头等函数

函数被当作头等公民。这意味着，函数可以作为别的函数的参数、函数的返回值，赋值给变量或存储在数据结构中。

have first-class functions if it treats functions as first-class citizens
- supports passing functions as arguments to other functions
- returning them as the values from other functions
- assigning them to variables or storing them in data structures
There are certain implementation difficulties in passing functions as arguments or returning them as results, especially in the presence of non-local variables introduced in nested and anonymous functions
- Historically, these were termed the funarg problems, the name coming from “function argument”

Concepts

Higher-order functions: passing functions as arguments
Anonymous and nested functions
Non-local variables and closures | 非局部变量和闭包
Higher-order functions: returning functions as results
Assigning functions to variables
Equality of functions
- Extensional equality | 外延相等
- Intensional equality | 内涵相等
- Reference equality | 引用相等