Re: [Haskell-beginners] a simple little problem

On Sat, Aug 13, 2011 at 02:47:04PM -0400, Brandon Allbery wrote:

On Sat, Aug 13, 2011 at 14:04, Dennis Raddle wrote:

...

Can someone suggest an elegant way to write the following?

fn :: [Maybe Float] -> Maybe Float

in which, if the input list has all Nothing, then the result is Nothing if the input list has one or more Just x, then the result is Just x (in which the x is picked arbitrarily, could be the first one or last one)

Isn't this just mconcat?

No. The Monoid instance for Maybe defined in Data.Monoid is the one which lifts a Monoid instance on a to a Monoid instance on Maybe a. That is, all the Nothings are discarded and all the Justs combined according to the Monoid instance on a. For example:

mconcat [Nothing, Just "hello", Nothing, Just "world"] Just "helloworld"

The MonadPlus instance for Maybe is what the OP wants (as I think someone else already pointed out), which is sensible, since the Maybe monad models possible failure, and hence the MonadPlus instance models choosing the first success. -Brent