https://gist.github.com/cojna/5857876 の答え的なもの． main関数をたくさん書いているので，このファイルはコンパイルできません．

AplusB.hs

import Control.Applicative ((<$>), (<*>), liftA2)
import Control.Monad (forM, replicateM)
import Data.Function (on)

main = do
  x <- getLine
  y <- getLine
  print $ read x + read y

main = do
  x <- fmap read getLine
  y <- read <$> getLine
  print $ x + y

main = do
  x <- readLn
  y <- readLn
  print $ x + y

main = do
  res <- (+) <$> readLn <*> readLn
  print res

main = do
  res <- liftA2 (+) readLn readLn
  print res

main = do
  f <- (+) <$> readLn
  x <- readLn
  return $ f x

main = do
  res <- ((+) `on` read) <$> getLine <*> getLine
  print res

main = do
  xs <- forM [1..2] $ \_ -> do
    x <- getLine
    return $ read x
  print $ sum xs

main = do
  xs <- replicateM 2 readLn
  print $ foldl (+) 0 xs

main = do
  [x, y] <- sequence [readLn, readLn]
  print $ x + y

main = do
  [x, y] <- map read . words <$> getContents
  print $ x + y

main=interact$show.sum.map read.words

main = (+) <$> readLn <*> readLn >>= print

main = replicateM 2 readLn >>= print . sum

main = getContents >>= print . foldr ((+) . read) 0 . words