content: --------------------------------------------------------------------------------
  -- MIT License
  --
  -- Copyright (C) 2024 Aaron Nguyen <mvngu [AT] gmx [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
  
  -- | Double a number.
  double :: Num a => a -> a
  double x = 2 * x
  
  -- | Apply the function "double" two times.
  quadruple = double . double
  
  -- | The successor of a number.
  next :: Num a => a -> a
  next x = x + 1
  
  -- | The successor of the successor.
  afterNext = next . next
  
  -- | Apply a given function twice.
  twice :: (a -> a) -> (a -> a)
  twice f = f . f
  
  -- | Composition of a function with itself.
  main = do
      let [x, y] = [2, 3] :: [Integer]
      let at = (twice double) x
      let bt = twice next y
      let ac = double . double $ x
      let bc = next . next $ y
      let testa = at == ac && at == quadruple x
      let testb = bt == bc && bt == afterNext y
      printf "%d -> %d, %s\n" x at $ show testa
      printf "%d -> %d, %s\n" y bt $ show testb
  