AsyncLazySequence proposal (#189)

Author: phausler

Evolution/NNNN-lazy.md

@@ -0,0 +1,48 @@
+# AsyncLazySequence
+
+* Proposal: [NNNN](NNNN-lazy.md)
+* Authors: [Philippe Hausler](https://github.com/phausler)
+* Status: **Implemented**
+
+* Implementation: 
+  [Source](https://github.com/apple/swift-async-algorithms/blob/main/Sources/AsyncAlgorithms/AsyncLazySequence.swift) | 
+  [Tests](https://github.com/apple/swift-async-algorithms/blob/main/Tests/AsyncAlgorithmsTests/TestLazy.swift)
+  
+## Introduction
+
+`AsyncLazySequence` converts a non-asynchronous sequence into an asynchronous one. 
+
+This operation is available for all `Sequence` types. 
+
+```swift
+let numbers = [1, 2, 3, 4].async
+let characters = "abcde".async
+```
+
+This transformation can be useful to test operations specifically available on `AsyncSequence` but also is useful 
+to combine with other `AsyncSequence` types to provide well known sources of data. 
+
+The `.async` property returns an `AsyncLazySequence` that is generic upon the base `Sequence` it was constructed from.
+
+```swift
+extension Sequence {
+  public var async: AsyncLazySequence<Self> { get }
+}
+
+public struct AsyncLazySequence<Base: Sequence>: AsyncSequence {
+  ...
+}
+
+extension AsyncLazySequence: Sendable where Base: Sendable { }
+extension AsyncLazySequence.Iterator: Sendable where Base.Iterator: Sendable { }
+```
+
+### Naming
+
+This property's and type's name match the naming approaches in the Swift standard library. The property is named with a 
+succinct name in inspiration from `.lazy`, and the type is named in reference to the lazy behavior of the constructed 
+`AsyncSequence`. 
+
+## Effect on API resilience
+
+`AsyncLazySequence` has a trivial implementation and is marked as `@frozen` and `@inlinable`. This removes the ability of this type and functions to be ABI resilient boundaries at the benefit of being highly optimizable.