flatMap(_:)

Declaration

@preconcurrency func flatMap<SegmentOfResult>(_ transform: @escaping  @Sendable (Self.Element) async -> SegmentOfResult) -> AsyncFlatMapSequence<Self, SegmentOfResult> where SegmentOfResult : AsyncSequence, Self.Failure == Never, SegmentOfResult.Failure == Never

Parameters

• transform:

  A mapping closure. transform accepts an element of this sequence as its parameter and returns an AsyncSequence.

Return Value

A single, flattened asynchronous sequence that contains all elements in all the asynchronous sequences produced by transform.

Discussion

Use this method to receive a single-level asynchronous sequence when your transformation produces an asynchronous sequence for each element.

In this example, an asynchronous sequence called Counter produces Int values from 1 to 5. The transforming closure takes the received Int and returns a new Counter that counts that high. For example, when the transform receives 3 from the base sequence, it creates a new Counter that produces the values 1, 2, and 3. The flatMap(_:) method “flattens” the resulting sequence-of-sequences into a single AsyncSequence.

let stream = Counter(howHigh: 5)
    .flatMap { Counter(howHigh: $0) }
for await number in stream {
    print(number, terminator: " ")
}
// Prints "1 1 2 1 2 3 1 2 3 4 1 2 3 4 5 "