haskell中的自定义concat（++）运算符 [英] Custom concat (++) operator in haskell

问题描述

是否可以在Haskell中为自定义数据类型定义自己的 ++ 运算符？

Is it possible to define my own ++ operator for a custom data type in Haskell?

I有：

I have:

data MyType = MyType [String]

，我想将自己的连接运算符定义为：

and I would like to define my own concatenation operator as:

instance ? MyType where
    (MyType x) ++ (MyType y) = MyType (x ++ y)

I can't seem to find the name of the instance class anywhere.

推荐答案

如果您不坚持要求，调用操作符（++），

If you don't insist on calling the operator (++),

import Data.Monoid

instance Monoid MyType where
    (MyType x) `mappend` (MyType y) = MyType (x ++ y)
    mempty = MyType []

然后您可以使用

Then you can use

(<>) :: Monoid m => m -> m -> m

这是 mappend 的别名（我认为它已经是一个类型成员，但它不是：/）。列出了一个 Monoid 实例，其中 mappend 是（++），这样会做你想做的事。 Monoid 实例也为您提供

which is an alias for mappend (I thought it was already a type class member, but it isn't :/). Lists hava a Monoid instance where mappend is (++), so that would do what you desire. The Monoid instance also gives you

mconcat :: Monoid m => [m] -> m

您可以使用它连接 MyType s。

which you can use to concatenate a list of MyTypes.

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