feat: refactor iterators (#41)

* fix: check for cancellation

* feat: extract iterator into own struct

Asynchrone/Source/Extensions/AsyncSequence+Extension.swift

@@ -125,6 +125,7 @@ extension AsyncSequence {
         Task(priority: priority) {
             for try await element in self {
                 await receiveValue(element)
+                try Task.checkCancellation()
             }
         }
     }

Asynchrone/Source/Sequences/RemoveDuplicatesAsyncSequence.swift

@@ -29,8 +29,6 @@ public struct RemoveDuplicatesAsyncSequence<Base: AsyncSequence>: AsyncSequence
     // Private
     private let base: Base
     private let predicate: Predicate
-    private var iterator: Base.AsyncIterator
-    private var previousElement: Base.Element?
 
     // MARK: Initialization
 
@@ -45,55 +43,65 @@ public struct RemoveDuplicatesAsyncSequence<Base: AsyncSequence>: AsyncSequence
     ) {
         self.base = base
         self.predicate = predicate
-        self.iterator = base.makeAsyncIterator()
     }
 
     // MARK: AsyncSequence
 
     /// Creates an async iterator that emits elements of this async sequence.
     /// - Returns: An instance that conforms to `AsyncIteratorProtocol`.
-    public func makeAsyncIterator() -> Self {
-        .init(base: self.base, predicate: self.predicate)
+    public func makeAsyncIterator() -> Iterator {
+        Iterator(base: self.base, predicate: self.predicate)
     }
 }
 
 extension RemoveDuplicatesAsyncSequence: Sendable
 where
-Base: Sendable,
-Base.AsyncIterator: Sendable,
-Base.Element: Sendable {}
+Base: Sendable {}
 
-// MARK: AsyncIteratorProtocol
+// MARK: Iterator
 
-extension RemoveDuplicatesAsyncSequence: AsyncIteratorProtocol {
-    /// Produces the next element in the sequence.
-    ///
-    /// Continues to call `next()` on it's base iterator and discard the
-    /// results if the predicate returns true.
-    ///
-    /// The first element of the sequence is always returned.
-    ///
-    /// If the base iterator returns `nil`, indicating the end of the sequence, this
-    /// iterator returns `nil`.
-    /// - Returns: The next element or `nil` if the end of the sequence is reached.
-    public mutating func next() async rethrows -> Element? {
-        let element = try await self.iterator.next()
-        let previousElement = self.previousElement
+extension RemoveDuplicatesAsyncSequence {
+    public struct Iterator: AsyncIteratorProtocol {
+        private let predicate: Predicate
+        private var iterator: Base.AsyncIterator
+        private var previousElement: Base.Element?
 
-        // Update previous element
-        self.previousElement = element
+        // MARK: Initialization
+
+        init(
+            base: Base,
+            predicate: @escaping Predicate
+        ) {
+            self.iterator = base.makeAsyncIterator()
+            self.predicate = predicate
+        }
 
-        guard let unwrappedElement = element,
-              let unwrappedPreviousElement = previousElement else { return element }
+        // MARK: AsyncIteratorProtocol
 
-        if self.predicate(unwrappedPreviousElement, unwrappedElement) {
-            return try await self.next()
-        } else {
-            return element
+        public mutating func next() async rethrows -> Element? {
+            let element = try await self.iterator.next()
+            let previousElement = self.previousElement
+
+            // Update previous element
+            self.previousElement = element
+
+            guard let unwrappedElement = element,
+                  let unwrappedPreviousElement = previousElement else { return element }
+
+            if self.predicate(unwrappedPreviousElement, unwrappedElement) {
+                return try await self.next()
+            } else {
+                return element
+            }
         }
     }
 }
 
+extension RemoveDuplicatesAsyncSequence.Iterator: Sendable
+where
+Base.AsyncIterator: Sendable,
+Base.Element: Sendable {}
+
 // MARK: Remove duplicates
 
 extension AsyncSequence where Element: Equatable {

Asynchrone/Source/Sequences/ReplaceErrorAsyncSequence.swift

@@ -20,7 +20,6 @@ public struct ReplaceErrorAsyncSequence<Base: AsyncSequence>: AsyncSequence {
     // Private
     private let base: Base
     private let replacement: Element
-    private var iterator: Base.AsyncIterator
 
     // MARK: Initialization
 
@@ -31,38 +30,56 @@ public struct ReplaceErrorAsyncSequence<Base: AsyncSequence>: AsyncSequence {
     public init(base: Base, output: Element) {
         self.base = base
         self.replacement = output
-        self.iterator = base.makeAsyncIterator()
     }
 
     // MARK: AsyncSequence
 
     /// Creates an async iterator that emits elements of this async sequence.
     /// - Returns: An instance that conforms to `AsyncIteratorProtocol`.
-    public func makeAsyncIterator() -> Self {
-        .init(base: self.base, output: self.replacement)
+    public func makeAsyncIterator() -> Iterator {
+        Iterator(base: self.base, output: self.replacement)
     }
 }
 
 extension ReplaceErrorAsyncSequence: Sendable
 where
 Base: Sendable,
-Base.Element: Sendable,
-Base.AsyncIterator: Sendable {}
+Base.Element: Sendable {}
 
-// MARK: AsyncIteratorProtocol
+// MARK: Iterator
 
-extension ReplaceErrorAsyncSequence: AsyncIteratorProtocol {
-    /// Produces the next element in the sequence.
-    /// - Returns: The next element or `nil` if the end of the sequence is reached.
-    public mutating func next() async -> Element? {
-        do {
-            return try await self.iterator.next()
-        } catch {
-            return self.replacement
+extension ReplaceErrorAsyncSequence {
+    public struct Iterator: AsyncIteratorProtocol {
+        private let replacement: Element
+        private var iterator: Base.AsyncIterator
+
+        // MARK: Initialization
+
+        init(
+            base: Base,
+            output: Element
+        ) {
+            self.iterator = base.makeAsyncIterator()
+            self.replacement = output
+        }
+
+        // MARK: AsyncIteratorProtocol
+
+        public mutating func next() async -> Element? {
+            do {
+                return try await self.iterator.next()
+            } catch {
+                return self.replacement
+            }
         }
     }
 }
 
+extension ReplaceErrorAsyncSequence.Iterator: Sendable
+where
+Base.AsyncIterator: Sendable,
+Base.Element: Sendable {}
+
 // MARK: Replace error
 
 extension AsyncSequence {