如何阅读以下的haskell定义“ Num（a→b）=> [a] - > [b]“ [英] How to read the following haskell definition " Num (a -> b) => [a] -> [b] "

问题描述

我试图理解下面的类型

  Prelude> ：t map（10 $）
 map（10 $）:: Num（a  - > b）=> [a]  - > [b] 
  

解决方案

例如 Num（a-> b）？可以更一般地回答：如何理解 Num SomeCompoundType ？

非常简单，这种约束意味着您尝试将某个类型视为数字类型，尽管它不是数字类型。这种类型只是假的，尽管它不是一个类型错误（理论上，有人可以添加一个 Num（a - > b）例如）。

特别是，<数字$ c>数字，视为数字通常意味着您写了一个数字字面在某个地方，例如预计会发挥功能。 （10 $）部分希望 10 是一个函数。由于Haskell数字文字是多态的，本质上没有什么不妥： 10 could 是一个函数，只要它具有合适的 instance Num（a - > b）。只是，这个实例不存在，但是因为有些人原则上可以在以后添加它，所以编译器将此视为一个约束条件。

I am trying to understand below type

Prelude> :t map(10$)
map(10$) :: Num (a -> b) => [a] -> [b]

解决方案

How to understand a constraint like Num (a->b)? can be answered more generally: how to understand Num SomeCompoundType?

Quite simple, such a constraint means that you tried to treat a type as a number type though it is not a number type. Such a type is just bogus, though it is not a type error (and theoretically, somebody could add a Num (a -> b) instance).

With Num in particular, "treated as a number" usually just means that you wrote a number literal in a place where e.g. a function is expected. The section (10$) wants 10 to be a function. Because Haskell number literals are polymorphic, there is per se nothing wrong with that: 10 could be a function if only it had the suitable instance Num (a -> b). Just, this instance does not exist, but because somebody could in principle yet add it later, the compiler leaves this as a constraint-to-be-fulfilled.

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