G'day all.
Quoting Remi Turk
According to http://www.haskell.org/hawiki/MonadPlus (see also the recent thread about MonadPlus) a MonadPlus instance should obey m >> mzero === mzero, which IO doesn't. IOW, the MonadPlus instance for IO (defined in Control.Monad.Error) probably shouldn't be there.
Clearly the wiki page has not been updated to reflect the current debate. :-) I've changed the wording to this. Anyone disagree? Note: There are theoretical reasons why ''mzero'' should be a right-zero for (>>=), but surprisingly few of the existing MonadPlus instances actually obey this law. {{{IO}}} does not, and neither do any ["MonadTransformer"]s, since they may be stacked on top of {{{IO}}}. This suggests that either some of the extant MonadPlus instances are inappropriate, or that the law itself might be incorrect. There is continuing debate over this, and the dust has not yet settled. Cheers, Andrew Bromage