In programming language design, a first-class citizen (also type, object, entity, or value) in a given programming language is an entity which supports all the operations generally available to other entities. These operations typically include being passed as an argument, returned from a function, modified, and assigned to a variable.[1]
The simplest scalar data types, such as integer and floating-point numbers, are nearly always first-class.
In many older languages, arrays and strings are not first-class: they cannot be assigned as objects or passed as parameters to a subroutine. For example, neither Fortran IV nor C supports array assignment, and when they are passed as parameters, only the position of their first element is actually passed—their size is lost. C appears to support assignment of array pointers, but in fact these are simply pointers to the array‘s first element, and again do not carry the array‘s size.
In most languages, data types are not first-class objects, though in some object-oriented languages, classes are first-class objects and used for metaclasses.
Few languages support continuations and GOTO-labels as objects at all, let alone as first-class objects.
| Concept | Description | Languages |
|---|---|---|
| first-class function | closures and anonymous functions | Scheme, ML, Haskell, F#, Scala, Swift, PHP, Perl 6, JavaScript |
| first-class control | continuations | Scheme, ML, F# |
| first-class type | Coq, Idris | |
| first-class data type | Generic Haskell. C++11 | |
| first-class polymorphism | impredicative polymorphism | |
| first-class message | dynamic messages (method calls) | Smalltalk,[7] Objective-C[7] |
| first-class class | metaclass | Smalltalk, Objective-C, Ruby, Python |
| proof object[8] | Coq, Agda |