MonadPlus(Monad + Monoid) http://d.hatena.ne.jp/sirocco/20110415/1302856463

monadplus.hs

import Control.Monad

-- list
:{
  do
    x <- [1,2,3] `mplus` [4,5,6]
    return x -- [1, 2, 3, 4, 5, 6]
:}

:{
  do
    x <- Nothing `mplus` Just 2
    return x -- Just 2
:}

-- リストをなぞって条件に一致した値をMonadPlusで返す。
:{
let {;
mLsearch :: MonadPlus m => (Int -> Bool) -> [Int] -> m Int;
mLsearch _ [] = mzero;
mLsearch isCond (x:xs)
  | isCond x  = return x `mplus` mLsearch isCond xs
  | otherwise = mLsearch isCond xs
}
:}

-- リストで返す。
mLsearch (==10) [0..20]  :: [Int]       -- => [10]
mLsearch (>10)  [0..20]  :: [Int]       -- => [11,12,13,14,15,16,17,18,19,20]
mLsearch (==99) [0..20]  :: [Int]       -- => []

-- Maybeで返す。
mLsearch (==10)  [0..20]  :: Maybe Int  -- => Just 10
mLsearch (==100) [0..20]  :: Maybe Int  -- => Nothing
mLsearch (>100)  [0..20]  :: Maybe Int  -- => Nothing