2016 Update

Task/Caesar-cipher/Haskell/caesar-cipher-2.hs

@@ -0,0 +1,32 @@
+{-# LANGUAGE LambdaCase #-}
+module Main where
+
+import Control.Error (tryRead, tryAt)
+import Control.Monad.Trans (liftIO)
+import Control.Monad.Trans.Except (ExceptT, runExceptT)
+
+import Data.Char
+import System.Exit (die)
+import System.Environment (getArgs)
+
+main :: IO ()
+main = runExceptT parseKey >>= \case
+           Left err -> die err
+           Right k  -> interact $ caesar k
+
+parseKey :: (Read a, Integral a) => ExceptT String IO a
+parseKey = liftIO getArgs >>=
+           flip (tryAt "Not enough arguments") 0 >>=
+           tryRead "Key is not a valid integer"
+
+caesar :: (Integral a) => a -> String -> String
+caesar k = map f
+    where f c = case generalCategory c of
+              LowercaseLetter -> addChar 'a' k c
+              UppercaseLetter -> addChar 'A' k c
+              _               -> c
+
+addChar :: (Integral a) => Char -> a -> Char -> Char
+addChar b o c = chr $ fromIntegral (b' + (c' - b' + o) `mod` 26)
+    where b' = fromIntegral $ ord b
+          c' = fromIntegral $ ord c