List[Try[T]] to Try[List[T]] in Scala

Question

I would like to know how to convert a List[Try[T]] into Try[List[T]] in Scala?

I have tried using an accumulator and folding right but it do

Answer 1

I would also recommend just using Cats...

But, if you do not want to add another (big) dependency to your project, just for one function.
You can implement it your self! - (cats implementation may be better)

import scala.util.{Try, Success, Failure}

def sequence[A](list: List[Try[A]]): Try[List[A]] = {
  @annotation.tailrec
  def loop(remaining: List[Try[A]], acc: List[A]): Try[List[A]] =
    remaining match {
      case Nil                => Success(acc.reverse)
      case Success(a) :: tail => loop(remaining = tail, acc = a :: acc)
      case Failure(e) :: _    => Failure(e)
    }
  loop(remaining = list, acc = List.empty)
}

Also, if you may use traverse instead of sequence if you did a map just before.

def traverse[A, B](list: List[A])(f: A => Try[B]): Try[List[B]] = {
  @annotation.tailrec
  def loop(remaining: List[A], acc: List[B]): Try[List[B]] =
    remaining match {
      case Nil          => Success(acc.reverse)
      case head :: tail => f(head) match {
        case Success(b) => loop(remaining = tail, acc = b :: acc)
        case Failure(e) => Failure(e)
      }
    }
  loop(remaining = list, acc = List.empty)
}

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Anyways, Cats (and FP in general) is very useful (as you have just seen).
Thus, I would recommend you to give it a try.