A basic Monoid definition gives “No instance for (Semigroup MyMonoid) arising from the superclasses

在定義一個自己的Monoid時haskell報了錯誤：

module MyMonoid where

import qualified Data.IntSet as S

data MyMonoid = MyMonoid S.IntSet

instance Monoid MyMonoid where
  mempty = MyMonoid S.empty
  MyMonoid m1 `mappend` MyMonoid m2 = MyMonoid (S.union m1 m2)

報錯：

• No instance for (Semigroup Markup)
    arising from the superclasses of an instance declaration
• In the instance declaration for ‘Monoid MyMonoid’

解決方法：

(<>)已從Monoid移到Semigroup，並且所有Monoid例項也必須也是Semigroup。mappend只是的同義詞(<>)。因此，需要兩個例項：

instance Semigroup MyMonoid where
  MyMonoid m1 <> MyMonoid m2 = MyMonoid (S.union m1 m2)

instance Monoid MyMonoid where
  mempty = MyMonoid S.empty