Haskell类型问题

Question

我写了代码：

{-# LANGUAGE InstanceSigs #-}
{-# LANGUAGE FlexibleInstances #-}

module MonoidApp where

class Monoid' a where 
    mempty'  :: a
    mappend' :: a -> a -> a

    mconcat' :: [a] -> a
    mconcat' = foldr mappend' mempty'

instance Monoid' Int where 
    mempty' :: Int a => a
    mempty' = 0

    mappend' :: Int a => a -> a -> a
    mappend' a b = (+) a b

但它遇到了错误：

‘Int’ is applied to too many type arguments
    In the type signature for ‘mempty'’: mempty' :: Int a => a
    In the instance declaration for ‘Monoid' Int’
Failed, modules loaded: none.

任何想法为什么？

Answer 1

应该是：

instance Monoid' Int where
    mempty' :: Int
    mempty' = 0

    mappend' :: Int -> Int -> Int
    mappend' a b = (+) a b