Contents

compactMap(_:)

Creates an asynchronous sequence that maps the given closure over the asynchronous sequence’s elements, omitting results that don’t return a value.

Declaration

@preconcurrency func compactMap<ElementOfResult>(_ transform: @escaping  @Sendable (Self.Element) async -> ElementOfResult?) -> AsyncCompactMapSequence<Self, ElementOfResult>

Parameters

  • transform:

    A mapping closure. transform accepts an element of this sequence as its parameter and returns a transformed value of the same or of a different type.

Return Value

An asynchronous sequence that contains, in order, the non-nil elements produced by the transform closure.

Discussion

Use the compactMap(_:) method to transform every element received from a base asynchronous sequence, while also discarding any nil results from the closure. Typically, you use this to transform from one type of element to another.

In this example, an asynchronous sequence called Counter produces Int values from 1 to 5. The closure provided to the compactMap(_:) method takes each Int and looks up a corresponding String from a romanNumeralDict dictionary. Because there is no key for 4, the closure returns nil in this case, which compactMap(_:) omits from the transformed asynchronous sequence.

let romanNumeralDict: [Int: String] =
    [1: "I", 2: "II", 3: "III", 5: "V"]
    
let stream = Counter(howHigh: 5)
    .compactMap { romanNumeralDict[$0] }
for await numeral in stream {
    print(numeral, terminator: " ")
}
// Prints "I II III V "