Question

我正在尝试通过并集运算将Monoid定义为Haskell整数集。

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)

我得到了错误

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

我在做什么错？这似乎很简单，我正在复制在this之类的示例中看到的语法，但是我看不到为什么会发生此错误。

Answer 1

自从编写游览以来，import React from "react"; import ReactDOM from "react-dom"; const Modal = ({ open, toggle }) => { if (open) { return ( <React.Fragment> <h1>Modal</h1> <button onClick={toggle}>Close</button> </React.Fragment> ); } return null; }; class App extends React.Component { state = { open: false }; toggle = () => { this.setState({ open: !this.state.open }); }; render() { return ( <React.Fragment> <button onClick={this.toggle}>Open</button> <Modal open={this.state.open} toggle={this.toggle} /> </React.Fragment> ); } } const rootElement = document.getElementById("root"); ReactDOM.render(<App />, rootElement);已从Monoid移到Semigroup，并且所有Monoid实例也必须是Semigroup。 (<>)只是mappend的同义词。因此，您需要两个实例：

(<>)

基本的Monoid定义给出“没有实例实例声明的超类引起的（Semigroup MyMonoid）实例”

1 个答案: