functor

English

Etymology

From function, modeled after factor.

Pronunciation

• (Received Pronunciation) IPA^(key): /ˈfʌŋktə/

Noun

functor (plural functors)

1. (grammar) A function word.
2. (object-oriented programming) A function object.
3. (category theory) A category homomorphism; a morphism from a source category to a target category which maps objects to objects and arrows to arrows (either covariantly or contravariantly), in such a way as to preserve morphism composition and identities.

   Hypernym: morphism

   Hyponym: endofunctor

   In the category of categories, $\mathbb {CAT}$, the objects are categories and the morphisms are functors.

   • 1991, Natalie Wadhwa (translator), Yu. A. Brudnyǐ, N. Ya. Krugljak, Interpolation Functors and Interpolation Spaces, Volume I, Elsevier (North-Holland), page 143,

     Choosing for $U$ the operation of closure, regularization or relative completion, we obtain from a given functor ${\mathcal {F}}\in {\mathcal {JF}}$ the functors

     ${\overline {F}}:{\overrightarrow {X}}\rightarrow {\overline {F({\overrightarrow {X}})}},F^{0}:{\overrightarrow {X}}\rightarrow F({\overrightarrow {X}})^{0},F^{c}:{\overrightarrow {X}}\rightarrow F({\overrightarrow {X}})^{c}$.

   • 2004, William G. Dwyer, Philip S. Hirschhorn, Daniel M. Kan, Jeffrey H. Smith, Homotopy Limit Functors on Model Categories and Homotopical Categories, American Mathematical Society, page 165:

     Given a homotopical category $X$ and a functor $u:A\rightarrow B$, a homotopical $u$-colimit (resp. $u$-limit) functor on $X$ will be a homotopically terminal (resp. initial) Kan extension of the identity (50.2) along the induced diagram functor $X^{u}:X^{B}\rightarrow X^{A}$ (47.1).

   • 2009, Benoit Fresse, Modules Over Operads and Functors, Springer, Lecture Notes in Mathematics: 1967, page 35,

     In this chapter, we recall the definition of the category of $\Sigma _{*}$-objects and we review the relationship between $\Sigma _{*}$-objects and functors. In short, a $\Sigma _{*}$-object (in English words, a symmetric sequence of objects, or simply a symmetric object) is the coefficient sequence of a generalized symmetric functor $S(M):X\rightarrow S(M,X)$, defined by a formula of the form

     $S(M,X)=\bigoplus _{r=0}^{\infty }\left(M(r)\otimes X^{\otimes r}\right)_{\Sigma _{r}}$.

4. (functional programming) A structure allowing a function to apply within a generic type, in a way that is conceptually similar to a functor in category theory.

Derived terms

• adjoint functor
• contravariant functor
• faithful functor
• forgetful functor
• full functor
• functor category
• group functor
• identity functor
• representable functor
• underlying functor
• Yoneda functor

 

Translations

grammar: function word — see function word

object-oriented programming: function object

• Finnish: funktio-olio
• French: foncteur (fr) m

category theory: category mapping

• Bashkir: функтор (funktor)
• Chinese:

  Mandarin: 函子 (hánzǐ)

• Finnish: funktori
• French: foncteur (fr) m
• German: Funktor m
• Hungarian: funktor
• Icelandic: varpi m
• Irish: feidhmeoir
• Italian: funtore m
• Japanese: 関手 (かんしゅ, kanshu)
• Kazakh: функтор (funktor)
• Kyrgyz: функтор (funktor)
• Polish: funktor m
• Portuguese: functor m, funtor m
• Russian: функтор (ru) (funktor)
• Spanish: funtor m
• Swedish: funktor (sv) c

Further reading

• function word on Wikipedia.Wikipedia
• functor on Wikipedia.Wikipedia
• functor (functional programming) on Wikipedia.Wikipedia
• “functor”, in Lexico, Dictionary.com; Oxford University Press, 2019–2022.

Portuguese

Alternative forms

• funtor

Noun

functor m (plural functores)

1. (category theory) functor (a mapping between categories)

Romanian

Etymology

Borrowed from French functeur.

Noun

functor m (plural functori)

1. functor

Declension

Declension of functor

	singular		plural
	indefinite articulation	definite articulation	indefinite articulation	definite articulation
nominative/accusative	(un) functor	functorul	(niște) functori	functorii
genitive/dative	(unui) functor	functorului	(unor) functori	functorilor
vocative	functorule		functorilor