A `Functor`

is a ubiquitous type class involving types that have one
"hole", i.e. types which have the shape `F[?]`

, such as `Option`

,
`List`

and `Future`

. (This is in contrast to a type like `Int`

which has
no hole, or `Tuple2`

which has two holes (`Tuple2[?,?]`

)).

The `Functor`

category involves a single operation, named `map`

:

`def map[A, B](fa: F[A])(f: A => B): F[B]`

This method takes a function `A => B`

and turns an `F[A]`

into an
`F[B]`

. The name of the method `map`

should remind you of the `map`

method that exists on many classes in the Scala standard library, for
example:

```
Option(1).map(_ + 1)
List(1, 2, 3).map(_ + 1)
Vector(1, 2, 3).map(_.toString)
```

We can trivially create a `Functor`

instance for a type which has a well
behaved `map`

method:

```
import cats._
implicit val optionFunctor: Functor[Option] = new Functor[Option] {
def map[A, B](fa: Option[A])(f: A => B) = fa map f
}
implicit val listFunctor: Functor[List] = new Functor[List] {
def map[A, B](fa: List[A])(f: A => B) = fa map f
}
```

However, functors can also be created for types which don't have a `map`

method. For example, if we create a `Functor`

for `Function1[In, ?]`

we can use `andThen`

to implement `map`

:

```
implicit def function1Functor[In]: Functor[Function1[In, ?]] =
new Functor[Function1[In, ?]] {
def map[A, B](fa: In => A)(f: A => B): Function1[In, B] = fa andThen f
}
```

This example demonstrates the use of the
kind-projector compiler plugin
This compiler plugin can help us when we need to change the number of type
holes. In the example above, we took a type which normally has two type holes,
`Function1[?,?]`

and constrained one of the holes to be the `In`

type,
leaving just one hole for the return type, resulting in `Function1[In,?]`

.
Without kind-projector, we'd have to write this as something like
`({type F[A] = Function1[In,A]})#F`

, which is much harder to read and understand.

`List`

is a functor which applies the function to each element of the list:

`Functor[List].map(List("qwer", "adsfg"))(_.length)`

`Option`

is a functor which only applies the function when the `Option`

value
is a `Some`

:

```
Functor[Option].map(Option("Hello"))(_.length) should be(res0)
Functor[Option].map(None: Option[String])(_.length) should be(res1)
```

We can use `Functor`

to "lift" a function from `A => B`

to `F[A] => F[B]`

:

```
val lenOption: Option[String] => Option[Int] = Functor[Option].lift(_.length)
lenOption(Some("abcd"))
```

We can now apply the `lenOption`

function to `Option`

instances.

```
val lenOption: Option[String] => Option[Int] = Functor[Option].lift(_.length)
lenOption(Some("Hello")) should be(res0)
```

`Functor`

provides an `fproduct`

function which pairs a value with the
result of applying a function to that value.

```
val source = List("Cats", "is", "awesome")
val product = Functor[List].fproduct(source)(_.length).toMap
product.get("Cats").getOrElse(0) should be(res0)
product.get("is").getOrElse(0) should be(res1)
product.get("awesome").getOrElse(0) should be(res2)
```

Functors compose! Given any functor `F[_]`

and any functor `G[_]`

we can
create a new functor `F[G[_]]`

by composing them:

`val listOpt = Functor[List] compose Functor[Option]`

In the previous example the resulting functor will apply the `map`

operation through the two
type constructors: `List`

and `Option`

.

```
val listOpt = Functor[List] compose Functor[Option]
listOpt.map(List(Some(1), None, Some(3)))(_ + 1) should be(res0)
```