Anonymous function

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.

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.

All of the code in the following sections is written in Python 2.x (not 3.x).

Sorting

When attempting to sort in a non-standard way, it may be easier to contain the comparison 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 comparison function that is supplied two items and the function indicates if they are equal or if one is greater or less than the other (usually indicated by returning a negative number, zero, or a positive number).

Consider sorting a list of strings by length of the string:

The anonymous function in this example is the lambda expression:

The anonymous function accepts two arguments, x and y, and returns the comparison between them using the built-in function cmp(). 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 it takes fewer inputs (in this case, transforming a function that performs division by any integer 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 "d" variable.

Higher-order functions

Python 2.x includes several functions that take anonymous functions as an argument. This section describes some of them.

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). Above form is discouraged by creators of the language, who maintains that 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 form below is considered as more appropriate:

Fold

The fold/reduce function runs over all elements in a list (usually left-to-right), accumulating a value as it goes. A common use of this is to combine all elements of a list into one value, for example:

This performs

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 conventional 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. 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. However, the new C++11 standard adds them to C++, even though this is a conventional, statically typed language.

Examples

Numerous languages support anonymous functions, or something similar.

C (non-standard extension)

The anonymous function is not supported by standard C programming language, but supported by some C dialects, such as GCC and Clang.

GCC

GNU Compiler Collection (GCC) supports anonymous functions, mixed by nested functions and statement expressions. It has the form:

The following example works only with GCC. Because of how macros are expanded, the l_body cannot contain any commas outside of parentheses; GCC treats the comma as a delimiter between macro arguments. The argument l_ret_type can be removed if __typeof__ is available; in the example below using __typeof__ on array would return testtype *, which can be dereferenced for the actual value if needed.

Clang (C, C++, Objective-C, Objective-C++)

Clang supports anonymous functions, called blocks, which have the form:

The type of the blocks above is return_type (^)(parameters).

Using the aforementioned blocks extension and Grand Central Dispatch (libdispatch), the code could look simpler:

The code with blocks should be compiled with -fblocks and linked with -lBlocksRuntime

C++ (since C++11)

C++11 supports anonymous functions, called lambda expressions, which have the form:

An example lambda function is defined as follows:

C++11 also supports closures. Closures are defined between square brackets [and ] in the declaration of lambda expression. The mechanism allows these variables to be captured by value or by reference. The following table demonstrates this:

Variables captured by value are constant by default. Adding mutable after the parameter list makes them non-constant.

The following two examples demonstrate use of a lambda expression:

This computes the total of all elements in the list. The variable total is stored as a part of the lambda function's closure. Since it is a reference to the stack variable total, it can change its value.

This will cause total to be stored as a reference, but value will be stored as a copy.

The capture of this is special. It can only be captured by value, not by reference. this can only be captured if the closest enclosing function is a non-static member function. The lambda will have the same access as the member that created it, in terms of protected/private members.

If this is captured, either explicitly or implicitly, then the scope of the enclosed class members is also tested. Accessing members of this does not need explicit use of this-> syntax.

The specific internal implementation can vary, but the expectation is that a lambda function that captures everything by reference will store the actual stack pointer of the function it is created in, rather than individual references to stack variables. However, because most lambda functions are small and local in scope, they are likely candidates for inlining, and thus need no added storage for references.

If a closure object containing references to local variables is invoked after the innermost block scope of its creation, the behaviour is undefined.

Lambda functions are function objects of an implementation-dependent type; this type's name is only available to the compiler. If the user wishes to take a lambda function as a parameter, the type must be a template type, or they must create a std::function or a similar object to capture the lambda value. The use of the auto keyword can help store the lambda function,

Here is an example of storing anonymous functions in variables, vectors, and arrays; and passing them as named parameters:

A lambda expression with an empty capture specification ([]) can be implicitly converted into a function pointer with the same type as the lambda was declared with. So this is legal:

The Boost library provides its own syntax for lambda functions as well, using the following syntax:

C#

In C#, support for anonymous functions has deepened through the various versions of the language compiler. The language v3.0, released in November 2007 with .NET Framework v3.5, has full support of anonymous functions. C# names them lambda expressions, following the original version of anonymous functions, the lambda calculus. See the C# 4.0 Language Specification, section 5.3.3.29, for more information.

