/Я language (β)/Glossary/Functor/

Functor is a mapping between categories. Depends the way functors map morphisms they could be either covariant or contravariant. Covariant functors map morphisms as it is, contravariant functors flip morphisms:

Functors which map from (source) and into (target) the same category are called endo functors.

Functor mapping operators in Я are Yonedified:

We compose functors by treating one of them as an argument of another one: