Re: [Haskell-cafe] Combining computations

On Sun, May 3, 2009 at 4:41 AM, Luke Palmer wrote:

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

On Sat, May 2, 2009 at 9:26 PM, michael rice 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

_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe