Re: [Haskell-beginners] Selecting single result of function application to list

On 11/03/2011 04:26 PM, Michael Xavier wrote:

Here's how I'd do it:

import Data.Maybe (catMaybes)

list = ["hi", "blah", "foo"]

firstJust = head . catMaybes

selectOne f = firstJust . map f

myFunction :: String -> Maybe Int myFunction = undefined

main = print $ selectOne myFunction list

catMaybes will take a list of Maybe a and reduce it to a list of a, throwing out all the Nothings.

As you'll learn from working with Maybe a lot, if you're casing off of a maybe value, there's probably a better way to do it. Functions like "catMaybes" and "maybe" and especially the Monad instance of Maybe are really helpful for avoiding this ugly branching logic.

Thanks Michael. I am planning to compose functions that return Maybe so I will, as you pointed out, be looking at the Maybe Monad. Rgards, Hugo F.

On Thu, Nov 3, 2011 at 9:07 AM, Hugo Ferreira mailto:hmf@inescporto.pt> wrote:

I am considering something like:

selectOne f = take 1 . filter (\e -> case e of Just _ -> True _ -> False ) . map f

-- Michael Xavier http://www.michaelxavier.net LinkedIn http://www.linkedin.com/pub/michael-xavier/13/b02/a26