[Haskell-cafe] MonadPlus

[Andrew Coppin]

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?

[In a somewhat unrelated question... I saw some code the other day that 
used Either as if it were a monad. And yet, I don't see an instance 
given in the standard libraries - even though there should be one. I can 
see Functor (Either a), but not Monad (Either a) or even Monad (Either 
String)...]