Many modern (and some older) languages, e.g. C++, Eiffel, Groovy, Lisp, Smalltalk, Perl, PHP, Python, Ruby, Scala, and many others, support first-class function objects and may even make significant use of them.
Consider the example of a sorting routine that uses a callback function to define an ordering relation between a pair of items.
To understand what power a functor provides more than a regular function, consider the common use case of sorting objects by a particular field.
Much of the C++ Standard Template Library (STL) makes heavy use of template-based function objects.
D also supports function literals, that allow a lambda-style definition: To allow the compiler to inline the code (see above), function objects can also be specified C++-style via operator overloading: In the Eiffel software development method and language, operations and objects are seen always as separate concepts.
Agents satisfy the range of application attributed to function objects, such as being passed as arguments in procedural calls or specified as callback routines.
The design of the agent mechanism in Eiffel attempts to reflect the object-oriented nature of the method and language.
An agent is an object that generally is a direct instance of one of the two library classes, which model the two types of routines in Eiffel: PROCEDURE and FUNCTION.
The routine extend referenced in the example above is a feature of a class in a graphical user interface (GUI) library to provide event-driven programming capabilities.
': When agents are created, the arguments to the routines they model and even the target object to which they are applied can be either closed or left open.
The assignment of values for open arguments and targets is deferred until some point after the agent is created.
The ability to close or leave open targets and arguments is intended to improve the flexibility of the agent mechanism.
Consider a class that contains the following procedure to print a string on standard output after a new line: The following snippet, assumed to be in the same class, uses print_on_new_line to demonstrate the mixing of open arguments and open targets in agents used as arguments to the same routine.
Java has no first-class functions, so function objects are usually expressed by an interface with a single method (most commonly the Callable interface), typically with the implementation being an anonymous inner class, or, starting in Java 8, a lambda.
An example is this accumulator mutable struct (based on Paul Graham's study on programming language syntax and clarity):[5] Such an accumulator can also be implemented using closure: In Lisp family languages such as Common Lisp, Scheme, and others, functions are objects, just like strings, vectors, lists, and numbers.
Since the programmer cannot directly construct a closure, they must define a class that has all of the necessary state variables, and also a member function.
Making objects callable using the same syntax as functions is a fairly trivial business.
Making a function call operator work with different kinds of function things, whether they be class objects or closures is no more complicated than making a + operator that works with different kinds of numbers, such as integers, reals or complex numbers.
While programmers who use closures are not surprised that an object is called like a function, they discover that multiple closures sharing the same environment can provide a complete set of abstract operations like a virtual table for single dispatch type OOP.
A single NSInvocation can be created and then called for each of any number of targets, for instance from an observable object.
An example of this in use: It is also possible in PHP 5.3+ to make objects invokable by adding a magic __invoke() method to their class:[6] In the Windows PowerShell language, a script block is a collection of statements or expressions that can be used as a single unit.
More recently, symbols (accessed via the literal unary indicator :) can also be converted to Procs.
[1] Because of the variety of forms, the term Functor is not generally used in Ruby to mean a Function object.
Just a type of dispatch delegation introduced by the Ruby Facets project is named as Functor.
The most basic definition of which is: This usage is more akin to that used by functional programming languages, like ML, and the original mathematical terminology.