While the function is anonymous, it cannot be assigned to an implicitly typed variable, because the lambda syntax may be used for denoting an anonymous function or an expression tree, and the choice cannot automatically be decided by the compiler. E.g., this does not work:

However, a lambda expression can take part in type inference and can be used as a method argument, e.g. to use anonymous functions with the Map capability available with System.Collections.Generic.List (in the ConvertAll() method):

Prior versions of C# had more limited support for anonymous functions. C# v1.0, introduced in February 2002 with the .NET Framework v1.0, provided partial anonymous function support through the use of delegates. This construct is somewhat similar to PHP delegates. In C# 1.0, delegates are like function pointers that refer to an explicitly named method within a class. (But unlike PHP, the name is unneeded at the time the delegate is used.) C# v2.0, released in November 2005 with the .NET Framework v2.0, introduced the concept of anonymous methods as a way to write unnamed inline statement blocks that can be executed in a delegate invocation. C# 3.0 continues to support these constructs, but also supports the lambda expression construct.

This example will compile in C# 3.0, and exhibits the three forms:

In the case of the C# 2.0 version, the C# compiler takes the code block of the anonymous function and creates a static private function. Internally, the function gets a generated name, of course; this generated name is based on the name of the method in which the Delegate is declared. But the name is not exposed to application code except by using reflection.

In the case of the C# 3.0 version, the same mechanism applies.

ColdFusion Markup Language (CFML)

CFML supports any statements within the function's definition, not simply expressions.

CFML supports recursive anonymous functions:

CFML anonymous functions implement closure.

D

D uses inline delegates to implement anonymous functions. The full syntax for an inline delegate is

If unambiguous, the return type and the keyword delegate can be omitted.

Since version 2.0, D allocates closures on the heap unless the compiler can prove it is unnecessary; the scope keyword can be used for forcing stack allocation. Since version 2.058, it is possible to use shorthand notation:

An anonymous function can be assigned to a variable and used like this:

Dart

Dart supports anonymous functions.

or

Delphi

Delphi introduced anonymous functions since version 2009.

Elixir

Elixir uses the closure fn for anonymous functions.

Erlang

Erlang uses a syntax for anonymous functions similar to that of named functions.

Go

Go supports anonymous functions.

Haskell

Haskell uses a concise syntax for anonymous functions (lambda expressions).

Lambda expressions are fully integrated with the type inference engine, and support all the syntax and features of "ordinary" functions (except for the use of multiple definitions for pattern-matching, since the argument list is only specified once).

The following are all equivalent:

Haxe

In Haxe, anonymous functions are called lambda, and use the syntax function(argument-list) expression; .

Java

Java supports anonymous functions, named Lambda Expressions, starting with JDK 8.

A lambda expression consists of a comma separated list of the formal parameters enclosed in parentheses, an arrow token (->), and a body. Data types of the parameters can always be omitted, as can the parentheses if there is only one parameter. The body can consist of one statement or a statement block.

Lambda expressions are converted to "functional interfaces" (defined as interfaces that contain only one abstract method in addition to one or more default or static methods), as in the following example:

In this example, a functional interface called IntegerMath is declared. Lambda expressions that implement IntegerMath are passed to the apply() method to be executed. Default methods like swap define methods on functions.

Java 8 introduced another mechanism named method reference (the :: operator) to create a lambda on an existing method. A method reference doesn't indicate the number or types of arguments because those are extracted from the abstract method of the functional interface.

In the example above, the functional interface IntBinaryOperator declares an abstract method int applyAsInt(int, int), so the compiler looks for a method int sum(int, int) in the class java.lang.Integer.

Java Limitations

Java 8 lambdas have the following limitations:

JavaScript

JavaScript/ECMAScript supports anonymous functions.

In ES6:

This construct is often used in Bookmarklets. For example, to change the title of the current document (visible in its window's title bar) to its URL, the following bookmarklet may seem to work.

However, as the assignment statement returns a value (the URL itself), many browsers actually create a new page to display this value.

Instead, an anonymous function, that does not return a value, can be used:

The function statement in the first (outer) pair of parentheses declares an anonymous function, which is then executed when used with the last pair of parentheses. This is almost equivalent to the following, which populates the environment with f unlike an anonymous function.

Use void() to avoid new pages for arbitrary anonymous functions:

or just:

JavaScript has syntactic subtleties for the semantics of defining, invoking and evaluating anonymous functions. These subliminal nuances are a direct consequence of the evaluation of parenthetical expressions. The following constructs which are called immediately-invoked function expression illustrate this:

and

Representing "function(){ ... }" by f, the form of the constructs are a parenthetical within a parenthetical (f()) and a parenthetical applied to a parenthetical (f)().

