Using Cats Traverse to turn a List of Try into a Try of List

sequence.scala

Welcome to the Ammonite Repl 0.8.0
(Scala 2.11.8 Java 1.8.0_91)

@ import scala.util.Try
import scala.util.Try

@ val listOfTries: List[Try[String]] = List(Try("a"), Try("b"), Try("c"))
listOfTries: List[Try[String]] = List(Success("a"), Success("b"), Success("c"))

// I have a List[Try[String]] but I actually want a Try[List[String]].
// I want it to be a Failure if any of the individual Trys was a Failure.
// Traverse to the rescue!

@ import $ivy.`org.typelevel::cats-core:0.8.1`
import $ivy.$

@ import cats.Traverse
import cats.Traverse

// This brings in an instance of Applicative for Try
@ import cats.instances.try_._
import cats.instances.try_._

// This brings in an instance of Traverse for List
@ import cats.instances.list._
import cats.instances.list._

@ val tryOfList = Traverse[List].sequence(listOfTries)
tryOfList: Try[List[String]] = Success(List("a", "b", "c"))

// Job done! Or, with a bit of syntax sugar...

@ import cats.syntax.traverse._
import cats.syntax.traverse._

@ val tryOfList2 = listOfTries.sequence
tryOfList2: Try[List[String]] = Success(List("a", "b", "c"))

// As pointed out by @philwills, the `traverse` method is also useful:
@ val listOfStrings = List("1", "2", "3")
listOfStrings: List[String] = List("1", "2", "3")

@ val tryOfList3 = listOfStrings.traverse(x => Try(x.toInt))
tryOfList3: Try[List[Int]] = Success(List(1, 2, 3))

@fancywriter: you need to use type Lambda:

https://stackoverflow.com/questions/55979167/scalaz-7-how-to-use-functor-with-function1