Finder Monoid

finder.hs

import Data.Monoid
import Data.Maybe
import Data.List


data Finder a = Finder ([a] -> Maybe a)


instance Monoid (Finder a) where
  mempty = Finder (const Nothing)
  (Finder f1) `mappend` (Finder f2) = Finder (\xs -> if isNothing (f1 xs) then f2 xs else f1 xs)


useFinder :: Finder a -> [a] -> Maybe a
useFinder (Finder f) = f


makeFinder :: (a -> Bool) -> Finder a
makeFinder = Finder . find


-- Example

finders = map makeFinder [(<1), (==7), (>8)]
finder = mconcat finders

xs = [2, 3, 4, 6, 9]

-- useFinder finder xs
-- => Just 9