[swift-evolution] [swift-evolution-announce] [Review] SE-0127: Cleaning up stdlib Pointer and Buffer Routines

[Xiaodi Wu]

On Fri, Jul 22, 2016 at 6:49 PM, Dave Abrahams via swift-evolution <
swift-evolution at swift.org> wrote:

>
> on Fri Jul 22 2016, Xiaodi Wu <swift-evolution at swift.org> wrote:
>
> > On Fri, Jul 22, 2016 at 5:06 PM, Dave Abrahams via swift-evolution <
> > swift-evolution at swift.org> wrote:
> >
> >>
> >> on Fri Jul 22 2016, Bob Wilson <swift-evolution at swift.org> 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
> >>
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>
> --
> Dave
>
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