# NonEmptyTraverse

API Documentation: NonEmptyTraverse

`NonEmptyTraverse`

is a non-empty version of the Traverse type class, just like Reducible is a non-empty version of Foldable.
As such, it extends both `Reducible`

and `Traverse`

in the type class hierarchy.
It provides the `nonEmptyTraverse`

and `nonEmptySequence`

methods that require an instance of `Apply`

instead of `Applicative`

:

```
def nonEmptyTraverse[G[_]: Apply, A, B](fa: F[A])(f: A => G[B]): G[F[B]]
def nonEmptySequence[G[_]: Apply, A](fga: F[G[A]]): G[F[A]]
```

In the Applicative tutorial we learned of `Apply`

as a weakened `Applicative`

lacking the `pure`

method.
One example type lacking an `Applicative`

instance is `Map[K, *]`

, it's impossible to implement a `pure`

method for it.

Knowing this, we can make use of `NonEmptyTraverse`

, to traverse over a sequence of `Map`

s.
One example application one could think of is, when we have a list of text snippets,
count the occurrence of each word in each snippet and return all the common words and their occurrences in each snippet:

```
import cats.syntax.all._
import cats.data.NonEmptyList
val snippets = NonEmptyList.of("What do you do", "What are you doing")
// snippets: NonEmptyList[String] = NonEmptyList(
// head = "What do you do",
// tail = List("What are you doing")
// )
def countWords(text: String): Map[String, Int] =
text.split(" ").groupBy(identity).view.mapValues(_.length).toMap
snippets.nonEmptyTraverse(countWords)
// res0: Map[String, NonEmptyList[Int]] = Map(
// "What" -> NonEmptyList(head = 1, tail = List(1)),
// "you" -> NonEmptyList(head = 1, tail = List(1))
// )
```

Note that, just like `traverse`

, `nonEmptyTraverse(f)`

is equivalent to `map(f).nonEmptySequence`

, so the above could be rewritten as:

```
snippets.map(countWords).nonEmptySequence
// res1: Map[String, NonEmptyList[Int]] = Map(
// "What" -> NonEmptyList(head = 1, tail = List(1)),
// "you" -> NonEmptyList(head = 1, tail = List(1))
// )
```

`NonEmptyTraverse`

also offers `flatNonEmptyTraverse`

and `flatNonEmptySequence`

methods that are analogous to `flatTraverse`

and `flatSequence`

in `Traverse`

.
Just like with `nonEmptyTraverse`

these methods don't require a `Monad`

instance, but only a `FlatMap`

, which is the weakened version of `Monad`

without `pure`

.