Integrate review feedback

This integrates all of the feedback from the review thread. Here is a quick summary:

- Change the trailing separator behaviour. We are no longer returning a separator before we are forwarding the error
- Add a synchronous and asynchronous closure based `interspersed` method.
- Support interspersing every n elements

Author: FranzBusch

Evolution/0011-interspersed.md

@@ -17,33 +17,84 @@ a separator element.
 ## Proposed solution
 
 We propose to add a new method on `AsyncSequence` that allows to intersperse
-a separator between each emitted element. This proposed API looks like this
+a separator between every n emitted element. This proposed API looks like this
 
 ```swift
-extension AsyncSequence {
-  /// Returns a new asynchronous sequence containing the elements of this asynchronous sequence, inserting
-  /// the given separator between each element.
-  ///
-  /// Any value of this asynchronous sequence's element type can be used as the separator.
-  ///
-  /// The following example shows how an async sequences of `String`s can be interspersed using `-` as the separator:
-  ///
-  /// ```
-  /// let input = ["A", "B", "C"].async
-  /// let interspersed = input.interspersed(with: "-")
-  /// for await element in interspersed {
-  ///   print(element)
-  /// }
-  /// // Prints "A" "-" "B" "-" "C"
-  /// ```
-  ///
-  /// - Parameter separator: The value to insert in between each of this async
-  ///   sequence’s elements.
-  /// - Returns: The interspersed asynchronous sequence of elements.
-  @inlinable
-  public func interspersed(with separator: Element) -> AsyncInterspersedSequence<Self> {
-    AsyncInterspersedSequence(self, separator: separator)
-  }
+public extension AsyncSequence {
+    /// Returns a new asynchronous sequence containing the elements of this asynchronous sequence, inserting
+    /// the given separator between each element.
+    ///
+    /// Any value of this asynchronous sequence's element type can be used as the separator.
+    ///
+    /// The following example shows how an async sequences of `String`s can be interspersed using `-` as the separator:
+    ///
+    /// ```
+    /// let input = ["A", "B", "C"].async
+    /// let interspersed = input.interspersed(with: "-")
+    /// for await element in interspersed {
+    ///   print(element)
+    /// }
+    /// // Prints "A" "-" "B" "-" "C"
+    /// ```
+    ///
+    /// - Parameters:
+    ///   - every: Dictates after how many elements a separator should be inserted.
+    ///   - separator: The value to insert in between each of this async sequence’s elements.
+    /// - Returns: The interspersed asynchronous sequence of elements.
+    @inlinable
+    func interspersed(every: Int = 1, with separator: Element) -> AsyncInterspersedSequence<Self> {
+        AsyncInterspersedSequence(self, every: every, separator: separator)
+    }
+
+    /// Returns a new asynchronous sequence containing the elements of this asynchronous sequence, inserting
+    /// the given separator between each element.
+    ///
+    /// Any value of this asynchronous sequence's element type can be used as the separator.
+    ///
+    /// The following example shows how an async sequences of `String`s can be interspersed using `-` as the separator:
+    ///
+    /// ```
+    /// let input = ["A", "B", "C"].async
+    /// let interspersed = input.interspersed(with: "-")
+    /// for await element in interspersed {
+    ///   print(element)
+    /// }
+    /// // Prints "A" "-" "B" "-" "C"
+    /// ```
+    ///
+    /// - Parameters:
+    ///   - every: Dictates after how many elements a separator should be inserted.
+    ///   - separator: A closure that produces the value to insert in between each of this async sequence’s elements.
+    /// - Returns: The interspersed asynchronous sequence of elements.
+    @inlinable
+    func interspersed(every: Int = 1, with separator: @Sendable @escaping () -> Element) -> AsyncInterspersedSequence<Self> {
+        AsyncInterspersedSequence(self, every: every, separator: separator)
+    }
+
+    /// Returns a new asynchronous sequence containing the elements of this asynchronous sequence, inserting
+    /// the given separator between each element.
+    ///
+    /// Any value of this asynchronous sequence's element type can be used as the separator.
+    ///
+    /// The following example shows how an async sequences of `String`s can be interspersed using `-` as the separator:
+    ///
+    /// ```
+    /// let input = ["A", "B", "C"].async
+    /// let interspersed = input.interspersed(with: "-")
+    /// for await element in interspersed {
+    ///   print(element)
+    /// }
+    /// // Prints "A" "-" "B" "-" "C"
+    /// ```
+    ///
+    /// - Parameters:
+    ///   - every: Dictates after how many elements a separator should be inserted.
+    ///   - separator: A closure that produces the value to insert in between each of this async sequence’s elements.
+    /// - Returns: The interspersed asynchronous sequence of elements.
+    @inlinable
+    func interspersed(every: Int = 1, with separator: @Sendable @escaping () async -> Element) -> AsyncInterspersedSequence<Self> {
+        AsyncInterspersedSequence(self, every: every, separator: separator)
+    }
 }
 ```
 
@@ -53,83 +104,166 @@ The bulk of the implementation of the new `interspersed` method is inside the ne
 `AsyncInterspersedSequence` struct. It constructs an iterator to the base async sequence
 inside its own iterator. The `AsyncInterspersedSequence.Iterator.next()` is forwarding the demand
 to the base iterator.
-There is one special case that we have to call out. When the base async sequence throws
-then `AsyncInterspersedSequence.Iterator.next()` will return the separator first and then rethrow the error.
 
