module Main ( main ) where import Data.Foldable (traverse_) import Data.List (intercalate) data Variance = Co | Contra | In deriving (Bounded, Enum) showVariance :: Variance -> String showVariance Co = "O" showVariance Contra = "I" showVariance In = "X" showParameter :: Int -> Variance -> String showParameter n Co = "(a" ++ show n ++ " -> b" ++ show n ++ ")" showParameter n Contra = "(b" ++ show n ++ " -> a" ++ show n ++ ")" showParameter n In = "(a" ++ show n ++ " -> b" ++ show n ++ ") -> " ++ "(b" ++ show n ++ " -> a" ++ show n ++ ")" showType :: [Variance] -> String showType variances = let parameters :: String parameters = intercalate " -> " . fmap (uncurry showParameter) $ [1 ..] `zip` variances arguments :: String -> String arguments t = intercalate " " . fmap ((t ++) . show) $ [1 .. length variances] in parameters ++ " -> f " ++ arguments "a" ++ " -> f " ++ arguments "b" functors :: [String] functors = [ "class Functor" ++ foldMap showVariance variances ++ " f where\n" ++ " map" ++ foldMap showVariance variances ++ " :: " ++ showType variances | count <- [1 .. 4] , variances <- traverse (const [minBound .. maxBound]) [1 .. count] ] main :: IO () main = traverse_ putStrLn functors