`Monad`

extends the `Applicative`

type class with a
new function `flatten`

. Flatten takes a value in a nested context (eg.
`F[F[A]]`

where F is the context) and "joins" the contexts together so
that we have a single context (ie. `F[A]`

).

The name `flatten`

should remind you of the functions of the same name on many
classes in the standard library.

```
Option(Option(1)).flatten should be(res0)
Option(None).flatten should be(res1)
List(List(1), List(2, 3)).flatten should be(res2)
```

If `Applicative`

is already present and `flatten`

is well-behaved,
extending the `Applicative`

to a `Monad`

is trivial. To provide evidence
that a type belongs in the `Monad`

type class, cats' implementation
requires us to provide an implementation of `pure`

(which can be reused
from `Applicative`

) and `flatMap`

.

We can use `flatten`

to define `flatMap`

: `flatMap`

is just `map`

followed by `flatten`

. Conversely, `flatten`

is just `flatMap`

using
the identity function `x => x`

(i.e. `flatMap(_)(x => x)`

).

```
import cats._
implicit def optionMonad(implicit app: Applicative[Option]) =
new Monad[Option] {
// Define flatMap using Option's flatten method
override def flatMap[A, B](fa: Option[A])(f: A => Option[B]): Option[B] =
app.map(fa)(f).flatten
// Reuse this definition from Applicative.
override def pure[A](a: A): Option[A] = app.pure(a)
}
```

Cats already provides a `Monad`

instance of `Option`

.

```
import cats._
import cats.implicits._
Monad[Option].pure(42) should be(res0)
```

`flatMap`

is often considered to be the core function of `Monad`

, and cats
follows this tradition by providing implementations of `flatten`

and `map`

derived from `flatMap`

and `pure`

.

```
implicit val listMonad = new Monad[List] {
def flatMap[A, B](fa: List[A])(f: A => List[B]): List[B] = fa.flatMap(f)
def pure[A](a: A): List[A] = List(a)
}
```

Part of the reason for this is that name `flatMap`

has special significance in
scala, as for-comprehensions rely on this method to chain together operations
in a monadic context.

```
import cats._
import cats.implicits._
Monad[List].flatMap(List(1, 2, 3))(x ⇒ List(x, x)) should be(res0)
```

`Monad`

provides the ability to choose later operations in a sequence based on
the results of earlier ones. This is embodied in `ifM`

, which lifts an `if`

statement into the monadic context.

```
import cats._
import cats.implicits._
Monad[Option].ifM(Option(true))(Option("truthy"), Option("falsy")) should be(res0)
Monad[List].ifM(List(true, false, true))(List(1, 2), List(3, 4)) should be(res1)
```

Unlike `Functor`

s and `Applicative`

s, you cannot derive a monad instance for a generic `M[N[_]]`

where both `M[_]`

and `N[_]`

have an instance of a monad.

However, it is common to want to compose the effects of both `M[_]`

and `N[_]`

. One way of expressing this
is to provide instructions on how to compose any outer monad (`F`

in the following example) with a specific
inner monad (`Option`

in the following example).

```
case class OptionT[F[_], A](value: F[Option[A]])
implicit def optionTMonad[F[_]](implicit F: Monad[F]) = {
new Monad[OptionT[F, *]] {
def pure[A](a: A): OptionT[F, A] = OptionT(F.pure(Some(a)))
def flatMap[A, B](fa: OptionT[F, A])(f: A => OptionT[F, B]): OptionT[F, B] =
OptionT {
F.flatMap(fa.value) {
case None => F.pure(None)
case Some(a) => f(a).value
}
}
def tailRecM[A, B](a: A)(f: A => OptionT[F, Either[A, B]]): OptionT[F, B] =
defaultTailRecM(a)(f)
}
}
```

This sort of construction is called a monad transformer. Cats already provides
a monad transformer for `Option`

called `OptionT`

.

```
import cats.implicits._
optionTMonad[List].pure(42) should be(OptionT(res0))
```