什么=>意味着在一个类型签名？ [英] What does => mean in a type signature?

问题描述

 <$ c 
 
 $ c> tell ::（Show a）=> [a]  - >字符串
  

这是什么意思，特别是 => ？我知道如果我用 - > 替换或者删除它，它就行不通了，但我不明白为什么。

>解决方案

它是对类型的约束，意味着 a 应该是类 Show 。

请参阅此处 a>

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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