Proposal for joined and joined(separator:) (#180)

* Proposal for `joined` and `joined(separator:)`

* Small fixes

* Update proposal # to 0004

Co-authored-by: Philippe Hausler <[email protected]>

Author: FranzBusch

Evolution/0004-joined.md

@@ -0,0 +1,67 @@
+# Joined
+
+* Proposal: [SAA-0004](https://github.com/apple/swift-async-algorithms/blob/main/Evolution/0004-joined.md)
+* Authors: [Philippe Hausler](https://github.com/phausler)
+* Review Manager: [Franz Busch](https://github.com/FranzBusch)
+* Status: **Implemented**
+
+* Implementation: [[Source](https://github.com/apple/swift-async-algorithms/blob/main/Sources/AsyncAlgorithms/AsyncJoinedSequence.swift) | 
+[Tests](https://github.com/apple/swift-async-algorithms/blob/main/Tests/AsyncAlgorithmsTests/TestJoin.swift)]
+* Decision Notes: 
+* Bugs: 
+
+## Introduction
+
+The `joined()` and `joined(separator:)` algorithms on `AsyncSequence`s provide APIs to concatenate an `AsyncSequence` of `AsyncSequence`s.
+
+```swift
+extension AsyncSequence where Element: AsyncSequence {
+  public func joined() -> AsyncJoinedSequence<Self>
+}
+
+extension AsyncSequence where Element: AsyncSequence {
+  public func joined<Separator: AsyncSequence>(separator: Separator) -> AsyncJoinedBySeparatorSequence<Self, Separator>
+}
+```
+
+## Detailed Design
+
+These algorithms iterate over the elements of each `AsyncSequence` one bye one, i.e. only after the iteration of one `AsyncSequence` has finished the next one will be started.
+
+```swift
+ let appleFeed = URL("http://www.example.com/ticker?symbol=AAPL").lines
+ let nasdaqFeed = URL("http://www.example.com/ticker?symbol=^IXIC").lines
+
+ for try await line in [appleFeed, nasdaqFeed].async.joined() {
+   print("\(line)")
+ }
+ ```
+
+ Given some sample inputs the following combined events can be expected.
+
+ | Timestamp   | appleFeed | nasdaqFeed | output                        |                 
+ | ----------- | --------- | ---------- | ----------------------------- |
+ | 11:40 AM    | 173.91    |            | 173.91                        |
+ | 12:25 AM    |           | 14236.78   |                               |
+ | 12:40 AM    |           | 14218.34   |                               |
+ |  1:15 PM    | 173.00    |            | 173.00                        |
+ |  1:15 PM    |           |            | 14236.78                      |
+ |  1:15 PM    |           |            | 14218.34                      |
+
+
+The `joined()` and `joined(separator:)` methods are available on `AsyncSequence`s with elements that are `AsyncSequence`s themselves and produce either an `AsyncJoinedSequence` or an `AsyncJoinedBySeparatorSequence`. 
+
+As soon as an inner `AsyncSequence` returns `nil` the algorithm continues with iterating the next inner `AsyncSequence`.
+
+The throwing behaviour of `AsyncJoinedSequence` and `AsyncJoinedBySeparatorSequence` is that if any of the inner `AsyncSequence`s throws, then the composed sequence throws on its iteration.
+
+### Naming
+
+The naming follows to current method naming of the standard library's [`joined`](https://developer.apple.com/documentation/swift/array/joined(separator:)-7uber) method.
+Prior art in the reactive community often names this method `concat`; however, we think that an alignment with the current method on `Sequence` is better.
+
+### Comparison with other libraries
+
+**ReactiveX** ReactiveX has an [API definition of Concat](https://reactivex.io/documentation/operators/concat.html) as a top level function for concatenating Observables.
+
+**Combine** Combine has an [API definition of append](https://developer.apple.com/documentation/combine/publisher/append(_:)-5yh02) which offers similar functionality but limited to concatenating two individual `Publisher`s.