The start of a solution to the tower of hanoi.

toh.hs

import System.Environment
import qualified Data.Bifunctor as BF (first)
import Text.Read (readEither)

numberOfRings :: [String] -> Either String Int
numberOfRings [] = Left "You didn't give me the number of rings!"
numberOfRings [n] = BF.first intError $ readEither n
  where intError _ = "That's not a valid integer..."
numberOfRings _ = Left "What's all this?"

moves :: Either String Int -> String
moves (Left s) = s
moves (Right n) = "Woo!"

main = do
  args <- getArgs
  putStrLn . moves $ numberOfRings args