Note the general syntactic ambiguity of a parenthetical expression, parenthesized arguments to a function and the parentheses around the formal parameters in a function definition. In particular, JavaScript defines a , (comma) operator in the context of a parenthetical expression. It is no mere coincidence that the syntactic forms coincide for an expression and a function's arguments (ignoring the function formal parameter syntax)! If f is not identified in the constructs above, they become (()) and ()(). The first provides no syntactic hint of any resident function but the second MUST evaluate the first parenthetical as a function to be legal JavaScript. (Aside: for instance, the ()'s could be ([],{},42,"abc",function(){}) as long as the expression evaluates to a function.)

Also, a function is an Object instance (likewise objects are Function instances) and the object literal notation brackets, {} for braced code, are used when defining a function this way (as opposed to using new Function(...)). In a very broad non-rigorous sense (especially since global bindings are compromised), an arbitrary sequence of braced JavaScript statements, {stuff}, can be considered to be a fixed point of

More correctly but with caveats,

Note the implications of the anonymous function in the JavaScript fragments that follow:

Julia

In Julia programming language anonymous functions are defined using the syntax (arguments)->(expression),

Lisp

Lisp and Scheme support anonymous functions using the "lambda" construct, which is a reference to lambda calculus. Clojure supports anonymous functions with the "fn" special form and #() reader syntax.

Common Lisp

Common Lisp has the concept of lambda expressions. A lambda expression is written as a list with the symbol "lambda" as its first element. The list then contains the argument list, documentation or declarations and a function body. Lambda expressions can be used inside lambda forms and with the special operator "function".

"function" can be abbreviated as #'. Also, macro lambda exists, which expands into a function form:

One typical use of anonymous functions in Common Lisp is to pass them to higher-order functions like mapcar, which applies a function to each element of a list and returns a list of the results.

The lambda form in Common Lisp allows a lambda expression to be written in a function call:

Anonymous functions in Common Lisp can also later be given global names:

Scheme

Interestingly, Scheme's named functions is simply syntactic sugar for anonymous functions bound to names:

expands (and is equivalent) to

Clojure

Clojure supports anonymous functions through the "fn" special form:

There is also a reader syntax to define a lambda:

Like Scheme, Clojure's "named functions" are simply syntactic sugar for lambdas bound to names:

expands to:

Lua

In Lua (much as in Scheme) all functions are anonymous. A named function in Lua is simply a variable holding a reference to a function object.

Thus, in Lua

is just syntactical sugar for

An example of using anonymous functions for reverse-order sorting:

Wolfram Language, Mathematica

The Wolfram Language is the programming language of Mathematica. Anonymous functions are important in programming the latter. There are several ways to create them. Below are a few anonymous functions that increment a number. The first is the most common. #1 refers to the first argument and & marks the end of the anonymous function.

So, for instance:

Also, Mathematica has an added construct to make recursive anonymous functions. The symbol '#0' refers to the entire function. The following function calculates the factorial of its input:

MATLAB, Octave

Anonymous functions in MATLAB or Octave are defined using the syntax @(argument-list)expression. Any variables that are not found in the argument list are inherited from the enclosing scope.

Maxima

In Maxima anonymous functions are defined using the syntax lambda(argument-list,expression),

ML

The various dialects of ML support anonymous functions.

F#

F# supports anonymous functions, as follows:

Standard ML

Standard ML supports anonymous functions, as follows:

Perl 5

Perl 5 supports anonymous functions, as follows:

Other constructs take bare blocks as arguments, which serve a function similar to lambda functions of one parameter, but don't have the same parameter-passing convention as functions -- @_ is not set.

Perl 6

In Perl 6, all blocks (even those associated with if, while, etc.) are anonymous functions. A block that is not used as an rvalue is executed immediately.

1. fully anonymous, called as created
2. assigned to a variable
3. currying

PHP

Before 4.0.1, PHP had no anonymous function support.

PHP 4.0.1 to 5.3

PHP 4.0.1 introduced the create_function which was the initial anonymous function support. This function call makes a new randomly named function and returns its name (as a string)

The argument list and function body must be in single quotes, or the dollar signs must be escaped. Otherwise, PHP assumes "$x" means the variable $x and will substitute it into the string (despite possibly not existing) instead of leaving "$x" in the string. For functions with quotes or functions with lots of variables, it can get quite tedious to ensure the intended function body is what PHP interprets.

