“First-class function” refers to a programming language’s ability to pass functions as arguments and assign them to variables. It allows for dynamic code generation and high abstraction. The term “function” is used loosely, but strict guidelines require handling functions as variables natively. First-class functions enable the implementation of design patterns and polymorphism.
“First-class function” is a term that can be applied in computer programming and computer science to a computer programming language if its treatment of functions adheres to a set of guidelines, although the full scope and depth of these guidelines are not formally agreed. To have a first-class computer programming language, the main feature that needs to be supported is the ability to pass functions as arguments to other functions. Also, a function must be capable of being assigned to a variable so that it can be stored. Other properties used to determine what constitutes a first-class function include the ability to generate a function dynamically at runtime and the language’s ability to have a function be the return value of another function. Languages that support the first-class function architecture allow for a high level of abstraction and, in some cases, also provide a mechanism for dynamic code generation at runtime.
When the term “function” is used in a loosely defined sense, the concept of passing a function into a program through variables is not necessarily unique to languages that natively implement first-class function support. The ability to pass blocks of code to functions or to return non-dynamic code from a function can be easily accomplished in many programming languages through different mechanisms. One of the strict parts of defining a first-class function language, however, is that handling functions as variables must be done natively, without the use of metadata such as conditional definitions, and without calling a compiler to recompile a piece of code. The term “function” is also used to refer to independent blocks of code that are called on their own, meaning they do not include the blocks of code known as methods in object-oriented programming languages or the blocks sometimes called procedures in other languages.
When a language is designed to allow first-class function code, some design patterns can be implemented more directly than other languages. A function can receive a function as a variable and then construct a new function and return the new function to the calling code, thus this creates a mechanism for run-time creation of program code. This may also make it possible to use real-time user-entered functions within the language without the use of processes such as reflection or evaluation.
Some of the more basic uses for first-class function architecture include creating generic functions that can be easily reused, and implementing recursive mathematical algorithms that can automatically modify their equations as they progress. A similar and more advanced use for first-class mechanisms is the implementation of polymorphism in languages where it is not specifically implemented. This can allow you to call functions that have the same signature as the function but execute code based on the context it was called in, sometimes passing a key function to the polymorphic function.
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