paulkoerbitz

data Lit = Lit Int
data Add l r = Add l r

class Eval x where
  eval :: x -> Int

instance Eval Lit where
  eval (Lit x) = x

instance (Eval l, Eval r) => Eval (Add l r) where

Javran

import Control.Monad
import System.FilePath
import System.Directory
import Data.Maybe

redoPath :: FilePath -> IO (Maybe FilePath)
redoPath target = listToMaybe `liftM` filterM doesFileExist candidates
    where candidates = map (++ ".do") $ target : fallbacks
          fallbacks = map (`replaceBaseName` "default") $ filter hasExtension [target]