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Concept
First-class citizen
Summary
In a given programming language design, a first-class citizen 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, and assigned to a variable.
The concept of first- and second-class objects was introduced by Christopher Strachey in the 1960s. He did not actually define the term strictly, but contrasted real numbers and procedures in ALGOL:
First and second class objects. In ALGOL, a real number may appear in an expression or be assigned to a variable, and either of them may appear as an actual parameter in a procedure call. A procedure, on the other hand, may only appear in another procedure call either as the operator (the most common case) or as one of the actual parameters. There are no other expressions involving procedures or whose results are procedures. Thus in a sense procedures in ALGOL are second class citizens—they always have to appear in person and can never be represented by a variable or expression (except in the case of a formal parameter)...
Robin Popplestone gave the following definition:
All items have certain fundamental rights.
All items can be the actual parameters of functions
All items can be returned as results of functions
All items can be the subject of assignment statements
All items can be tested for equality.
During the 1990s, Raphael Finkel proposed definitions of second and third class values, but these definitions have not been widely adopted.
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.
Official source
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