可変長引数の実験

punpukapuun.hs

{-# LANGUAGE FlexibleInstances #-}

class PType t where
  hoge :: [Int] -> t

instance PType Int where
  hoge = sum

instance PType r => PType (Int -> r) where
  hoge xs = (\x -> hoge $ x:xs)

total :: PType r => r
total = hoge []

main = do
  print $ (total :: Int)
  print $ (total (1 :: Int) :: Int)
  print $ (total (1 :: Int) (2 :: Int) :: Int)
  print $ (total (1 :: Int) (2 :: Int) (10 :: Int) :: Int)
  print $ (total (1 :: Int) (2 :: Int) (10 :: Int) (100 :: Int) :: Int)