mplus requires both arguments to be in the same monad (the same type, even). Fortunately, the empty list behaves like Nothing, and a singleton list behaves like Just. So convert the Maybe before composing, using:
maybeToList Nothing = []
maybeToList (Just x) = [x]
(The maybeToList function can be found in Data.Maybe)
Keep in mind that this will give you:
Just 1 `mplus` [2,3,4] ==> [1,2,3,4]
Silly me:
maybeToList (Just 1) `mplus` [2,3,4] ==> [1,2,3,4]
Which may not be what you want...
Luke
I posted something similar about an hour ago but it seems to have gotten lost. Very strange.
I've read that Monads can combine computations. Can a Maybe monad be combined with a List monad such that
Nothing `mplus` [] ==> []
Just 1 `mplus` [] ==> [1]
If not, can someone supply a simple example of combining computations?
Michael
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