 Below is the implementation of the `AsyncInterspersedSequence`.
 ```swift
 /// An asynchronous sequence that presents the elements of a base asynchronous sequence of
 /// elements with a separator between each of those elements.
 public struct AsyncInterspersedSequence<Base: AsyncSequence> {
-  @usableFromInline
-  internal let base: Base
-
-  @usableFromInline
-  internal let separator: Base.Element
-
-  @usableFromInline
-  internal init(_ base: Base, separator: Base.Element) {
-    self.base = base
-    self.separator = separator
-  }
-}
+    @usableFromInline
+    internal enum Separator {
+        case element(Element)
+        case syncClosure(@Sendable () -> Element)
+        case asyncClosure(@Sendable () async -> Element)
+    }
 
-extension AsyncInterspersedSequence: AsyncSequence {
-  public typealias Element = Base.Element
+    @usableFromInline
+    internal let base: Base
 
-  /// The iterator for an `AsyncInterspersedSequence` asynchronous sequence.
-  public struct AsyncIterator: AsyncIteratorProtocol {
     @usableFromInline
-    internal enum State {
-      case start
-      case element(Result<Base.Element, Error>)
-      case separator
-    }
+    internal let separator: Separator
 
     @usableFromInline
-    internal var iterator: Base.AsyncIterator
+    internal let every: Int
 
     @usableFromInline
-    internal let separator: Base.Element
+    internal init(_ base: Base, every: Int, separator: Element) {
+        precondition(every > 0, "Separators can only be interspersed ever 1+ elements")
+        self.base = base
+        self.separator = .element(separator)
+        self.every = every
+    }
 
     @usableFromInline
-    internal var state = State.start
+    internal init(_ base: Base, every: Int, separator: @Sendable @escaping () -> Element) {
+        precondition(every > 0, "Separators can only be interspersed ever 1+ elements")
+        self.base = base
+        self.separator = .syncClosure(separator)
+        self.every = every
+    }
 
     @usableFromInline
-    internal init(_ iterator: Base.AsyncIterator, separator: Base.Element) {
-      self.iterator = iterator
-      self.separator = separator
+    internal init(_ base: Base, every: Int, separator: @Sendable @escaping () async -> Element) {
+        precondition(every > 0, "Separators can only be interspersed ever 1+ elements")
+        self.base = base
+        self.separator = .asyncClosure(separator)
+        self.every = every
     }
+}
 
-    public mutating func next() async rethrows -> Base.Element? {
-      // After the start, the state flips between element and separator. Before
-      // returning a separator, a check is made for the next element as a
-      // separator is only returned between two elements. The next element is
-      // stored to allow it to be returned in the next iteration. However, if
-      // the checking the next element throws, the separator is emitted before
-      // rethrowing that error.
-      switch state {
-        case .start:
-          state = .separator
-          return try await iterator.next()
-        case .separator:
-          do {
-            guard let next = try await iterator.next() else { return nil }
-            state = .element(.success(next))
-          } catch {
-            state = .element(.failure(error))
-          }
-          return separator
-        case .element(let result):
-          state = .separator
-          return try result._rethrowGet()
-      }
+extension AsyncInterspersedSequence: AsyncSequence {
+    public typealias Element = Base.Element
+
+    /// The iterator for an `AsyncInterspersedSequence` asynchronous sequence.
+    public struct Iterator: AsyncIteratorProtocol {
+        @usableFromInline
+        internal enum State {
+            case start(Element?)
+            case element(Int)
+            case separator
+            case finished
+        }
+
+        @usableFromInline
+        internal var iterator: Base.AsyncIterator
+
+        @usableFromInline
+        internal let separator: Separator
+
+        @usableFromInline
+        internal let every: Int
+
+        @usableFromInline
+        internal var state = State.start(nil)
+
+        @usableFromInline
+        internal init(_ iterator: Base.AsyncIterator, every: Int, separator: Separator) {
+            self.iterator = iterator
+            self.separator = separator
+            self.every = every
+        }
+
+        public mutating func next() async rethrows -> Base.Element? {
+            // After the start, the state flips between element and separator. Before
+            // returning a separator, a check is made for the next element as a
+            // separator is only returned between two elements. The next element is
+            // stored to allow it to be returned in the next iteration. However, if
+            // the checking the next element throws, the separator is emitted before
+            // rethrowing that error.
+            switch state {
+            case var .start(element):
+                do {
+                    if element == nil {
+                        element = try await self.iterator.next()
+                    }
+
+                    if let element = element {
+                        if every == 1 {
+                            state = .separator
+                        } else {
+                            state = .element(1)
+                        }
+                        return element
+                    } else {
+                        state = .finished
+                        return nil
+                    }
+                } catch {
+                    state = .finished
+                    throw error
+                }
+
+            case .separator:
+                do {
+                    if let element = try await iterator.next() {
+                        state = .start(element)
+                        switch separator {
+                        case let .element(element):
+                            return element
+
+                        case let .syncClosure(closure):
+                            return closure()
+
+                        case let .asyncClosure(closure):
+                            return await closure()
+                        }
+                    } else {
+                        state = .finished
+                        return nil
+                    }
+                } catch {
+                    state = .finished
+                    throw error
+                }
+
+            case let .element(count):
+                do {
+                    if let element = try await iterator.next() {
+                        let newCount = count + 1
+                        if every == newCount {
+                            state = .separator
+                        } else {
+                            state = .element(newCount)
+                        }
+                        return element
+                    } else {
+                        state = .finished
+                        return nil
+                    }
+                } catch {
+                    state = .finished
+                    throw error
+                }
+
+            case .finished:
+                return nil
+            }
+        }
     }
-  }
 
-  @inlinable
-  public func makeAsyncIterator() -> AsyncInterspersedSequence<Base>.AsyncIterator {
-        AsyncIterator(base.makeAsyncIterator(), separator: separator)
-  }
+    @inlinable
+    public func makeAsyncIterator() -> AsyncInterspersedSequence<Base>.Iterator {
+        Iterator(base.makeAsyncIterator(), every: every, separator: separator)
+    }
 }
 ```