Each invocation of create_function makes a new function, which exists for the rest of the program, and cannot be garbage collected, using memory in the program irreversibly. If this is used to create anonymous functions many times, e.g., in a loop, it can cause problems such as memory bloat.

PHP 5.3

PHP 5.3 added a new class called Closure and magic method __invoke() that makes a class instance invocable.

In this example, $func is an instance of Closure and echo $func() is equivalent to $func->__invoke($z). PHP 5.3 mimics anonymous functions but it does not support true anonymous functions because PHP functions are still not first-class objects.

PHP 5.3 does support closures but the variables must be explicitly indicated as such:

The variable $x is bound by reference so the invocation of $func modifies it and the changes are visible outside of the function.

Logtalk

Logtalk uses the following syntax for anonymous predicates (lambda expressions):

A simple example with no free variables and using a list mapping predicate is:

Currying is also supported. The above example can be written as:

Visual Prolog

Anonymous functions (in general anonymous predicates) were introduced in Visual Prolog in version 7.2. Anonymous predicates can capture values from the context. If created in an object member, it can also access the object state (by capturing This).

mkAdder returns an anonymous function, which has captured the argument X in the closure. The returned function is a function that adds X to its argument:

Python

Python supports simple anonymous functions through the lambda form. The executable body of the lambda must be an expression and can't be a statement, which is a restriction that limits its utility. The value returned by the lambda is the value of the contained expression. Lambda forms can be used anywhere ordinary functions can. However these restrictions make it a very limited version of a normal function. Here is an example:

In general, Python convention encourages the use of named functions defined in the same scope as one might typically use an anonymous functions in other languages. This is acceptable as locally defined functions implement the full power of closures and are almost as efficient as the use of a lambda in Python. In this example, the built-in power function can be said to have been curried:

R

In GNU R the anonymous functions are defined using the syntax function(argument-list)expression .

Ruby

Ruby supports anonymous functions by using a syntactical structure called block. There are two data types for blocks in Ruby. Procs behave similarly to closures, whereas lambdas behave more analogous to an anonymous function. When passed to a method, a block is converted into a Proc in some circumstances.

Scala

In Scala, anonymous functions use the following syntax:

In certain contexts, like when an anonymous function is a parameter being passed to another function, the compiler can infer the types of the parameters of the anonymous function and they can be omitted in the syntax. In such contexts, it is also possible to use a shorthand for anonymous functions using the underscore character to introduce unnamed parameters.

Smalltalk

In Smalltalk anonymous functions are called blocks

Swift

In Swift, anonymous functions are called closures. The syntax has following form:

For example:

For sake of brevity and expressiveness, the parameter types and return type can be omitted if these can be inferred:

Similarly, Swift also supports implicit return statements for one-statement closures:

Finally, the parameter names can be omitted as well; when omitted, the parameters are referenced using shorthand argument names, consisting of the $ symbol followed by their position (e.g. $0, $1, $2, etc.):

Tcl

In Tcl, applying the anonymous squaring function to 2 looks as follows:

This example involves two candidates for what it means to be a function in Tcl. The most generic is usually called a command prefix, and if the variable f holds such a function, then the way to perform the function application f(x) would be

where {*} is the expansion prefix (new in Tcl 8.5). The command prefix in the above example is apply {x {expr {$x*$x}}} Command names can be bound to command prefixes by means of the interp alias command. Command prefixes support currying. Command prefixes are very common in Tcl APIs.

The other candidate for "function" in Tcl is usually called a lambda, and appears as the {x {expr {$x*$x}}} part of the above example. This is the part which caches the compiled form of the anonymous function, but it can only be invoked by being passed to the apply command. Lambdas do not support currying, unless paired with an apply to form a command prefix. Lambdas are rare in Tcl APIs.

Visual Basic .NET

Visual Basic .NET 2008 introduced anonymous functions through the lambda form. Combined with implicit typing, VB provides an economical syntax for anonymous functions. As with Python, in VB.NET, anonymous functions must be defined on one line; they cannot be compound statements. Further, an anonymous function in VB.NET must truly be a VB.NET Function - it must return a value.

Visual Basic.NET 2010 added support for multiline lambda expressions and anonymous functions without a return value. For example, a function for use in a Thread.