什么 =>在类型签名中意味着什么? [英] What does => mean in a type signature?

问题描述

我刚刚开始向你学习 Haskell，我在一个没有解释的例子中看到了这个:

tell :: (Show a) =>[a] ->细绳

这是什么意思，特别是 =>?我知道如果我替换 -> 或删除它就行不通，但我真的不明白为什么.

解决方案

这是对类型的约束，意味着 a 应该是类 Show 的一个实例.

参见此处

I just starting Learn you a Haskell, and I saw this used in an example without explanation:

tell :: (Show a) => [a] -> String

What does this mean, in particular the =>? I know it won't work if I substitute -> or delete it, but I don't really understand why.

解决方案

It's a constraint on the type, means a should be an instance of class Show.

see for example here

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