> On Feb 17, 2017, at 2:52 PM, Jonathan Hull via swift-evolution > <swift-evolution@swift.org> wrote: > > Out of curiosity, what are the benefits to being able to define that a > closure must be pure as a parameter/type definition, as opposed to defining a > particular closure to being pure while being passed? What guarantees does it > give you as the caller of the closure?
If you only accept pure closures and otherwise meet the criteria of a pure function then you are pure. If you have a function like that and want to accept both pure and impure closures and receive the purity of the closure provided then we need syntax indicating something similar to `rethrows`, but for purity. > > Thanks, > Jon > > >> On Feb 16, 2017, at 1:18 PM, T.J. Usiyan <griotsp...@gmail.com >> <mailto:griotsp...@gmail.com>> wrote: >> >> I am ok with a keyword but `pure` in front of func doesn't work well with >> inline closures. >> >> A few people talked through many of these issues starting with this tweet. >> https://twitter.com/griotspeak/status/832247545325842432 >> <https://twitter.com/griotspeak/status/832247545325842432> >> >> On Thu, Feb 16, 2017 at 4:13 PM, Jonathan Hull <jh...@gbis.com >> <mailto:jh...@gbis.com>> wrote: >> +1 for the idea of pure functions in swift. Seems like it would enable a >> lot of good optimizations (in some cases even just evaluating the function >> at compile time). >> >> -1 on the specific notation. I would much rather just put the word ‘pure’ >> in front of ‘func’, the same way we put ‘mutating' in front of mutating >> functions… it seems to me like these are part of the same family. >> >> I agree we should allow inout. >> >> Thanks, >> Jon >> >>> On Feb 16, 2017, at 9:03 AM, T.J. Usiyan via swift-evolution >>> <swift-evolution@swift.org <mailto:swift-evolution@swift.org>> 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 {…} >>> 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. >>> >>> _______________________________________________ >>> swift-evolution mailing list >>> swift-evolution@swift.org <mailto:swift-evolution@swift.org> >>> https://lists.swift.org/mailman/listinfo/swift-evolution >>> <https://lists.swift.org/mailman/listinfo/swift-evolution> >> >> > > _______________________________________________ > swift-evolution mailing list > swift-evolution@swift.org > https://lists.swift.org/mailman/listinfo/swift-evolution
_______________________________________________ swift-evolution mailing list swift-evolution@swift.org https://lists.swift.org/mailman/listinfo/swift-evolution
