non-exhaustive patterns in lambda

[Michael Weber]

Hi!

\begin{code}
module Exh3 where
class Monad m => StateMonad s m where
   getState :: m s

setState0 :: forall b.(StateMonad (a,b) m => m a)
setState0 = getState >>= \ (l,_r) -> return l
\end{code}

yields:  TcType.lhs:313: Non-exhaustive patterns in lambda


Cheers,
Michael
BTW: Yes, sometimes I prefer randomly shuffling source code 
     instead of thinking about why this can't work ;-)
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