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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  --------------------------------------------------------------------------------
  
  import Text.Printf
  
  -- | Meaning of each traffic light.
  trafficg :: String -> String
  trafficg light
      | light == "red" = "stop"
      | light == "orange" = "wait"
      | light == "green" = "go"
      | otherwise = "Invalid traffic light"
  
  -- | Meaning of each traffic light.
  trafficp :: String -> String
  trafficp "red"    = "stop"
  trafficp "orange" = "wait"
  trafficp "green"  = "go"
  trafficp _        = "Invalid traffic light"
  
  -- | Guarded equation versus pattern matching.
  main = do
      let a = "red"
      let b = "orange"
      let c = "green"
      let d = "purple"
      -- Guarded equation.
      printf "%s -> %s\n" a $ trafficg a
      printf "%s -> %s\n" b $ trafficg b
      printf "%s -> %s\n" c $ trafficg c
      printf "%s -> %s\n" d $ trafficg d
      -- Pattern matching.
      printf "%s -> %s\n" a $ trafficp a
      printf "%s -> %s\n" b $ trafficp b
      printf "%s -> %s\n" c $ trafficp c
      printf "%s -> %s\n" d $ trafficp d
  