什么是头等公民职能？ [英] What is a first class citizen function?

问题描述

什么是头等公民职能？

Java是否支持头等公民职能？

Does Java supports first class citizen function?

编辑：

如 Wikepedia 所述

函数编程风格的第一类函数是必需的
。

First class functions are a necessity for the functional programming style.

第一类函数还有其他用途吗？

Is there any other use of first class functions?

推荐答案

考虑的语言程序为一流 允许函数像任何其他值一样传递 。

A language that considers procedures to be "first-class" allows functions to be passed around just like any other value.

语言像Java 7（及更早版本）和C种类具有这种能力：C允许传递函数指针，但是你不能动态地定义这些语言中的函数并突然传递到其他地方。版本8之前的Java可以在某种程度上使用匿名类模拟，但它在技术上并不具有一流的功能。

Languages like Java 7 (and earlier) and C "kind of" have this capability: C allows function pointers to be passed around, but you can't dynamically define a function in those languages and suddenly pass that somewhere else. Java before version 8 can simulate this to a certain extent with anonymous classes, but it doesn't technically have first-class functions.

On另一方面，C ++，D，C＃，Visual Basic .NET，Java 8+和函数式语言（如Scheme和Haskell） do 允许您传递变量等函数。例如，下面的代码返回一个函数，该函数将 addend 添加到其输入中：

On the other hand, C++, D, C#, Visual Basic .NET, Java 8+, and functional languages (like Scheme and Haskell) do allow you to pass around functions like variables. For example, the code below returns a function that adds addend to its input:

写在D：

int delegate(int) makeAdder(int addend) //Returns a function
{
    return delegate int(int x) //Long way
    {
        return x + addend; //Notice that addend came from _outside_ the function
    };

    return (int x) { return x + addend; }; //Short way

    return x => addend + x; //Super-short way, introduced in D 2.058
}

用C＃编写：

Func<int, int> MakeAdder(int addend) //Returns a function
{
    return delegate(int x) //The long way. Note: Return type is implicitly 'int'
    {
        return x + addend;
    };

    return x => x + addend; //Short way: x "goes to" (x + addend); inferred types
}

用C ++编写：

#include <functional>

std::function<int(int)> make_adder(int addend)
{
    return [=](int x)
    {
        return addend + x;
    };
}

用Scala写的：

def makeAdder(addend: Int) = (x: Int) => addend + x

用Python编写：

def make_adder(addend):
    def f(x):
        return addend + x
    return f
    # or...
    return lambda x: addend + x

用Erlang编写：

make_adder(Addend) ->
    fun(X) -> Addend + X end.

用JavaScript编写：

Written in JavaScript:

function makeAdder(addend) {
    return function(x) {
        return addend + x;
    };
}

用JavaScript编写（ES2015箭头函数语法）：

Written in JavaScript (ES2015 arrow function syntax):

const makeAdder = addend => x => addend + x;

写在Scheme中：

(define (makeAdder addend)
  (lambda (x)
    (+ x addend)))

用Haskell编写：

Written in Haskell:

makeAdder :: Int -> (Int -> Int)
makeAdder addend = \x -> addend + x

用Visual Basic 2008编写：

Written in Visual Basic 2008:

Function MakeAdder(addend As Integer) As Func(Of Integer, Integer)
    Return Function(x) (x + addend)
End Function

用Swift编写（详细和短手实现）：

Written in Swift (both verbose and short-hand implementations):

func makeAdder(append: Int) -> (x: Int) -> Int {
    return { (x: Int) -> Int in
        return x + append
    };
}

func makeAdder(append: Int) -> (Int) -> Int {
    return {$0 + append};
}

（顺便说一句，lambda只是一个没有名字的函数Lambda只支持支持一流功能的语言。）

(By the way, a "lambda" is just a function without a name. Lambdas are only supported in languages that support first-class functions.)

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