Hi, this law apparently fails for a MonadPlus instance that has more than one possible failure value. Consider: runIdentity . runErrorT $ ((ErrorT . Identity $ Left "failure") >> mzero :: ErrorT String Identity ()) evaluates to `Left "failure"`, which isn't equal to ErrorT's mzero `Left ""`. This isn't just the case of ErrorT, it fails for any MonadPlus with multiple failure values. For example lift (tell "foo") >> mzero :: MaybeT (Writer String) () is again distinct from mzero. Actually, no monad transformer with a MonadPlus instance can satisfy the law, because the first part in front of `>> mzero` can introduce side effects in the underlying monad. I'm not sure what should be the proper solution. Perhaps to change the laws to: return x >> mzero = mzero (v >> mzero) >>= f = (v >> mzero)` That is, if an expression ends with `mzero`, it behaves like `mzero`. Petr