import Control.Monad
import Control.Monad.Identity
import Control.Monad.Trans.Maybe

import System.Environment

tryR :: Monad m => ([a] -> MaybeT m [a]) -> ([a] -> m [a])
tryR f x = do
  m <- runMaybeT (f x)
  case m of
    Just t -> return t
    Nothing -> return x

check :: MonadPlus m => Int -> m Int
check x = if x `mod` 2 == 0 then return (x `div` 2) else mzero

foo :: MonadPlus m => [Int] -> m [Int]
foo [] = return []
foo (x:xs) = liftM2 (:) (check x) (tryR foo xs)


runFoo :: [Int] -> [Int]
runFoo x = runIdentity $ tryR foo x

import Data.Time
bogoBench :: [a] -> IO NominalDiffTime
bogoBench x = do
   t₀<-getCurrentTime
   last (reverse x) `seq` (fmap (`diffUTCTime`t₀) getCurrentTime)

{-# LANGUAGE TupleSections #-}
timings <- mapM (\n -> (n,) <$> bogoBench (runFoo [2,4..n])) [2,4 .. 50]

import Graphics.Rendering.Chart.Easy
toRenderable . plot
    $ line "t" [[(fromIntegral n, realToFrac t) :: (Double, LogValue) | (n,t) <- timings]]