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In computer programming, an anonymous function (function literal, 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.
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. 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.
Uses
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. Anonymous functions often provide a briefer notation than defining named functions. In languages that do not permit the definition of named functions in local scopes, anonymous functions may provide encapsulation via localized scope, however the code in the body of such anonymous function may not be re-usable, or amenable to separate testing. Short/simple anonymous functions used in expressions may be easier to read and understand than separately defined named functions, though without a descriptive name they may be more difficult to understand.
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 in a Dynamic programming language, more readable, and less error-prone than calling a 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:
The anonymous function in this example is the lambda expression:
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
The expression returned by the lambda function can be assigned to a variable and used in the code at multiple places.
Another example would be sorting items in a list by the name of their class (in Python, everything has a class):
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.
This can be used as a sort of generator of comparison functions:
It would be impractical to create a function for every possible comparison function and may be too inconvenient to keep the threshold around for further use. Regardless of the reason why a closure is used, the anonymous function is the entity that contains the functionality that does the comparing.
= Currying
=
Currying is the process of changing a function so that rather than taking multiple inputs, it takes a single input and returns a function which accepts the second input, and so forth. In this example, a function that performs division by any integer is transformed into one that performs division by a set integer.
While the use of anonymous functions is perhaps not common with currying, it still can be used. In the above example, the function divisor generates functions with a specified divisor. The functions half and third curry the divide function with a fixed divisor.
The divisor function also forms a closure by binding the variable d.
= Higher-order functions
=
A higher-order function is a function that takes a function as an argument or returns one as a result. This is commonly used to customize the behavior of a generically defined function, often a looping construct or recursion scheme. Anonymous functions are a convenient way to specify such function arguments. The following examples are in Python 3.
Map
The map function performs a function call on each element of a list. The following example squares every element in an array with an anonymous function.
The anonymous function accepts an argument and multiplies it by itself (squares it). The above form is discouraged by the creators of the language, who maintain that the form presented below has the same meaning and is more aligned with the philosophy of the language:
Filter
The filter function returns all elements from a list that evaluate True when passed to a certain function.
The anonymous function checks if the argument passed to it is even. The same as with map, the form below is considered more appropriate:
Fold
A fold function runs over all elements in a structure (for lists usually left-to-right, a "left fold", called reduce in Python), accumulating a value as it goes. This can be used to combine all elements of a structure into one value, for example:
This performs
(
(
(
1
×
2
)
×
3
)
×
4
)
×
5
=
120.
{\displaystyle \left(\left(\left(1\times 2\right)\times 3\right)\times 4\right)\times 5=120.}
The anonymous function here is the multiplication of the two arguments.
The result of a fold need not be one value. Instead, both map and filter can be created using fold. In map, the value that is accumulated is a new list, containing the results of applying a function to each element of the original list. In filter, the value that is accumulated is a new list containing only those elements that match the given condition.
List of languages
The following is a list of programming languages that support unnamed anonymous functions fully, or partly as some variant, or not at all.
This table shows some general trends. First, the languages that do not support anonymous functions (C, Pascal, Object Pascal) are all statically typed languages. However, statically typed languages can support anonymous functions. For example, the ML languages are statically typed and fundamentally include anonymous functions, and Delphi, a dialect of Object Pascal, has been extended to support anonymous functions, as has C++ (by the C++11 standard). Second, the languages that treat functions as first-class functions (Dylan, Haskell, JavaScript, Lisp, ML, Perl, Python, Ruby, Scheme) generally have anonymous function support so that functions can be defined and passed around as easily as other data types.
Examples of anonymous functions
See also
First-class function
Lambda calculus definition
References
External links
Anonymous Methods - When Should They Be Used? (blog about anonymous function in Delphi)
Compiling Lambda Expressions: Scala vs. Java 8
php anonymous functions php anonymous functions
Lambda functions in various programming languages
Functions in Go
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What does "function anonymous" mean in Javascript?
Feb 2, 2012 · If I do Function('test'), I receive: function anonymous() { test }. This still returns function with name anonymous. Last argument is body of the function, all arguments before it are function arguments. –
language agnostic - What is a lambda (function)? - Stack Overflow
Aug 19, 2008 · A Lambda Function, or a Small Anonymous Function, is a self-contained block of functionality that can be passed around and used in your code. Lambda has different names in different programming languages – Lambda in Python and Kotlin , Closure in Swift , or Block in C and Objective-C .
coding style - What are the benefits to using anonymous functions ...
Apr 23, 2012 · Anonymous functions are declared inline and inline functions have advantages in that they can access variables in the parent scopes. Yes, you can put a name on an anonymous function, but that's usually pointless if it's declared inline. So inline has a significant advantage and if you're doing inline, there's little reason to put a name on it.
terminology - Difference between "anonymous function" and …
May 2, 2011 · This type of function is often referred to as an anonymous function because the function itself isn't directly declared or named. Is this the correct definition of an "anonymous function" in JavaScript? If not, what is an anonymous function, and is there any difference between an anonymous function and a function literal?
Explain the encapsulated anonymous function syntax
Oct 28, 2009 · is a function literal that defines an (anonymous) function. An additional ()-pair, which is interpreted as an expression, is not expected at toplevel, only literals. (function() { alert( 2 + 2 ); })(); is in an expression statement that invokes an anonymous function.
How to pass and execute anonymous function as parameter in …
A call through a std::function has a run time cost roughly equal to a virtual function call (caused by the above type erasure), and when you create it it has to copy the state of the function object (aka functor) passed in (which can be cheap -- stateless lambdas, or lambdas capturing arguments by reference -- or expensive in some other cases ...
Await / async on anonymous function - Stack Overflow
Aug 2, 2018 · async function hello(){ return "hello"; } let x = await hello(); console.log(x); result : "Hello" This is the way i want it to work : let x = await async function() {return "hello"}; console.log(x); result : [AsyncFunction] What am i missing ? I'm new to promises. EDIT : I tried adding the after the anonymous function to call it. Here is the ...
Event Listener with anonymous function - Stack Overflow
After some experimentation I figured out when I try to register an event listener with a one-parameter function like this (without an anonymous function) target.addEventListener(type, doSomething(parameter)); the listener function executes even when an event didn't happen, but when I wrap it up in an anonymous function
javascript - Naming an anonymous function - Stack Overflow
An anonymous function is a function without a name, it is executed from where it is defined. Alternatively, you can define the debugging function before using it. function debuggingName() { alert("x"); } $("object").bind("click", debuggingName);
Why define an anonymous function and pass it jQuery as the …
As a minor sidenote, sending in $ as an argument to an anonymous function makes $ local to that function which has a small positive performance implication if the $ function is called a lot. This is because javascript searches the local scope for variables first and then traverses down all the way to the window scope (where $ usually lives).
