This looks cool. Quick nit-pick inline:
> On 16 Feb 2017, at 18:03, T.J. Usiyan via swift-evolution
> <[email protected]> wrote:
>
> # Pure Functions
>
> * Proposal:
> [SE-NNNN](https://github.com/apple/swift-evolution/blob/master/proposals/NNNN-name.md
>
> <https://github.com/apple/swift-evolution/blob/master/proposals/NNNN-name.md>)
> * Author(s): [TJ Usiyan](https://github.com/griotspeak
> <https://github.com/griotspeak>)
> * Status: **Awaiting review**
> * Review manager: TBD
>
> ## Introduction
>
> Some functions are, essentially, only meant to be transformations of their
> input and–as such–do not and should not reference any variables other than
> those passed in. These same functions are not meant to have any effects other
> than the aforementioned transformation of input. Currently, Swift cannot
> assist the developer and confirm that any given function is one of these
> 'pure' functions. To facilitate this, this proposal adds syntax to signal
> that a function is 'pure'.
>
> 'pure', in this context, means:
> 1. The function must have a return value
> 1. This function can only call other pure functions
> 1. This function cannot access/modify global or static variables.
>
> ## Motivation
>
> Consider the following example where `_computeNullability(of:)` is meant to
> create its output solely based on the provided recognizer.
>
> ```
> class Recognizer {
> var nullabilityMemo: Bool?
> var isNullable: Bool {
> func _computeNullability(of recognizer: Recognizer) -> Bool {…}
> if let back = nullabilityMemo {
> return back
> } else {
> let back = _computeNullability(of: self)
> nullabilityMemo = back
> return back
> }
> }
> }
> ```
> if `_computeNullability(of:)` is recursive at all, there exists a real
> potential to accidentally reference `self` in its body and the mistake,
> depending on circumstance, can be terribly subtle. Converting
> `_computeNullability(of:)` to a `static` function is an option but obfuscates
> the fact that it is *only* to be called within `isNullable`.
>
>
> ## Proposed solution
>
> Given the ability to indicate that `_computeNullability(of:)` is a 'pure'
> function, the developer gains assurance from the tooling that it doesn't
> reference anything or cause any side effects.
>
>
> ```
> class Recognizer {
> var nullabilityMemo: Bool?
> var isNullable: Bool {
> pfunc _computeNullability(of recognizer: Recognizer) -> Bool {…}
You still use `pfunc` in the example above. Shouldn’t it be using => ?
> if let back = nullabilityMemo {
> return back
> } else {
> let back = _computeNullability(of: self)
> nullabilityMemo = back
> return back
> }
> }
> }
> ```
>
> ## Detailed design
>
> This proposal introduces a new annotation `=>`, which is to be accepted
> everywhere `->` currently is. Members created using this kewyord must follow
> the rules listed in the introduction.
>
> ## Impact on existing code
>
> This is an additive feature unless alternative 2 is chosen and, as such,
> should not require an effect on existing code. It could be used to annotate
> closures accepted by methods in the standard library such as `map`, `filter`,
> and `reduce`. While this would fit well with their typical use, such a change
> is not necessarily part of this proposal.
>
> ## Alternatives considered
>
> It should be noted that neither of these alternatives can remain consistent
> for inline closures.
> 1. keyword `pfunc` (pronounciation: pifəŋk) for 'pure' functions.
> 2. `proc` keyword for 'impure' functions and 'func' for 'pure' functions.
> This would be a massively source breaking change and, as such, is unlikely to
> have any feasibility. It is, however, the most clean semantically, in my
> opinion.
>
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