix: add an instance declaration for (Num [()]) In the expression: negate [()] In the definition of `f': f _ = negate [()] *Main> let f :: Num [()] => t -> [()]; f _ = negate [()] *Main> :t f <interactive>:1:0: No instance for (Num [()]) arising from a use of `f' at <interactive>:1:0 Possible fix: add an instance declaration for (Num [()]) In the expression: f This does look like a bug to me? Compare with Hugs (Hugs mode): Main> :t let f _ = negate [] in f let {...} in f :: Num [a] => b -> [a] Main> :t let f _ = negate [()] in f let {...} in f :: Num [()] => a -> [()] Claus