content: --------------------------------------------------------------------------------
  -- MIT License
  --
  -- Copyright (C) 2024 Duck McSouls <quacksouls [AT] gmail [DOT] com>
  --
  -- Permission is hereby granted, free of charge, to any person obtaining a copy
  -- of this software and associated documentation files (the "Software"), to deal
  -- in the Software without restriction, including without limitation the rights
  -- to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
  -- copies of the Software, and to permit persons to whom the Software is
  -- furnished to do so, subject to the following conditions:
  --
  -- The above copyright notice and this permission notice shall be included in
  -- all copies or substantial portions of the Software.
  --
  -- THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
  -- IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
  -- FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
  -- AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
  -- LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
  -- OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
  -- SOFTWARE.
  --------------------------------------------------------------------------------
  
  import Text.Printf
  
  -- | The first elements in a list of 2-tuples.
  first :: [(a, b)] -> [a]
  first xs = [x | (x, _) <- xs]
  
  -- | The second elements in a list of 2-tuples.
  second :: [(a, b)] -> [b]
  second xs = [y | (_, y) <- xs]
  
  -- | An implementation of the function "zip".
  zipper :: [a] -> [b] -> [(a, b)]
  zipper [] []         = []
  zipper [] _          = []
  zipper _ []          = []
  zipper (x:xs) (y:ys) = (x, y) : zipper xs ys
  
  -- | An implementation of the function "unzip".
  unzipper :: [(a, b)] -> ([a], [b])
  unzipper [] = ([], [])
  unzipper xs = (first xs, second xs)
  
  -- | Implementations of the functions "zip" and "unzip".
  main = do
      let a = [0 .. 9]
      let b = ['a' .. 'j']
      let c = ['Z','Y' .. 'A']
      let zipab = zipper a b
      let zipac = zipper a c
      let zipcb = zipper c b
      printf "%s -> %s\n" (show a) $ show b
      printf "zip -> %s\n" $ show zipab
      printf "unzip -> %s\n" $ show . unzipper $ zipab
      printf "\n%s -> %s\n" (show a) $ show c
      printf "zip -> %s\n" $ show zipac
      printf "unzip -> %s\n" $ show . unzipper $ zipac
      printf "\n%s -> %s\n" (show c) $ show b
      printf "zip -> %s\n" $ show zipcb
      printf "unzip -> %s\n" $ show . unzipper $ zipcb
  