``` //given private fun getUserRole(user: User): E...

# arrow

[Fred Friis]

//given
private fun getUserRole(user: User): Either<IError, SearchUserResponse> {
}

//this generates an explosion of calls
users
  .asSequence()
  .map { getUserRole(it) }
  .sequence(Either.applicative())
  .map { it.fix() }
  .map { it.toSortedSet(comparator) }

//all we want is to call getUserRole one by one
//AND short circuit on the first error
//ie if there's 100 users and we get an error on
//the 2nd one, we don't want to call getUserRole 
//another 98 times

[simon.vergauwen]

users.traverse(Either.applicative()) { user ->
  getUserRole(user)
}.map { it.fix().toSortedSet(comparator) }

I'm assuming users is of type Set? If it's of type Set then you need to first convert to a data type that guarantees order, or won't discard results like Set does.

[simon.vergauwen]

traverse is like sequence but it combines itself with the map operation, so that you can do.

List<User> -> Either<IError, List<SearchUserResponse> with a single function call.

[simon.vergauwen]

In 0.11.0-SNAPSHOT you could also simply do the following:

either {
  users.map { !getUserRole(it) }
       .toSortedSet(comparator)
}

Here ! will short-circuit on the first Left as well, returning it's IError.

[Jannis]

What version are you on?
This is a bug where traverse evaluated all elements of a sequence before it short-circuits. It should be fixed on the latest snapshot 0.10.5. @simon.vergauwen's versions also all need the latest snapshot at least 0.10.5 to be lazy, although they are nicer code^^
On 0.11 you can either use traverse/sequence or just straight up use the either dsl and any inline function.
If you don't want to upgrade you can probably also use the normal eitherfx dsl if sequence map is inline Either.fx { seq.map { stuff().bind() } }.

[Jannis]

Actually never mind that, 0.10.5 should have the fixed traverse instance as well

[Fred Friis]

users are a set, yes. Awesome, big thanks for the info!!!