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AsyncThrowingFlatMapSequence.swift
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AsyncThrowingFlatMapSequence.swift
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//===----------------------------------------------------------------------===//
//
// This source file is part of the Swift.org open source project
//
// Copyright (c) 2021 Apple Inc. and the Swift project authors
// Licensed under Apache License v2.0 with Runtime Library Exception
//
// See https://swift.org/LICENSE.txt for license information
// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
//
//===----------------------------------------------------------------------===//
import Swift
@available(SwiftStdlib 5.1, *)
extension AsyncSequence {
/// Creates an asynchronous sequence that concatenates the results of calling
/// the given error-throwing transformation with each element of this
/// sequence.
///
/// 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`. However, when the closure receives `4`, it throws an
/// error, terminating the sequence.
///
/// do {
/// let stream = Counter(howHigh: 5)
/// .flatMap { (value) -> Counter in
/// if value == 4 {
/// throw MyError()
/// }
/// return Counter(howHigh: value)
/// }
/// for try await number in stream {
/// print(number, terminator: " ")
/// }
/// } catch {
/// print(error)
/// }
/// // Prints "1 1 2 1 2 3 MyError() "
///
/// - Parameter transform: An error-throwing mapping closure. `transform`
/// accepts an element of this sequence as its parameter and returns an
/// `AsyncSequence`. If `transform` throws an error, the sequence ends.
/// - Returns: A single, flattened asynchronous sequence that contains all
/// elements in all the asynchronous sequences produced by `transform`. The
/// sequence ends either when the last sequence created from the last
/// element from base sequence ends, or when `transform` throws an error.
@preconcurrency
@inlinable
public __consuming func flatMap<SegmentOfResult: AsyncSequence>(
_ transform: @Sendable @escaping (Element) async throws -> SegmentOfResult
) -> AsyncThrowingFlatMapSequence<Self, SegmentOfResult> {
return AsyncThrowingFlatMapSequence(self, transform: transform)
}
}
/// An asynchronous sequence that concatenates the results of calling a given
/// error-throwing transformation with each element of this sequence.
@available(SwiftStdlib 5.1, *)
public struct AsyncThrowingFlatMapSequence<Base: AsyncSequence, SegmentOfResult: AsyncSequence> {
@usableFromInline
let base: Base
@usableFromInline
let transform: (Base.Element) async throws -> SegmentOfResult
@usableFromInline
init(
_ base: Base,
transform: @escaping (Base.Element) async throws -> SegmentOfResult
) {
self.base = base
self.transform = transform
}
}
@available(SwiftStdlib 5.1, *)
extension AsyncThrowingFlatMapSequence: AsyncSequence {
/// The type of element produced by this asynchronous sequence.
///
/// The flat map sequence produces the type of element in the asynchronous
/// sequence produced by the `transform` closure.
public typealias Element = SegmentOfResult.Element
/// The type of error produced by this asynchronous sequence.
///
/// The flat map sequence produces errors from either the base
/// sequence or the `transform` closure.
public typealias Failure = any Error
/// The type of iterator that produces elements of the sequence.
public typealias AsyncIterator = Iterator
/// The iterator that produces elements of the flat map sequence.
public struct Iterator: AsyncIteratorProtocol {
@usableFromInline
var baseIterator: Base.AsyncIterator
@usableFromInline
let transform: (Base.Element) async throws -> SegmentOfResult
@usableFromInline
var currentIterator: SegmentOfResult.AsyncIterator?
@usableFromInline
var finished = false
@usableFromInline
init(
_ baseIterator: Base.AsyncIterator,
transform: @escaping (Base.Element) async throws -> SegmentOfResult
) {
self.baseIterator = baseIterator
self.transform = transform
}
/// Produces the next element in the flat map sequence.
///
/// This iterator calls `next()` on its base iterator; if this call returns
/// `nil`, `next()` returns `nil`. Otherwise, `next()` calls the
/// transforming closure on the received element, takes the resulting
/// asynchronous sequence, and creates an asynchronous iterator from it.
/// `next()` then consumes values from this iterator until it terminates.
/// At this point, `next()` is ready to receive the next value from the base
/// sequence. If `transform` throws an error, the sequence terminates.
@inlinable
public mutating func next() async throws -> SegmentOfResult.Element? {
while !finished {
if var iterator = currentIterator {
do {
guard let element = try await iterator.next() else {
currentIterator = nil
continue
}
// restore the iterator since we just mutated it with next
currentIterator = iterator
return element
} catch {
finished = true
throw error
}
} else {
guard let item = try await baseIterator.next() else {
return nil
}
let segment: SegmentOfResult
do {
segment = try await transform(item)
var iterator = segment.makeAsyncIterator()
guard let element = try await iterator.next() else {
currentIterator = nil
continue
}
currentIterator = iterator
return element
} catch {
finished = true
currentIterator = nil
throw error
}
}
}
return nil
}
/// Produces the next element in the flat map sequence.
///
/// This iterator calls `next(isolation:)` on its base iterator; if this
/// call returns `nil`, `next(isolation:)` returns `nil`. Otherwise,
/// `next(isolation:)` calls the transforming closure on the received
/// element, takes the resulting asynchronous sequence, and creates an
/// asynchronous iterator from it. `next(isolation:)` then consumes values
/// from this iterator until it terminates. At this point,
/// `next(isolation:)` is ready to receive the next value from the base
/// sequence. If `transform` throws an error, the sequence terminates.
@available(SwiftStdlib 6.0, *)
@inlinable
public mutating func next(isolation actor: isolated (any Actor)?) async throws(Failure) -> SegmentOfResult.Element? {
while !finished {
if var iterator = currentIterator {
do {
guard let element = try await iterator.next(isolation: actor) else {
currentIterator = nil
continue
}
// restore the iterator since we just mutated it with next
currentIterator = iterator
return element
} catch {
finished = true
throw error
}
} else {
guard let item = try await baseIterator.next(isolation: actor) else {
return nil
}
let segment: SegmentOfResult
do {
segment = try await transform(item)
var iterator = segment.makeAsyncIterator()
guard let element = try await iterator.next(isolation: actor) else {
currentIterator = nil
continue
}
currentIterator = iterator
return element
} catch {
finished = true
currentIterator = nil
throw error
}
}
}
return nil
}
}
@inlinable
public __consuming func makeAsyncIterator() -> Iterator {
return Iterator(base.makeAsyncIterator(), transform: transform)
}
}
@available(SwiftStdlib 5.1, *)
extension AsyncThrowingFlatMapSequence: @unchecked Sendable
where Base: Sendable,
Base.Element: Sendable,
SegmentOfResult: Sendable,
SegmentOfResult.Element: Sendable { }
@available(SwiftStdlib 5.1, *)
extension AsyncThrowingFlatMapSequence.Iterator: @unchecked Sendable
where Base.AsyncIterator: Sendable,
Base.Element: Sendable,
SegmentOfResult: Sendable,
SegmentOfResult.Element: Sendable,
SegmentOfResult.AsyncIterator: Sendable { }