Re: [Haskell-cafe] About mplus

G'day all. Slight nit... Quoting ok :

I've been thinking about making a data type an instance of MonadPlus. From the Haddock documentation at haskell.org, I see that any such instance should satisfy

mzero `mplus` x = x x `mplus` mzero = x mzero >>= f = mzero v >> mzero = mzero

As discussed previously, that last "law" is wrong. In particular, it can't be true of any monad transformer: lift fireMissiles >> mzero /= mzero

but is that all there is to it? Are there no other requirements for MonadPlus to make sense?

It's proposed to split nondeterminism-like monads and error catch-like monads to allow for some other laws: http://haskell.org/haskellwiki/MonadPlus_reform_proposal Cheers, Andrew Bromage