Haskell Type Class Study

TypeClassTest.hs

{-# LANGUAGE FlexibleInstances #-}

main = do
    print $ mySum ([1, 2, 3] :: [Int])
    print $ mySum (["hoge", "piyo", "huga"] :: [String])

class AddMonoid a where
    unit :: a
    add :: a -> a -> a

instance AddMonoid Int where
    unit = 0
    add x y = x + y

instance AddMonoid [Char] where
    unit = ""
    add x y = x ++ y

mySum :: AddMonoid a => [a] -> a
mySum [] = unit
mySum (x: xs) = add x (mySum xs)