A simple example showing that IO doesn't satisfy the monad laws?

Question

I\'ve seen mentioned that IO doesn\'t satisfy the monad laws, but I didn\'t find a simple example showing that. Anybody knows an example? Thanks.

Answer 1

m >>= return ≡ m

is broken:

sequence_ $ take 100000 $ iterate (>>=return) (return ()) :: IO ()

clutters memory and increases computation time, while

sequence_ $ take 100000 $ iterate (>>=return) (return ()) :: Maybe ()

does not.

AFAIR there is a Monad Transformer which solves this problem; if I guess right, it is the Codensity Monad Transformer.