26 Bottles of Functional Programming

Beer.hs

module Beer where

import           Data.Char (toUpper)

main = putStrLn (song [100, 99 .. 0])

song = unlines . map verse

verse x = unlines [firstLine x, secondLine x]

firstLine x = unwords [capIfZero (beerOnTheWall x) ++ ",", beer x, "of beer."]
  where
    capIfZero (c:cs) = if x == 0 then toUpper c : cs else c : cs

secondLine 0 = "Go to the store and buy some more, " ++ beerOnTheWall 99 ++ "!"
secondLine n = unwords ["Take", it, "down and pass it around,", beerOnTheWall (n - 1) ++ "!"]
  where
    it = if n == 1 then "it" else "one"

beer 0 = "no more " ++ bottle 0
beer x = unwords [show x, bottle x]

bottle 1 = "bottle"
bottle _ = "bottles"

beerOnTheWall n = beer n ++ " of beer on the wall"