Base64.hs

import Data.Bits
import Data.Word

base64Chars = zip [(0::Word8)..] "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/"
getFst ((b,c):xs) a = if a == b then c else getFst xs a
getBase64Char = getFst base64Chars

encodeTriple :: Word8 -> Word8 -> Word8 -> String
encodeTriple a b c =
  map getBase64Char $ zipWith (.|.)
    (map transform [(a, -2, 0x3f), (a,  4, 0x30), (b,  2, 0x3c), (c, 0, 0x3f)])
    (map transform [(0,  0,    0), (b, -4, 0x0f), (c, -6, 0x03), (0, 0,    0)])
  where
    transform (a, s, m) = a `shift` s .&. m

bytesToBase64 :: [Word8] -> String
bytesToBase64 [] = []
bytesToBase64 [a] = take 2 (encodeTriple a 0 0) ++ "=="
bytesToBase64 [a, b] = take 3 (encodeTriple a b 0) ++ "="
bytesToBase64 (a:b:c:xs) = encodeTriple a b c ++ bytesToBase64 xs