haskell - 一个基本的 Monoid 定义给出 "No instance for (Semigroup MyMonoid) arising from the superclasses of an instance declaration"

Question

我正在尝试使用联合操作将 Haskell 整数集定义为 Monoid .

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)

我得到错误

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

我究竟做错了什么？这看起来很简单，我正在复制我在 this 等示例中看到的语法。 ，但我不明白为什么会发生此错误。

Answer 1

自从那次旅行写完后，(<>)已从 Monoid 移至 Semigroup，并且所有 Monoid 实例也必须是 Semigroup。 mappend只是 (<>) 的同义词.因此，您需要两个实例:

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

instance Monoid MyMonoid where
  mempty = MyMonoid S.empty