<html><head><meta http-equiv="Content-Type" content="text/html charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><div class=""><br class=""></div><br class=""><div><blockquote type="cite" class=""><div class="">On Feb 16, 2017, at 12:30 PM, Rien via swift-evolution <<a href="mailto:swift-evolution@swift.org" class="">swift-evolution@swift.org</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><div class="">In essence this is about assistance from the compiler that a function marked ‘pure’ is indeed pure?<br class="">I.e. an error message should be generated when a function marked as ‘pure’ is in fact not ‘pure’?<br class=""><br class="">If the answer to both questions is ‘yes’ then -not surprising- its a -1 from me.<br class=""><br class="">Unless there are other benefits?<br class=""></div></div></blockquote><div><br class=""></div><div>This feature<a href="https://github.com/apple/swift/blob/master/docs/proposals/OptimizerEffects.rst" class=""> already exists</a> in a certain sense - via the @effects annotation - but is undocumented, highly unstable, and does not entail any semantic checking. I think that at least indicates a desire, even if it’s only in the lower-level parts of SIL now, to have some way to determine the “purity” of a function to perhaps guide an inliner or future block fusion pass. Perhaps Andrew Trick can speak more about the goals of the annotation and whether it would be ready for prime time as it were. </div><br class=""><blockquote type="cite" class=""><div class=""><div class=""><br class="">Regards,<br class="">Rien<br class=""><br class="">Site: <a href="http://balancingrock.nl" class="">http://balancingrock.nl</a><br class="">Blog: <a href="http://swiftrien.blogspot.com" class="">http://swiftrien.blogspot.com</a><br class="">Github: <a href="http://github.com/Balancingrock" class="">http://github.com/Balancingrock</a><br class="">Project: <a href="http://swiftfire.nl" class="">http://swiftfire.nl</a><br class=""><br class=""><br class=""><br class=""><br class=""><br class=""><blockquote type="cite" class="">On 16 Feb 2017, at 18:03, T.J. Usiyan via swift-evolution <<a href="mailto:swift-evolution@swift.org" class="">swift-evolution@swift.org</a>> wrote:<br class=""><br class=""># Pure Functions<br class=""><br class="">* Proposal: [SE-NNNN](<a href="https://github.com/apple/swift-evolution/blob/master/proposals/NNNN-name.md" class="">https://github.com/apple/swift-evolution/blob/master/proposals/NNNN-name.md</a>)<br class="">* Author(s): [TJ Usiyan](<a href="https://github.com/griotspeak" class="">https://github.com/griotspeak</a>)<br class="">* Status: **Awaiting review**<br class="">* Review manager: TBD<br class=""><br class="">## Introduction<br class=""><br class="">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'.<br class=""><br class="">'pure', in this context, means:<br class="">1. The function must have a return value<br class="">1. This function can only call other pure functions<br class="">1. This function cannot access/modify global or static variables.<br class=""><br class="">## Motivation<br class=""><br class="">Consider the following example where `_computeNullability(of:)` is meant to create its output solely based on the provided recognizer.<br class=""><br class="">```<br class="">class Recognizer {<br class=""><span class="Apple-tab-span" style="white-space:pre"> </span>var nullabilityMemo: Bool?<br class=""><span class="Apple-tab-span" style="white-space:pre"> </span>var isNullable: Bool {<br class=""><span class="Apple-tab-span" style="white-space:pre"> </span><span class="Apple-tab-span" style="white-space:pre"> </span>func _computeNullability(of recognizer: Recognizer) -> Bool {…}<br class=""><span class="Apple-tab-span" style="white-space:pre"> </span><span class="Apple-tab-span" style="white-space:pre"> </span>if let back = nullabilityMemo {<br class=""><span class="Apple-tab-span" style="white-space:pre"> </span><span class="Apple-tab-span" style="white-space:pre"> </span><span class="Apple-tab-span" style="white-space:pre"> </span>return back<span class="Apple-tab-span" style="white-space:pre"> </span><span class="Apple-tab-span" style="white-space:pre"> </span><br class=""><span class="Apple-tab-span" style="white-space:pre"> </span><span class="Apple-tab-span" style="white-space:pre"> </span>} else {<br class=""><span class="Apple-tab-span" style="white-space:pre"> </span><span class="Apple-tab-span" style="white-space:pre"> </span><span class="Apple-tab-span" style="white-space:pre"> </span>let back = _computeNullability(of: self)<br class=""><span class="Apple-tab-span" style="white-space:pre"> </span><span class="Apple-tab-span" style="white-space:pre"> </span><span class="Apple-tab-span" style="white-space:pre"> </span>nullabilityMemo = back<br class=""><span class="Apple-tab-span" style="white-space:pre"> </span><span class="Apple-tab-span" style="white-space:pre"> </span><span class="Apple-tab-span" style="white-space:pre"> </span>return back<br class=""><span class="Apple-tab-span" style="white-space:pre"> </span><span class="Apple-tab-span" style="white-space:pre"> </span>}<br class=""><span class="Apple-tab-span" style="white-space:pre"> </span>}<br class="">}<br class="">```<br class="">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`.<br class=""><br class=""><br class="">## Proposed solution<br class=""><br class="">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.<br class=""><br class=""><br class="">```<br class="">class Recognizer {<br class=""><span class="Apple-tab-span" style="white-space:pre"> </span>var nullabilityMemo: Bool?<br class=""><span class="Apple-tab-span" style="white-space:pre"> </span>var isNullable: Bool {<br class=""><span class="Apple-tab-span" style="white-space:pre"> </span><span class="Apple-tab-span" style="white-space:pre"> </span>pfunc _computeNullability(of recognizer: Recognizer) -> Bool {…}<br class=""><span class="Apple-tab-span" style="white-space:pre"> </span><span class="Apple-tab-span" style="white-space:pre"> </span>if let back = nullabilityMemo {<br class=""><span class="Apple-tab-span" style="white-space:pre"> </span><span class="Apple-tab-span" style="white-space:pre"> </span><span class="Apple-tab-span" style="white-space:pre"> </span>return back<span class="Apple-tab-span" style="white-space:pre"> </span><span class="Apple-tab-span" style="white-space:pre"> </span><br class=""><span class="Apple-tab-span" style="white-space:pre"> </span><span class="Apple-tab-span" style="white-space:pre"> </span>} else {<br class=""><span class="Apple-tab-span" style="white-space:pre"> </span><span class="Apple-tab-span" style="white-space:pre"> </span><span class="Apple-tab-span" style="white-space:pre"> </span>let back = _computeNullability(of: self)<br class=""><span class="Apple-tab-span" style="white-space:pre"> </span><span class="Apple-tab-span" style="white-space:pre"> </span><span class="Apple-tab-span" style="white-space:pre"> </span>nullabilityMemo = back<br class=""><span class="Apple-tab-span" style="white-space:pre"> </span><span class="Apple-tab-span" style="white-space:pre"> </span><span class="Apple-tab-span" style="white-space:pre"> </span>return back<br class=""><span class="Apple-tab-span" style="white-space:pre"> </span><span class="Apple-tab-span" style="white-space:pre"> </span>}<br class=""><span class="Apple-tab-span" style="white-space:pre"> </span>}<br class="">}<br class="">```<br class=""><br class="">## Detailed design<br class=""><br class="">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.<br class=""><br class="">## Impact on existing code<br class=""><br class="">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.<br class=""><br class="">## Alternatives considered<br class=""><br class="">It should be noted that neither of these alternatives can remain consistent for inline closures.<br class="">1. keyword `pfunc` (pronounciation: pifəŋk) for 'pure' functions. <br class="">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.<br class=""><br class="">_______________________________________________<br class="">swift-evolution mailing list<br class=""><a href="mailto:swift-evolution@swift.org" class="">swift-evolution@swift.org</a><br class="">https://lists.swift.org/mailman/listinfo/swift-evolution<br class=""></blockquote><br class="">_______________________________________________<br class="">swift-evolution mailing list<br class=""><a href="mailto:swift-evolution@swift.org" class="">swift-evolution@swift.org</a><br class="">https://lists.swift.org/mailman/listinfo/swift-evolution<br class=""></div></div></blockquote></div><br class=""></body></html>