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 On Thu, Feb 16, 2017 at 4:13 PM, Jonathan Hull <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> wrote: > > # Pure Functions > > * Proposal: [SE-NNNN](https://github.com/apple/swift-evolution/blob/ > master/proposals/NNNN-name.md) > * Author(s): [TJ Usiyan](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 > https://lists.swift.org/mailman/listinfo/swift-evolution > > >
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