I tried out the recently released and very nice binary library and tripped up by laziness. Muchly simplified my problem was: data Bad = Bad instance Binary Bad where put = return () get = do fail "this is bad" return Bad Here I expected decoding to Bad to fail, but to my surprise: ghci> decode (encode Bad) :: Bad Bad From the example, it is clear that my brain has the following law for fail built in: fail err >> m === fail err Some testing reveals that this law is violated not only by the Get monad, but also by State, Reader, and Writer. (There was a discussion about the strictness of the state monad a while back, I can't recall if this came up...) So, where am I going wrong? Am I mixing up fail and mzero? Are there other laws for fail that actually hold? / Ulf