Andrew asked,
...so it's a kind of choice operator? Run all actions until you get to one that succeeds and return the result from that?
The eternal bit of trickiness for Haskell is that type classes group often together things that people don't immediately see as similar - monads probably being the best example. So it's probably best not to immediately try to find an all-encompassing summary of MonadPlus like "a kind of choice operator" but instead concentrate on the definition. The key thing is the line
mplus :: m a -> m a -> m a
You can combine two m a's together with a binary operation to get another m a. Absolutely any binary operation (with an identity) that fits the rather lax laws that come with MonadPlus will do. Depending on context mplus might mean "either/or", or "and", or "this first, or else that" or something entirely different that nobody here has anticipated yet. For each monad m, just try to think about how you might combine a pair of m a's and what might serve as an identity. -- Dan