AsyncThrowingFilterSequence.swift

//===----------------------------------------------------------------------===//
//
// 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 contains, in order, the elements of
  /// the base sequence that satisfy the given error-throwing predicate.
  ///
  /// In this example, an asynchronous sequence called `Counter` produces `Int`
  /// values from `1` to `10`. The `filter(_:)` method returns `true` for even
  /// values and `false` for odd values, thereby filtering out the odd values,
  /// but also throws an error for values divisible by 5:
  ///
  ///     do {
  ///         let stream = Counter(howHigh: 10)
  ///             .filter {
  ///                 if $0 % 5 == 0 {
  ///                     throw MyError()
  ///                 }
  ///                 return $0 % 2 == 0
  ///             }
  ///         for try await number in stream {
  ///             print(number, terminator: " ")
  ///         }
  ///     } catch {
  ///         print("Error: \(error)")
  ///     }
  ///     // Prints "2 4 Error: MyError() "
  ///
  /// - Parameter isIncluded: An error-throwing closure that takes an element
  ///   of the asynchronous sequence as its argument and returns a Boolean value
  ///   that indicates whether to include the element in the filtered sequence.
  /// - Returns: An asynchronous sequence that contains, in order, the elements
  ///   of the base sequence that satisfy the given predicate. If the predicate
  ///   throws an error, the sequence contains only values produced prior to
  ///   the error.
  @preconcurrency
  @inlinable
  public __consuming func filter(
    _ isIncluded: @Sendable @escaping (Element) async throws -> Bool
  ) -> AsyncThrowingFilterSequence<Self> {
    return AsyncThrowingFilterSequence(self, isIncluded: isIncluded)
  }
}

/// An asynchronous sequence that contains, in order, the elements of
/// the base sequence that satisfy the given error-throwing predicate.
@available(SwiftStdlib 5.1, *)
public struct AsyncThrowingFilterSequence<Base: AsyncSequence> {
  @usableFromInline
  let base: Base
  
  @usableFromInline
  let isIncluded: (Element) async throws -> Bool

  @usableFromInline
  init(
    _ base: Base, 
    isIncluded: @escaping (Base.Element) async throws -> Bool
  ) {
    self.base = base
    self.isIncluded = isIncluded
  }
}

@available(SwiftStdlib 5.1, *)
extension AsyncThrowingFilterSequence: AsyncSequence {
  /// The type of element produced by this asynchronous sequence.
  ///
  /// The filter sequence produces whatever type of element its base
  /// sequence produces.
  public typealias Element = Base.Element
  /// The type of iterator that produces elements of the sequence.
  public typealias AsyncIterator = Iterator

  /// The iterator that produces elements of the filter sequence.
  public struct Iterator: AsyncIteratorProtocol {
    @usableFromInline
    var baseIterator: Base.AsyncIterator

    @usableFromInline
    let isIncluded: (Base.Element) async throws -> Bool

    @usableFromInline
    var finished = false

    @usableFromInline
    init(
      _ baseIterator: Base.AsyncIterator,
      isIncluded: @escaping (Base.Element) async throws -> Bool
    ) {
      self.baseIterator = baseIterator
      self.isIncluded = isIncluded
    }

    /// Produces the next element in the filter sequence.
    ///
    /// This iterator calls `next()` on its base iterator; if this call returns
    /// `nil`, `next()` returns nil. Otherwise, `next()` evaluates the
    /// result with the `predicate` closure. If the closure returns `true`,
    /// `next()` returns the received element; otherwise it awaits the next
    /// element from the base iterator. If calling the closure throws an error,
    /// the sequence ends and `next()` rethrows the error.
    @inlinable
    public mutating func next() async throws -> Base.Element? {
      while !finished {
        guard let element = try await baseIterator.next() else {
          return nil
        }
        do {
          if try await isIncluded(element) {
            return element
          }
        } catch {
          finished = true
          throw error
        }
      }

      return nil
    }
  }

  @inlinable
  public __consuming func makeAsyncIterator() -> Iterator {
    return Iterator(base.makeAsyncIterator(), isIncluded: isIncluded)
  }
}

@available(SwiftStdlib 5.1, *)
extension AsyncThrowingFilterSequence: @unchecked Sendable 
  where Base: Sendable, 
        Base.Element: Sendable { }

@available(SwiftStdlib 5.1, *)
extension AsyncThrowingFilterSequence.Iterator: @unchecked Sendable 
  where Base.AsyncIterator: Sendable, 
        Base.Element: Sendable { }