什么是lambda(函数)? [英] What is a lambda (function)?

问题描述

对于没有计算机科学背景的人来说，计算机科学世界中的lambda是什么?

For a person without a comp-sci background, what is a lambda in the world of Computer Science?

推荐答案

Lambda来自 Lambda微积分并指代编程中的匿名函数.

Lambda comes from the Lambda Calculus and refers to anonymous functions in programming.

为什么这个很酷?它允许您编写快速抛出的函数而无需命名它们.它还提供了一种编写闭包的好方法.有了这种能力，您就可以做这样的事情.

Why is this cool? It allows you to write quick throw away functions without naming them. It also provides a nice way to write closures. With that power you can do things like this.

Python

def adder(x):
    return lambda y: x + y
add5 = adder(5)
add5(1)
6

从Python片段中可以看到，函数加法器接受一个参数x，并返回一个带另一个参数y的匿名函数或lambda.该匿名函数使您可以从函数创建函数.这是一个简单的示例，但它应该传达lambda和闭包所具有的功能.

As you can see from the snippet of Python, the function adder takes in an argument x, and returns an anonymous function, or lambda, that takes another argument y. That anonymous function allows you to create functions from functions. This is a simple example, but it should convey the power lambdas and closures have.

其他语言的示例

Perl 5

sub adder {
    my ($x) = @_;
    return sub {
        my ($y) = @_;
        $x + $y
    }
}

my $add5 = adder(5);
print &$add5(1) == 6 ? "ok\n" : "not ok\n";

JavaScript

var adder = function (x) {
    return function (y) {
        return x + y;
    };
};
add5 = adder(5);
add5(1) == 6

JavaScript(ES6)

const adder = x => y => x + y;
add5 = adder(5);
add5(1) == 6

方案

(define adder
    (lambda (x)
        (lambda (y)
           (+ x y))))
(define add5
    (adder 5))
(add5 1)
6

C#3.5或更高版本

C# 3.5 or higher

Func<int, Func<int, int>> adder = 
    (int x) => (int y) => x + y; // `int` declarations optional
Func<int, int> add5 = adder(5);
var add6 = adder(6); // Using implicit typing
Debug.Assert(add5(1) == 6);
Debug.Assert(add6(-1) == 5);

// Closure example
int yEnclosed = 1;
Func<int, int> addWithClosure = 
    (x) => x + yEnclosed;
Debug.Assert(addWithClosure(2) == 3);

快速

func adder(x: Int) -> (Int) -> Int{
   return { y in x + y }
}
let add5 = adder(5)
add5(1)
6

PHP

$a = 1;
$b = 2;

$lambda = fn () => $a + $b;

echo $lambda();

Haskell

(\x y -> x + y) 

Java (请参阅这篇文章

// The following is an example of Predicate : 
// a functional interface that takes an argument 
// and returns a boolean primitive type.

Predicate<Integer> pred = x -> x % 2 == 0; // Tests if the parameter is even.
boolean result = pred.test(4); // true

卢阿

adder = function(x)
    return function(y)
        return x + y
    end
end
add5 = adder(5)
add5(1) == 6        -- true

科特林

val pred = { x: Int -> x % 2 == 0 }
val result = pred(4) // true

Ruby

Ruby与稍有不同，因为您不能使用与调用函数完全相同的语法来调用lambda，但是它仍然具有lambda.

Ruby is slightly different in that you cannot call a lambda using the exact same syntax as calling a function, but it still has lambdas.

def adder(x)
  lambda { |y| x + y }
end
add5 = adder(5)
add5[1] == 6

Ruby是Ruby，它是lambda的简写形式，因此您可以通过以下方式定义adder:

Ruby being Ruby, there is a shorthand for lambdas, so you can define adder this way:

def adder(x)
  -> y { x + y }
end

R

adder <- function(x) {
  function(y) x + y
}
add5 <- adder(5)
add5(1)
#> [1] 6

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