Editing MonadPlus reform proposal (section)

== Discussion ==
Given that Control.Applicative(Alternative) now defines a class which seems innately bound to '''Left Catch''', at least in spirit, it seems to make sense to clean up MonadPlus such that all instances obey '''Left Distribution'''? --sclv

I'd actually suggest almost the opposite, that MonadPlus be dispensed with and merged into Monad.  The (controversial) fail method looks no different than an mzero, except the string argument; indeed, so far as I know <tt>fail s</tt> is just mzero for any MonadPlus.  MonadPlus is also barely made use of; just guard and msum in the standard?  To be concrete, I would make the following the default definitions (in Monad):

<haskell>
mzero = fail "something"
mplus a b = a
</haskell>

These are thus somewhat trivial by default, but having msum=head and guard=assert (roughly; more like <tt>(`assert` return ())</tt>) for less-flexible monads doesn't seem actually wrong and could be useful fallbacks.

I also question the claim that Maybe and IO should be thought of as "left catch".  IO is not even in MonadPlus, and I don't see how it can be meaningfully in any way other than the above.  Maybe does satisfy Left Catch, but it seems almost like that's only because it's such a simple monad (holding only one value).  It is a useful observation that it fails Left Distribution, but that may only call for weaker Monad/Plus conditions.

The MonadOr idea is a solid one, but it seems to be taking the monad in a different direction.  So if there's a good match in Control.Applicative or Parsec, that might be the best place to develop that idea. -- Galen

The default <hask>mplus</hask> doesn't satisfy <hask>mplus mzero b = b</hask>, so you lose Monoid which seems to be the only thing people actually agree on :) -- [[User:Benmachine|Benmachine]]

[[Category:Proposals]] [[Category:Monad]]