Re: [Haskell-cafe] Fwd: incorrect MonadPlus law "v >> mzero = mzero"?

On Mon, Feb 3, 2014 at 3:00 PM, Brandon Allbery wrote:

On Mon, Feb 3, 2014 at 5:51 PM, Dan Burton wrote:

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Indeed this issue is not limited merely to multiple failure values.

>>> runMaybeT $ lift (putStrLn "effect") >> mzero effect >>> runMaybeT mzero

So you're right. This law is being violated

I thought it was fairly well known that IO violates one of the monad laws, in a way that would lead to this?

The choice of IO for the underlying monad is irrelevant. The issue is that a later mzero in the transformer cannot undo an earlier action in a lower monad. For example:

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Prelude Control.Monad.Maybe Control.Monad.State> runStateT (runMaybeT $ (lift $ put "bar") >> mzero) "foo" (Nothing,"bar") Prelude Control.Monad.Maybe Control.Monad.State> runStateT (runMaybeT mzero) "foo" (Nothing,"foo")