content: --------------------------------------------------------------------------------
  -- MIT License
  --
  -- Copyright (C) 2023 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 second element of a list.
  secondA :: [a] -> a
  secondA []       = error "Empty list"
  secondA [x]      = error "Singleton"
  secondA (x:y:xs) = y
  
  -- | The second element of a list.
  secondB :: [a] -> a
  secondB []      = error "Empty list"
  secondB [x]     = error "Singleton"
  secondB (_:x:_) = x
  
  -- | Various implementations for extracting the second element of a list.
  main = do
      let a = 2 :: Integer
      let b = 3 :: Integer
      let c = 4 :: Integer
      -- First version.
      printf "[%d, %d] -> %d\n" a b $ secondA [a, b]
      printf "[%d, %d] -> %d\n" b c $ secondA [b, c]
      printf "[%d, %d, %d] -> %d\n" b a c $ secondA [b, a, c]
      printf "[%d, %d, %d, %d] -> %d\n" b (-a) c (-b) $ secondA [b, -a, c, -b]
      -- Second version.
      printf "[%d, %d] -> %d\n" a b $ secondB [a, b]
      printf "[%d, %d] -> %d\n" b c $ secondB [b, c]
      printf "[%d, %d, %d] -> %d\n" b a c $ secondB [b, a, c]
      printf "[%d, %d, %d, %d] -> %d\n" b (-a) c (-b) $ secondB [b, -a, c, -b]
  