I've been thinking about making a data type an instance of MonadPlus.
From the Haddock documentation at haskell.org, I see that any such
instance should satisfy
mzero `mplus` x = x
x `mplus` mzero = x
mzero >>= f = mzero
v >> mzero = mzero
but is that all there is to it? Are there no other requirements for
MonadPlus to make sense?
I also wondered why, once MonadPlus was added to the language, the
definition of ++ wasn't changed to
(++) = MonadPlus
(with the MonadPlus instance for [] defined directly).
Aside from getting msum and guard, is there any point in bothering to
make something an instance of MonadPlus?
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