SE-0377: borrow and take parameter ownership modifiers

Though that is better aligned with other function modifiers (such as async and throws), it's worth noting that mutating with the former syntax already exists in the language, so to do that would be a breaking change. If the first syntax even becomes so much as a warning, we may see libraries having to do this:

#if swift(>=5.8) // or whatever version
    func foo() mutates {
        // ...body...
    mutating func foo() {
        // ...same body...

So I'm not sure that would be worth it.

This would introduce inconsistency with the fact that async and throws are keywords for denoting effects and already have their own set of "rules". Both are parts of their type signatures, (Int) -> String and (Int) async -> String are different (even though they have a certain subtyping relationships), while (Int) mutates -> String does not make sense as a standalone type signature. Effectful read-only properties were introduced in SE-0310, allowing get async and get throws in property declarations. In the light of that, what would get mutates even mean?

Additionally, throws has counterparts in throw and rethrows. async has await and potentially reasync, as discussed in future directions of the corresponding proposal. I don't think mutate and remutates make sense even at a conceptual level. In this sense, taking, borrowing, and mutating are not function/property effect modifiers.


Rust’s notion of “mutable borrow” does indeed correspond to Swift’s inout. They’re slightly different (and so are the two “borrow”s) because Rust’s references are first-class values that promise a pointer representation and Swift’s are calling conventions that do not expose the representation. (For example, a borrowed Int in Swift will still be passed by value because it is a small frozen trivially-copyable struct.)

Isn't mutating get already a thing? I'm 90% sure that lazy variables have an implicit { mutating get set } attached to them.

1 Like

Yes, that one's a weird special case, but note that it's then consistent with mutating func. It still stands far from effect keywords in all other aspects.

If we want a consistency argument, mutating and the new proposed keywords affect how self behaves within the function, while also changing the self the function is called on. What other modifiers on functions affect self like this? static/class, and maybe convenience as a stretch. These also appear at the start of the declaration.

(I don’t put a lot of stock in consistency here, though, cause it’s clear that “before the declaration” is the default place to put modifiers in Swift, and throws and async are the unusual ones.)


Since mutating get was brought up: what about nonmutating set? Would that be respelled borrowing set now?
Similarly, would it make sense to allow taking get or taking set?

Edit: it seems the compiler doesn't currently reject __consuming get and __consuming set. It even allows __consuming willSet and __consuming didSet, though I'm not sure why you'd use those.

Do we want to allow setters to borrow the new value? Currently, a setter can't mark its argument as __shared or __owned. However, allowing that opens up a whole can of worms:

  • What if a set, willSet, or didSet tries to have an inout argument?
  • What does it mean for didSet to take its argument (oldValue) by anything other than take?
  • Should willSet be allowed to take its argument (newValue)?

Maybe this was already discussed and I just missed it, but willSet and didSet don't seem to be mentioned in the proposal at all.

That's an interesting question. borrowing set would indeed be equivalent to nonmutating set, but we should probably keep the existing spelling working for compatibility. As you noted, __consuming is currently allowed on get and set accessors, and I would say taking and borrowing ought to be allowed as well. It doesn't really make sense to allow them on didSet or willSet since those get absorbed into the setter and modify coroutine ABI-wise, and they always follow the ownership semantics for setting a stored property of the containing type (which would be mutating for a struct or borrowing for a class).

It might be interesting to allow set to borrow or inout its newValue, but I think that doing so requires further language design to work out what that means in relation to the borrow/inout access on the base of the property being set. (Similar issues cloud our ability to put throws on set, for instance.) So I think it makes sense to explicitly subset setters out of this initial proposal. Since willSet and didSet are tied to the default assignment behavior of stored properties, I don't think that modifying the semantics of oldValue/newValue is particularly useful.


I was always in favour of prefix spelling for async/throw as well:

async func foo() -> Int {}
throwing func foo() -> Int {}

but OTOH as mentioned above async/throw is part of function signature so I am not so sure about it.


Taking is an action. Owning is a state. Taking means a transference of possession.

If an owned object can be borrowed, then an owning function is a contradiction in terms, because it explicitly cuts off this possibility: 1. by scope 2. by voiding the argument.


Actually, the term of borrowing is also not quite there. In actuality, how many borrowers have access to the same thing? How can multiple borrowers get the same thing?
Only 1. by reproduction, i.e. copy onto parameter or 2. by glimpsing into its argument in place, not moving or copying.

Only single-access borrowing should actually be considered borrowing.
(Considering that its intention is to change the value, it’s not perfect as well. You wouldn’t expect a thing you lend to return changed. The contract is usually precisely that trust is given to not change it. But at least changing it is not precluded.)

3 a musing, not even a suggestion

I myself enjoy a different analogy – that of flows. Terms like conduit, flow, tapping, filling.
One could imagine the value as a quantum of energy. Then borrowing is tapping, and mutable borrowing is switching or swapping. Or mutating.
I like the similarity of Take and Tap and how it separates itself from mutation in situ, which is the most special case and the main antagonist.
(I also like the mut of rust, as it does not only mutate, but ‘mute’ the value for the time being – not other place can have it). But it’s too poetic for the other tastes.

1 Like

Whatever modifier we choose to put in (front of?) the function, I think mutating needs to match inout. If we go with a naming scheme like takes, borrows, mutates then an inout parameter should also be spelled mutate. Because as it currently stands, take is a verb, borrow will be a verb and inout will have an adjective role.

1 Like

I wonder if an -ing prefix is at all good.

  1. appears in relation to value type methods only,
  2. has dual target – mutates property = mutates self
  3. modifies value
  4. most importantly, declares its behaviour towards the target, not the argument. i.e. it declares a side-effect from its body, with exactly zero relation to its arguments.

None of these points apply to ownership transfer; it

  1. appears in relation to any function,
  2. has a single target, no self pointer,
  3. either modifies place, or does no modification at all (value is modified by inout only)
  4. declares its behaviour towards its arguments, not its scope.

For the sake of consistency, it would necessitate a bunch of changes to the grammar of mutating, as well.

Thanks everyone for your feedback so far! I've kicked off a combined re-review of SE-0366 and SE-0377 focused on naming here:

Please provide further feedback on the new review thread. Thank you!

Holly Borla
Review Manager