Combine framework: how to process each element of array asynchronously before proceeding

With your latest edit and this comment below:

I literally am asking is there a Combine equivalent of "don't proceed to the next step until this step, involving multiple asynchronous steps, has finished"

I think this pattern can be achieved with .flatMap to an array publisher (Publishers.Sequence), which emits one-by-one and completes, followed by whatever per-element async processing is needed, and finalized with a .collect, which waits for all elements to complete before proceeding

So, in code, assuming we have these functions:

func getFoos() -> AnyPublisher<[Foo], Error>
func getPartials(for: Foo) -> AnyPublisher<[Partial], Error>
func getMoreInfo(for: Partial, of: Foo) -> AnyPublisher<MoreInfo, Error>

We can do the following:

getFoos()
.flatMap { fooArr in 
    fooArr.publisher.setFailureType(to: Error.self)
 }

// per-foo element async processing
.flatMap { foo in

  getPartials(for: foo)
    .flatMap { partialArr in
       partialArr.publisher.setFailureType(to: Error.self)
     }

     // per-partial of foo async processing
    .flatMap { partial in

       getMoreInfo(for: partial, of: foo)
         // build completed partial with more info
         .map { moreInfo in
            var newPartial = partial
            newPartial.moreInfo = moreInfo
            return newPartial
         }
     }
     .collect()
     // build completed foo with all partials
     .map { partialArr in
        var newFoo = foo
        newFoo.partials = partialArr
        return newFoo
     }
}
.collect()

(Deleted the old answer)

Using the accepted answer, I wound up with this structure:

head // [Entity]
    .flatMap { entities -> AnyPublisher<Entity, Error> in
        Publishers.Sequence(sequence: entities).eraseToAnyPublisher()
    }.flatMap { entity -> AnyPublisher<Entity, Error> in
        self.makeFuture(for: entity) // [Derivative]
            .flatMap { derivatives -> AnyPublisher<Derivative, Error> in
                Publishers.Sequence(sequence: derivatives).eraseToAnyPublisher()
            }
            .flatMap { derivative -> AnyPublisher<Derivative2, Error> in
                self.makeFuture(for: derivative).eraseToAnyPublisher() // Derivative2
        }.collect().map { derivative2s -> Entity in
            self.configuredEntity(entity, from: derivative2s)
        }.eraseToAnyPublisher()
    }.collect()

That has exactly the elegant tightness I was looking for! So the idea is:

We receive an array of something, and we need to process each element asynchronously. The old way would have been a DispatchGroup and a for...in loop. The Combine equivalent is:

• The equivalent of the for...in line is flatMap and Publishers.Sequence.

• The equivalent of the DispatchGroup (dealing with asynchronousness) is a further flatMap (on the individual element) and some publisher. In my case I start with a Future based on the individual element we just received.

• The equivalent of the right curly brace at the end is collect(), waiting for all elements to be processed and putting the array back together again.

So to sum up, the pattern is:

1. flatMap the array to a Sequence.
2. flatMap the individual element to a publisher that launches the asynchronous operation on that element.
3. Continue the chain from that publisher as needed.
4. collect back into an array.

By nesting that pattern, we can take advantage of Swift scoping rules to keep the thing we need to process in scope until we have acquired enough information to produce the processed object.