Asynchronous iteration using Swift Combine

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!

See this recent answer of mine. testStrings should not return Future. — Sweeper
– Sweeper, Commented Dec 4, 2020 at 9:35
What should it return then? I tried replacing it with AnyPublisher<Void,Error> and adding .eraseToAnyPublisher() but that gives me other errors — Joris
– Joris, Commented Dec 4, 2020 at 9:44
Yes that's what you should do. What other error did it give you? Please edit your question with what you changed, and the new error. — Sweeper
– Sweeper, Commented Dec 4, 2020 at 9:45
The issue was I needed to replace the map with flatMap (don't ask me why...) — Joris
– Joris, Commented Dec 4, 2020 at 9:47
@JorisMans, map and flatMap are quite different. map emits a mapped value for each upstream value. flatMap maps a publisher for each upstream and emits all the values of that publisher. — New Dev
– New Dev, Commented Dec 4, 2020 at 15:50

Joris · Accepted Answer · 2020-12-04 09:47:21Z

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)

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