On Fri, 2008-05-09 at 12:47 -0700, David Roundy wrote: > On Fri, May 09, 2008 at 08:39:38PM +0100, Andrew Coppin wrote: > > OK, so I feel I understand monads fine. I regularly use Maybe, [] and > > IO, and I've even constructed a few monads of my own. But here's a > > question: what is the purpose of the MonadPlus class? > > > > Clearly it defines a binary operation over monadic values and an > > identity element for that operation. But... what is this supposed to > > *do*? (For example, (>>) has an almost identical signature to mplus. But > > presumably they don't both do the same thing...) What functionallity is > > this supposed to give you? > > MonadPlus is a crude way of achieving the catching of exceptional cases, > and you should think of mplus as being equivalent to (catch . const). The > trouble is that MonadPlus doesn't provide any mechanism for finding out > what went wrong, so I've only ever found it useful when using the Maybe > monad (which itself has no way of identifying what went wrong).
MonadPlus is a crude way of doing that, but that isn't the motivating example of MonadPlus, indeed, that arguably shouldn't be a MonadPlus instance. Cale Gibbard suggests (perhaps not originally) splitting MonadPlus into MonadPlus (and MonadZero) and MonadOrElse. They'd have essentially the same operations, but would be intended to satisfy different laws. Maybe, Either e, IO are better instances of MonadOrElse while [], Parser, LogicT, other non-determinism, parser, or concurrency monads would more appropriately be MonadPlus. _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
