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]

Which may not be what you want...

Luke

On Sat, May 2, 2009 at 9:26 PM, michael rice <nowgate@yahoo.com> wrote:
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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