On Mon, 14 Feb 2005 19:01:53 -0500, ajb@spamcop.net
I was thinking more along the lines of Ralf Hinze's nondeterminism transformer monad:
http://haskell.org/hawiki/NonDeterminism
The relevant instance is this:
instance (Monad m) => MonadPlus (NondetT m)
That is, if m is a Monad, then NondetT m is a MonadPlus. This is not true if a requirement for MonadPlus is that it include the "mzero is a right zero for bind" law. Indeed, such a transformer is impossible to write if that law is a requirement.
Ah, I see. You are quite right.
You claimed that monad transformers break the mzero-is-right-identity-for-bind law because they can be applied to IO. I say, it's not the monad transformers fault. They cannot possibly be expected to repair the law if they are given a faulty monad.
IO is not a faulty monad. It satisfies all of the laws that a monad is supposed to satisfy.
Sloppy terminology on my side again. What I meant to say is that any MonadPlus instance of IO is faulty if we insist on the mzero-is-right-identity-for-bind law. I agree with you that the law should be dropped. Cheers, /Josef