[slavapestov]

You can think of inout parameters in Swift as something analogous to a mutable borrow in Rust. Until Swift 4 we allowed overlapping inout access, for example:

    var counter = 0
    func foo(x: inout Int) {
      x += 1
      print(counter)
    }
    foo(x: &counter)

Note how 'counter' is read by 'foo(x:)' during an inout access of the same value. This is now prohibited by Swift 4, using a combination of static and dynamic checks.

This fixes some undefined behavior and will also enable more aggressive compiler optimizations to be added in the future.

[leshow]

Isn't the mutable borrow ended after the end of the x += 1 line? leaving you free to read the contents of the ptr?

I don't know swift at all. but from their document on swapAt() it looks like they are trying to prevent two fn(&p, &p) where func fn(a: inout Type, b: inout Type)

[hellofunk]

> Isn't the mutable borrow ended after the end of the x += 1 line?

No, see Mike's comment here.

[hellofunk]

> Note how 'counter' is read by 'foo(x:)' during an inout access of the same value

It's not clear to me in your example why reading the value of counter after mutating it is bad; why is this now prohibited?

[mikeash]

Swift specifies inout parameters as copying the value that's passed in, giving the copy to the callee as a mutable value, and then writing back the value to the original storage after the function returns. & is not an "address of" operator.

Of course, it would be inefficient to do this all the time, so Swift will optimize copy-then-writeback to just passing a pointer to the original storage whenever it can. But this is an optimization operating under the "as if" rule: as long as it works as if it does a copy-then-writeback, the compiler can make it actually do whatever it wants.

If the example code were legal, then it would have to print `0`, because the writeback to `counter` doesn't happen until the end of the function. That means the compiler couldn't just pass in a pointer to `counter`, but would have to actually go through the copy-then-writeback procedure it's supposed to do, so you'd lose out on optimizations.

Instead, Swift makes it illegal. You can't access a value while this call is happening. That allows the language semantics to coexist with optimizations.

[leshow]

what does & do in Swift then? And what's the purpose of even having inout and & if you can't count on the code to actually be an address?

[mikeash]

& is just a sigil saying "I acknowledge that I am passing this as an inout parameter, and therefore the value may be modified by the function I am calling." Note that & is only legal on a function parameter. You cannot write, for example, `let b = &a`.

The purpose is to allow for out-parameters. A classic example would be the `+=` operator. (Swift operators are just normal functions with special call syntax.) It takes its first parameter as `inout` so that it can mutate the value.

Note that inout parameters work with expressions where it would be impossible to take the address. For example, you can use & on a computed property that has a setter. In that case it has to read the initial value, pass that to the function, then write back the new value, because it has no idea where the computed property actually stores the value, if anywhere.

Edit: because I'm obsessive and weird, I made a quick example of this computed property stuff:

http://swift.sandbox.bluemix.net/#/repl/59c284376cbea87f72c4...

Click the play triangle at the bottom to see the output.

[leshow]

> (Swift operators are just normal functions with special call syntax.)

Coming from Haskell and Rust it's nice to see this trend catching on.

Is Swift planning to introduce a distinction between borrows and mutable borrows to the user? From what you describe it seems like right now syntax-wise a borrow and a mutable borrow look the same, and the runtime makes some decision about it.

edit: Or I guess it could be the opposite. Since Swift passes by value always unless the runtime can optimize (right?), you could just not write & and inout and cross your fingers it gets optimized to a borrow rather than a copy?

Stuff like this makes me prefer the explicitness of Rust. It seems like here on the surface it's abstracted from you, but really you need to know the rules anyway or you could get into trouble.

[pjmlp]

> Coming from Haskell and Rust it's nice to see this trend catching on.

It was already like that in older languages like Lisp, Smalltalk and CLU, just C++ made them in a different way.

[mikeash]

What's the use of a non-mutable borrow? Is it just for speed, to avoid copying a large structure? Large structures are rare in Swift and probably not important to optimize. (Value types like String and Array are actually just one reference under the hood, and that's all that gets "copied" when you pass those by value.)

[hellofunk]

Still new to Swift but I believe & is an explicit syntax necessary to make clear in the code that the function being called is mutating the argument, and thus the variable passed into that function could be mutated. It must be used wherever an argument is in/out. It makes mutation explicit both in the function declaration and also in the function call. Which is nice!

It's good for code readability but also prevents accidentally passing a variable to a function that could mutate it when you weren't expecting that, and vice-versa.

[favorited]

Values are not guaranteed to even have an address, IIRC.

[hellofunk]

Thanks Mike, very clear.