Concurrent async version of map on any Sequence

AsyncMap.swift

extension Sequence {

    func mapAsync<T>(_ handler: @escaping (Element) async -> T) async -> [T] {
        return await withTaskGroup(of: (Int, T).self) { group in
            var results: [Int: T] = [:]
            for (index, item) in self.enumerated() {
                group.async { (index, await handler(item)) }
            }
            for await (index, transformed) in group {
                results[index] = transformed
            }
            return self.enumerated().map { index, _ in results[index]! }
        }
    }
}

// Array of user ids
let ids = [...]

// Concurrently retrieve all users
let users = await ids.mapAsync { id in
  return await getUserAsync(id)
}