Re: [Haskell-cafe] What is MonadPlus good for?

On Sun, 13 Feb 2005 17:59:57 -0500, ajb@spamcop.net wrote:

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.

I think it's unfair to the monad transformers to simply say that they don't obey the law. The interesting thing is whether they *preserve* the law. A monad transformer T preserves a law if given a monad M which obeys the law holds then the monad T M obeys the law. I haven't checked if this is the case for any of the monad transformers in the hierarchical libraries though. But I think that the wording should be changed so that they aren't blamed for breaking the law. (I can't believe I'm taking sides with monad transformers as if they where human. I spend too much time hacking haskell I guess...) /Josef