On Fri, Jul 22, 2016 at 6:49 PM, Dave Abrahams via swift-evolution < [email protected]> wrote:
> > on Fri Jul 22 2016, Xiaodi Wu <[email protected]> wrote: > > > On Fri, Jul 22, 2016 at 5:06 PM, Dave Abrahams via swift-evolution < > > [email protected]> wrote: > > > >> > >> on Fri Jul 22 2016, Bob Wilson <[email protected]> wrote: > >> > >> > It is not so clear what to do about SR-1956. (Charlie and I had some > >> > comments on this in https://github.com/apple/swift-evolution/pull/437 > >> > <https://github.com/apple/swift-evolution/pull/437>.) Jordan raised > >> > the objection that when using withUnsafePointer with a global, there > >> > is an expectation that you’ll get the same address every > >> > time. Removing inout would cause the argument to be passed by value > >> > and the address would refer to a copy. Dmitri agreed that this could > >> > be a problem. On the other hand, if you don’t care about the address, > >> > or if you’re not using a value type, it would indeed be convenient to > >> > have a version of withUnsafePointer that does not require an inout > >> > argument. > >> > > >> > Option 1: Keep inout (not addressing SR-1956). In this case, there’s > >> > no reason to have both withUnsafePointer and > >> > withUnsafeMutablePointer. If you want to call a function that expects > >> > an UnsafePointer, you can give it an UnsafeMutablePointer and there > >> > will be an implicit conversion to make it work. I discussed this with > >> > Apple’s stdlib team and they recommended that if we have only one > >> > function we use the shorter name “withUnsafePointer” and have it use > >> > an UnsafeMutablePointer. > >> > >> Very much in favor of Option 1. > >> > > > > Ditto, except that I think there is some value in keeping both (i.e. > doing > > nothing): allowing the user to document intent. It would be inconsistent > > and potentially confusing to call the function that returns an > > `UnsafeMutablePointer` `withUnsafePointer`. > > It doesn't return an `UnsafeMutablePointer`, it passes an > `UnsafeMutablePointer` to the body of the closure. > Brainfart. Yes, that's what I meant to write. Sorry. > > It's rarely used enough, and the shorter name needlessly raises the > > question of where I'm really "supposed to be" mutating the > > pointee. > > I don't understand; you only have the pointee inside the closure. > That's where you mutate it (obviously?) > If my closure does not mutate the pointee, `withUnsafePointer(_:)` allows me to document that. Everything *works* with `withUnsafeMutablePointer(_:)`, but I cannot read the code and understand that no mutation has happened within the body of the closure. [Am I wrong on this?] For instance, I've been working with some of the Accelerate.framework functions and the arguments are often cryptic. Take this call: ``` cblas_sgemm(CblasColMajor, CblasNoTrans, CblasNoTrans, m, n, k, 1, matrix, m, b, k, 1, &c, m) ``` There are times when I'd want to call `cblas_sgemm(_:_:_:_:_:_:_:_:_:_:_:_:_:_:)` inside an `withUnsafe[Mutable]Pointer(_:)` closure. Distinguishing `withUnsafePointer(_:)` and `withUnsafeMutablePointer(_:)` would allow a reader to know from the outset if `$0.pointee` is mutated without having to know that the second-from-last argument is the one that stores the result (it is not consistently second-from-last; for vDSP_* functions, it's often the third-from-last argument, and for others it can be the first argument). Removing the current `withUnsafePointer(_:)` would decrease clarity for the reader here. > I've not had to use these functions much, but the distinction between > > `Array.withUnsafeBufferPointer(_:)` and > > `Array.withUnsafeMutableBufferPointer(_:)` has conditioned me to > > mutate the pointee using only "mutable" functions. > > Not sure if you're just drawing an analogy, I was trying to. I guess ineffectively. > but if not, those two > methods are not under discussion here. They are meaningfully different, > whereas the existing functions are not, and the one currently called > withUnsafePointer is always going to cause people to complain about > having to pass a mutable variable. > > As a fallback position, I would suggest we only provide the mutating > one, but with its existing name. But I still prefer the shorter name. > > > > >> > > >> > Option 2: Fix SR-1956 and have two functions, one with inout and the > >> > other not. This would address the inconvenience of not being able to > >> > use withUnsafePointer with immutable values, while still supporting > >> > the existing behavior. The question then would be what to call these > >> > two functions. > >> > >> We do not need to support new use-cases in this release, and this would > >> be unsatisfying because the “address of a global” property that Jordan > >> argued for would not hold for the immutable version. > >> > >> > - Option 2a. Combine the two existing functions as in Option 1 and use > >> > a new name for the non-inout version, e.g., > >> > withUnsafePointer(toCopyOf:), so that it won’t be confused with the > >> > old function. (That particular name doesn’t work very well when > >> > dealing with references to objects, since the object itself would not > >> > be copied. I haven’t yet come up with a better name, though.) One > >> > advantage of this approach is that we would not need to rush the new > >> > function into Swift 3 since it would be an additive change. > >> > >> Not rushing that into Swift 3 is the same as Option 1. > >> > >> > - Option 2b. Switch to use withUnsafeMutablePointer for all the cases > >> > where you care about the getting the same address. Change > >> > withUnsafePointer to be the non-inout version. Charlie suggested that > >> > we could have the migrator convert all existing uses on > >> > withUnsafePointer in Swift 2 code to use withUnsafeMutablePointer in > >> > Swift 3, but I’m not sure how well that would work. > >> > >> That's exactly the same outcome, with respect to the language/library > >> surface, as Option 2 AFAICT. Can we simplify this list of options? > >> > >> -- > >> Dave > >> > >> _______________________________________________ > >> swift-evolution mailing list > >> [email protected] > >> https://lists.swift.org/mailman/listinfo/swift-evolution > >> > > _______________________________________________ > > swift-evolution mailing list > > [email protected] > > https://lists.swift.org/mailman/listinfo/swift-evolution > > > > -- > Dave > > _______________________________________________ > swift-evolution mailing list > [email protected] > https://lists.swift.org/mailman/listinfo/swift-evolution >
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