First-class function

Wikipedia First-class function

In computer science, a programming language is said to have first-class functions if it treats functions as first-class citizens. Specifically, this means the language supports passing functions as arguments to other functions, returning them as the values from other functions, and assigning them to variables or storing them in data structures.[1] Some programming language theorists require support for anonymous functions (function literals) as well.[2]

In languages with first-class functions, the names of functions do not have any special status; they are treated like ordinary variables with a function type.[3] The term was coined by Christopher Strachey in the context of "functions as first-class citizens" in the mid-1960s.[4]

First-class functions are a necessity for the functional programming style, in which the use of higher-order functions is a standard practice.

NOTE: first-class function是functional programming的必要条件

There are certain implementation difficulties in passing functions as arguments and 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".[5]

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