Asynchronous iteration using Swift Combine - swift

Question

I am trying to do multiple async operations, in sequence, on an array of data. However I am having problems with the return values of map.
Here is the test code:
import Combine
func getLength(_ string: String) -> Future<Int,Error> {
return Future<Int,Error>{ promise in
print("Length \(string.count)")
promise(.success(string.count))
}
}
func isEven(_ int: Int) -> Future<Bool,Error> {
return Future<Bool,Error>{ promise in
print("Even \(int % 2 == 0)")
promise(.success(int % 2 == 0))
}
}
let stringList = ["a","bbb","c","dddd"]
func testStrings(_ strings:ArraySlice<String>) -> Future<Void,Error> {
var remaining = strings
if let first = remaining.popFirst() {
return getLength(first).map{ length in
return isEven(length)
}.map{ even in
return testStrings(remaining)
}
} else {
return Future { promise in
promise(.success(()))
}
}
}
var storage = Set<AnyCancellable>()
testStrings(ArraySlice<String>(stringList)).sink { _ in } receiveValue: { _ in print("Done") }.store(in: &storage)
This generates the following error:
error: MyPlayground.playground:26:11: error: cannot convert return expression of type 'Publishers.Map<Future<Int, Error>, Future<Void, Error>>' to return type 'Future<Void, Error>'
}.map{ even in
I thought we could use map to convert from one publisher type to the other, but it seems it's wrapped inside a Publishers.Map. How do I get rid of this?
Thanks!

Answer 1

Well it seems that this works:
import Combine
func getLength(_ string: String) -> Future<Int,Error> {
return Future<Int,Error>{ promise in
print("Length \(string.count)")
promise(.success(string.count))
}
}
func isEven(_ int: Int) -> Future<Bool,Error> {
return Future<Bool,Error>{ promise in
print("Even \(int % 2 == 0)")
promise(.success(int % 2 == 0))
}
}
let stringList = ["a","bbb","c","dddd"]
func testStrings(_ strings:ArraySlice<String>) -> AnyPublisher<Void,Error> {
var remaining = strings
if let first = remaining.popFirst() {
return getLength(first).flatMap{ length in
return isEven(length)
}.flatMap{ even in
return testStrings(remaining)
}.eraseToAnyPublisher()
} else {
return Future<Void,Error> { promise in
promise(.success(()))
}.eraseToAnyPublisher()
}
}
var storage = Set<AnyCancellable>()
testStrings(ArraySlice<String>(stringList)).sink { _ in } receiveValue: { _ in print("Done") }.store(in: &storage)