Functor (functional programming)
In functional programming, a Functor is a design pattern inspired by definition from category theory, that allows for generic type to apply function inside without changing the structure of generic type.
This idea is encoded in Haskell using [type class]
class Functor f where
fmap :: (a -> b) -> f a -> f b
with conditions
fmap id = id
fmap (g . h) = (fmap g) . (fmap h)
In Scala [higher kinded types] are used
trait Functor[F[_]] {
def map[A,B](a: F[A])(f: A => B): F[B]
}
Simple examples of this are Option and collection types. Functor is very useful to model functional effects to apply a function to computations that did not yet finish.
Functor is base for more complex abstractions like Applicative, Monad, Comonad.
In C++ name functor is incorrectly refers to function object.