WriterMonad example

WriterMonad.hs

import Control.Monad.Writer

-- | Starts a value off.
-- This function doesn't perform any calculation at all, it just prepares an
-- initial value to start in the calculation pipeline
--
start :: Int -> Writer [String] Int
start x = do
  tell (["Starting with " ++ show x])
  return x

-- | Halve a value
-- Any value passed into this function gets halved
--
half :: Int -> Writer [String] Int
half x = do
  tell (["Halving " ++ show x])
  return (x `div` 2)

-- | Squares a value
-- Any value passed into this function gets squared
--
sqr :: Int -> Writer [String] Int
sqr x = do
  tell (["Squaring " ++ show x])
  return (x * x)

main :: IO ()
main = do
  let work = runWriter $ start 10 >>= half >>= sqr >>= half
  let ans  = fst work
  let log  = snd work

  putStrLn $ "Answer: " ++ show ans
  putStrLn ""
  putStrLn " ==== Log ==== "

  mapM_ putStrLn log