Standard library behavior change for LazyMapCollection prefix to act as a Sequence

The Swift standard library provides lazy variations of its Sequence and Collection types, on which operations such as map and filter are implemented lazily. This means that the actual computations will only occur when the results are needed. In this Swift evolution pitch, Greg Titus proposes an addition to the standard library that improves the behavior of the LazyMapCollection type.

Extend implicit member syntax to cover chains of member references

This Swift evolution proposal suggests an expansion of implicit member syntax to more complex expressions than just a single static member or function. What this means is that when the type of an expression is implied by the context, developers would be able to use leading dot syntax, even when the expression consists of chains of member references. This is a very logical language evolution in my opinion.

[Concurrency] Add `async` to the Swift type system.

Modern development involves a lot of asynchronous programming, implemented in Swift using closures and completion handlers. However, implementing asynchronous operations using closures can quickly become problematic, and tasks such as error handling and conditional execution become harder to do correctly. Coroutines are a specialized type of subroutine/function which allows for execution to be suspended and resumed, and the coroutine model has long been proposed as a Swift language extension to make asynchronous programming more natural and less error-prone. In this pull request, Doug Gregor begins the introduction of coroutines to Swift, adding async to the Swift type system. A good first step towards a better asynchronous future.

Memoization of Swift properties

Memoization is an optimization technique where the results of expensive computations (e.g function calls or computed properties) are stored, and these cached results are returned when the inputs to future computations are unchanged. In this Swift evolution pitch, Justin Reusch proposes adding keywords to Swift which tell the compiler when to synthesize boilerplate code to support the memoization of computed properties. I think this would be a good addition to the language.