An interesting revelation just occurred to Simon P.J. and myself while wondering about issues to do with exceptions in the IO monad (see discussion on glasgow-haskell-users@haskell.org if you're interested). The question we were considering was whether the following should hold in the IO monad: (return () >>= \_ -> undefined) `seq` 42 == undefined as we understand the IO monad it certainly shouldn't be the case. But according to the monad laws: (law) return a >>= k == k a so (return () >>= \_ -> undefined) `seq` 42 => ((\_ -> undefined) ()) `seq` 42 => undefined `seq` 42 => undefined So the IO monad in Haskell, at least as we understand it, doesn't satisfy the monad laws (or, depending on your point of view, seq breaks the monad laws). This discrepancy applies to any state monad. Suppose we define return a = \s -> (s, a) m >>= k = \s -> case m s of (s', a) -> k a s' now return a >>= k => \s -> case (return a) s of (s', a') -> k a' s' => \s -> case (s, a) of (s', a') -> k a' s' => \s -> k a s but (\s -> k a s) /= (k a) in Haskell, because seq can tell the difference. What should the report say about this? Cheers, Simon