Editing First-class citizen (section)

==History==

The concept of first- and second-class objects was introduced by [[Christopher Strachey]] in the 1960s.<ref>Rod Burstall, "Christopher Strachey—Understanding Programming Languages", ''Higher-Order and Symbolic Computation'' '''13''':52 (2000)</ref><ref>Harold Abelson and Gerald Jay Sussman, ''Structure and Interpretation of Computer Programs'', 2nd edition, section 1.3.4 [http://mitpress.mit.edu/sicp/full-text/book/book-Z-H-12.html#footnote_Temp_121 footnote 64] {{Webarchive|url=https://web.archive.org/web/20150309062836/http://mitpress.mit.edu/sicp/full-text/book/book-Z-H-12.html#footnote_Temp_121 |date=2015-03-09 }}</ref> He did not actually define the term strictly, but contrasted real numbers and procedures in [[ALGOL]]:
<blockquote>'''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)...
<ref>Christopher Strachey, "[http://www.ics.uci.edu/~jajones/INF102-S18/readings/05_stratchey_1967.pdf Fundamental Concepts in Programming Languages]" in ''Higher-Order and Symbolic Computation'' '''13''':11 (2000); though published in 2000, these are notes from lectures Strachey delivered in August, 1967</ref></blockquote>
[[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.<ref>R. J. Popplestone: The Design Philosophy of [[POP-2]]. in: D. Michie: Machine Intelligence 3, Edinburgh at the University Press, 1968</ref>

During the 1990s, [[Raphael Finkel]]<ref>Finkel, R. ''Advanced Programming language Design'', p 73</ref> proposed definitions of second and third class values, but these definitions have not been widely adopted.<ref>{{cite web|title=About first-,second- and third-class value|author=Norman Ramsey|url=https://stackoverflow.com/a/2582804|work=stackoverflow.com|access-date=14 September 2013}}</ref>{{better source needed|date=November 2021}}