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
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  -- OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
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  --------------------------------------------------------------------------------
  
  import Text.Printf
  
  -- | Summing a list of numbers.
  add :: Num a => [a] -> a
  add []     = 0
  add (x:xs) = x + add xs
  
  -- | Summing a list of numbers.  Use the fold operation.
  addf :: Num a => [a] -> a
  addf xs = foldl (+) 0 xs
  
  -- | Add together all numbers in a list.
  main = do
      let a = [] :: [Int]
      let b = [1, 2, 3] :: [Integer]
      let c = [3, 5, 7, 11, 13] :: [Float]
      let d = [1,4 .. 100] :: [Double]
      let testa = sum a == add a && sum a == addf a
      let testb = sum b == add b && sum b == addf b
      let testc = sum c == add c && sum c == addf c
      let testd = sum d == add d && sum d == addf d
      printf "%s: %s\n" (show $ add a) $ show testa
      printf "%s: %s\n" (show $ add b) $ show testb
      printf "%s: %s\n" (show $ add c) $ show testc
      printf "%s: %s\n" (show $ add d) $ show testd
  