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first class citizen wikipedia jump to content main menu main menu move to sidebar hide navigation main page contents current events random article about wikipedia contact us contribute help learn to edit community portal recent changes upload file special pages search search appearance donate create account log in personal tools donate create account log in contents move to sidebar hide top 1 history 2 examples 3 functions 4 reflection 5 see also 6 notes 7 references toggle the table of contents first class citizen 12 languages čeština deutsch français 日本語 한국어 polski português русский српски srpski українська tiếng việt 中文 edit links article talk english read edit view history tools tools move to sidebar hide actions read edit view history general what links here related changes upload file permanent link page information cite this page get shortened url switch to legacy parser print export download as pdf printable version in other projects wikidata item appearance move to sidebar hide from wikipedia the free encyclopedia redirected from first class object concept in programming language design for the usage in society see second class citizen in a given programming language design a first class citizen a 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 1 history edit the concept of first and second class objects was introduced by christopher strachey in the 1960s 2 3 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 4 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 5 during the 1990s raphael finkel 6 proposed definitions of second and third class values but these definitions have not been widely adopted 7 better source needed examples edit 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 citation needed in most languages data types are not first class objects though in some object oriented languages classes are first class objects and are instances of metaclasses languages in the functional programming family often also feature first class types in the form of for example generalized algebraic data types or other metalanguage amenities enabling programs to implement extensions to their own implementation language 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 smalltalk dart scheme ml haskell f kotlin scala swift perl php python raku javascript delphi rust common lisp c first class control continuations scheme ml f first class type dependent types rocq idris agda first class data type generic haskell c 8 first class polymorphism impredicative polymorphism first class message dynamic messages method calls smalltalk 9 objective c 9 common lisp first class class metaclass and metaobject smalltalk objective c ruby python delphi common lisp first class proofs proof object 10 rocq agda functions edit main article first class functions many programming languages support passing and returning function values which can be applied to arguments whether this suffices to call function values first class is disputed some authors require it be possible to create new functions at runtime to call them first class citation needed under this definition functions in c are not first class objects instead they are sometimes called second class objects because they can still be manipulated in most of the above fashions via function pointers in smalltalk functions methods are first class objects just like smalltalk classes since smalltalk operators etc are methods they are also first class objects reflection edit main article reflection computer programming some languages such as java and php have an explicit reflection subsystem which allow access to internal implementation structures even though they are not accessible or manipulable in the same way as ordinary objects in other languages such as those in the lisp family reflection is a central feature of the language rather than a special subsystem typically this takes the form of some set of the following features syntactic macros or fexprs which allow the user to write code which handles code as data and evaluates it by discretion enabling for example programs to write programs or rewrite themselves inside of the compiler interpreter or even the parser reader macros a meta circular evaluator which provides a definition of the language s evaluator as a compiled tautologisation of itself facilitating straightforward modification of the language without requiring a metalanguage different from itself a metaobject protocol a special form of meta circular evaluator for object oriented programming in which the object system implements itself recursively via a system of metaclasses and metaobjects which are themselves classes and objects these allow varying forms of first class access to the language implementation and are in general manipulable in the same way as and fully indistinguishable from ordinary language objects because of this their usage generally comes with some cultural stipulations and advice as untested modification of the core programming system by users can easily undermine performance optimisations made by language implementers see also edit first class function reification notes edit also known as first class type first class entity first class object or first class value references edit scott michael 2006 programming language pragmatics san francisco ca morgan kaufmann publishers p 140 isbn 9780126339512 rod burstall christopher strachey understanding programming languages higher order and symbolic computation 13 52 2000 harold abelson and gerald jay sussman structure and interpretation of computer programs 2nd edition section 1 3 4 footnote 64 archived 2015 03 09 at the wayback machine christopher strachey 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 r j popplestone the design philosophy of pop 2 in d michie machine intelligence 3 edinburgh at the university press 1968 finkel r advanced programming language design p 73 norman ramsey about first second and third class value stackoverflow com retrieved 14 september 2013 ivan čukić 2018 functional programming in c simon and schuster isbn 9781638355663 1 2 paritosh shroff scott f smith type inference for first class messages with match functions bove ana dybjer peter 2009 dependent types at work pdf language engineering and rigorous software development lecture notes in computer science vol 5520 pp 57 99 doi 10 1007 978 3 642 03153 3_2 isbn 978 3 642 03152 6 archived pdf from the original on april 2 2014 retrieved 8 june 2015 also archived retrieved from https en wikipedia org w index php title first class_citizen oldid 1374699469 categories programming constructs programming language design object computer science hidden categories articles with short description short description is different from wikidata webarchive template wayback links all articles lacking reliable references articles lacking reliable references from november 2021 all articles with unsourced statements articles with unsourced statements from january 2022 this page was last edited on 13 september 2026 at 16 14 utc page was rendered with parsoid text is available under the creative commons attribution sharealike 4 0 license 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