presentation.hs

import Prelude hiding (elem, map)

elem :: Eq a => a -> [a] -> Bool
elem _ []       = False
elem x (y:ys)
    | x == y    = True
    | otherwise = elem x ys

factorial :: Int -> Int
factorial 1 = 1
factorial x = x * factorial(x-1)

factorial' :: Int -> Int
factorial' x = foldr (*) 1 [1..x]

factorial'' :: Int -> Int
factorial'' x = if x == 1
    then 1
    else x * factorial'' (x-1)

map :: (a -> a) -> [a] -> [a]
map _ [] = []
map f (x:xs) = f x : map f xs

-- Quicksort
quicksort :: Ord a => [a] -> [a]
quicksort [] = []
quicksort (x:xs) = quicksort small ++ [x] ++ quicksort large
    where small = [y | y <- xs, y <= x]
          large = [y | y <- xs, y > x]

-- Reverse Words
reverseWords :: String -> String
reverseWords = unwords . map reverse . words

-- Cesar Cipher
import Data.Char

encode :: Int -> String -> String
encode shift msg =
    let ords = map ord msg
        shifted = map (+ shift) ords
    in  map chr shifted

encode' :: Int -> String -> String
encode' shift msg = map (chr . (+ shift) . ord) msg

decode :: Int -> String -> String
decode shift msg =
    let ords = map ord msg
        shifted = map (+(negate shift)) ords
    in  map chr shifted

decode'  :: Int -> String -> String
decode'  shift msg = map (chr . (+(negate shift)) . ord) msg

decode'' :: Int -> String -> String
decode'' shift msg = encode (negate shift) msg

-- Python
-- chr(ord('a')+3)