エキゾチックなNumインスタンス

Main.hs

module Main where

newtype IODouble = IODouble { runIODouble :: IO Double }

instance Num IODouble where
  fromInteger x = IODouble $ do
    print x
    pure $ fromInteger x
  (IODouble x) + (IODouble y) = IODouble $ do
    x' <- x
    y' <- y
    putStrLn $ "(+ " <> show x' <> " " <> show y' <>")"
    pure (x' + y')
  (IODouble x) * (IODouble y) = IODouble $ do
    x' <- x
    y' <- y
    putStrLn $ "(* " <> show x' <> " " <> show y' <>")"
    pure (x' * y')
  abs (IODouble x) = IODouble $ do
    x' <- x
    putStrLn $ "(abs" <> show x' <> ")"
    pure (abs x')
  signum (IODouble x) = IODouble $ do
    x' <- x
    putStrLn $ "(signum " <> show x' <> ")"
    pure (signum x')
  negate (IODouble x) = IODouble $ do
    x' <- x
    putStrLn $ "(negate " <> show x' <> ")"
    pure (negate x')

instance Fractional IODouble where
  fromRational x = IODouble $ do
    print x
    pure $ fromRational x
  recip (IODouble x) = IODouble $ do
    x' <- x
    putStrLn $ "(recip " <> show x' <> ")"
    pure (recip x')

main = do
  x <- runIODouble $ 10 / 3
  print x