Haskell State Monad solution to https://github.com/tonymorris/happy-numbers

Happy.hs

module Happy where

import Control.Monad
import Control.Monad.State -- Brian

happy ::
  Int
  -> Bool
happy =
  -- Start of Brian's code
  (\n -> evalState (happy' n) [])

happy' n = do
  state <- get
  let answer = sum $ map (^ 2) $ toDigits n
  if answer == 1
    then return True
    else if answer `elem` state
         then return False
         else do
           put $ answer:state
           happy' answer

toDigits = (map (readDigit . (:[]))) . show

readDigit :: String -> Int
readDigit = read

-- End of Brian's code

main :: 
  IO ()
main =
  let happys = [1, 7, 10, 13, 19, 23, 28, 31, 32, 44, 49, 68, 70, 79, 82, 86, 91, 94, 97, 100, 103, 109, 129, 130, 133, 139, 167, 176, 188, 190, 192, 193, 203, 208, 219, 226, 230, 236, 239, 262, 263, 280, 291, 293, 301, 302, 310, 313, 319, 320, 326, 329, 331, 338, 356, 362, 365, 367, 368, 376, 379, 383, 386, 391, 392, 397, 404, 409, 440, 446, 464, 469, 478, 487, 490, 496]
  in mapM_ (\(i, p) -> let passes = p == (i `elem` happys)
                       in unless passes $ putStrLn ("happy fails for: " ++ show i)) $ map (\n -> (n, happy n)) [1..500]