In computer programming, an anonymous function (function literal, lambda abstraction, lambda function, lambda expression or block) is a function definition that is not bound to an identifier. Anonymous functions are often arguments being passed to higher-order functions or used for constructing the result of a higher-order function that needs to return a function.^[1] If the function is only used once, or a limited number of times, an anonymous function may be syntactically lighter than using a named function. Anonymous functions are ubiquitous in functional programming languages and other languages with first-class functions, where they fulfil the same role for the function type as literals do for other data types.

Anonymous functions originate in the work of Alonzo Church in his invention of the lambda calculus, in which all functions are anonymous, in 1936, before electronic computers.^[2] In several programming languages, anonymous functions are introduced using the keyword lambda, and anonymous functions are often referred to as lambdas or lambda abstractions. Anonymous functions have been a feature of programming languages since Lisp in 1958, and a growing number of modern programming languages support anonymous functions.

Names

The names "lambda abstraction", "lambda function", and "lambda expression" refer to the notation of function abstraction in lambda calculus, where the usual function f(x) = M would be written (λx.M), and where M is an expression that uses x. Compare to the Python syntax of lambda x: M.

The name "arrow function" refers to the mathematical "maps to" symbol, x ↦ M. Compare to the JavaScript syntax of x => M.^[3]

Uses

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Anonymous functions can be used for containing functionality that need not be named and possibly for short-term use. Some notable examples include closures and currying.

The use of anonymous functions is a matter of style. Using them is never the only way to solve a problem; each anonymous function could instead be defined as a named function and called by name. Some programmers use anonymous functions to encapsulate specific, non-reusable code without littering the code with a lot of little one-line normal functions.

In some programming languages, anonymous functions are commonly implemented for very specific purposes such as binding events to callbacks or instantiating the function for particular values, which may be more efficient, more readable, and less error-prone than calling a more-generic named function.

The following examples are written in Python 3.

Sorting

When attempting to sort in a non-standard way, it may be easier to contain the sorting logic as an anonymous function instead of creating a named function. Most languages provide a generic sort function that implements a sort algorithm that will sort arbitrary objects. This function usually accepts an arbitrary function that determines how to compare whether two elements are equal or if one is greater or less than the other.

Consider this Python code sorting a list of strings by length of the string:

>>> a = 
>>> a.sort(key=lambda x: len(x))
>>> a

The anonymous function in this example is the lambda expression:

lambda x: len(x)

The anonymous function accepts one argument, x, and returns the length of its argument, which is then used by the sort() method as the criteria for sorting.

Basic syntax of a lambda function in Python is

lambda arg1, arg2, arg3, ...: <operation on the arguments returning a value>

The expression returned by the lambda function can be assigned to a variable and used in the code at multiple places.

>>> add = lambda a: a + a
>>> add(20)
40

Another example would be sorting items in a list by the name of their class (in Python, everything has a class):

>>> a = 
>>> a.sort(key=lambda x: x.__class__.__name__)
>>> a

Note that 11.2 has class name "float", 10 has class name "int", and 'number' has class name "str". The sorted order is "float", "int", then "str".

Closures

Closures are functions evaluated in an environment containing bound variables. The following example binds the variable "threshold" in an anonymous function that compares the input to the threshold.

def comp(threshold):
    return lambda x: x < threshold

This can be used as a sort of generator of comparison functions:

>>> func_a = comp(10)
>>> func_b = comp(20)

>>> print(func_a(5), func_a(8), func_a(13), func_a(21))
True True False False

>>> print(func_b(5), func_b(8), func_b(13), func_b(21))
True True True False
Zdroj:https://en.wikipedia.org?pojem=Anonymous_function
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