[swift-evolution] Optional Assignment Operator

[Jacob Bandes-Storch]

If no uses are found (which I suspect will be the case), it becomes hard to
also find evidence of harm other than in contrived scenarios. Perhaps
contrived will be all we can find. Anyway, this is a bit off-topic for this
thread...
On Wed, Jan 25, 2017 at 11:24 AM John McCall <rjmccall at apple.com> wrote:

> On Jan 25, 2017, at 1:48 PM, Xiaodi Wu <xiaodi.wu at gmail.com> wrote:
> Given lack of evidence of harm, is it really important to make such a
> source-breaking change?
>
>
> My first instinct is that, no, it's not important.  However, we haven't
> actually *tried* to find any evidence of harm, so it's a bit conclusory.
> If someone wants to make an evidence-based argument that it's harmful and
> that almost nobody is using it (intentionally/correctly), the balance could
> swing the other way.  That's for someone else to prove, though, since yes,
> at this point the bias has to be towards leaving things be.
>
> John.
>
>
> On Wed, Jan 25, 2017 at 12:45 John McCall via swift-evolution <
> swift-evolution at swift.org> wrote:
>
> On Jan 25, 2017, at 1:35 PM, Jacob Bandes-Storch <jtbandes at gmail.com>
> wrote:
>
> Agreed, IMO it would be quite dangerous for "a ??= b" to mean anything
> other than "a = a ?? b".
>
> On another note, I don't see the value of "a? = b". I had never realized
> before that this works. Is this feature actually used in the wild? Should
> we consider removing it? (I could perhaps see some value if the assignment
> operator were overloadable, but it's not.)
>
>
> The core semantics (that ? on an l-value still produces an l-value) fall
> out from the ability to call a mutating method with a?.foo().  Once you
> have that, you have to decide what it means to put such an l-value to the
> left of an assignment operator, and we decided to make it Just Work™.  I
> agree that it is not a particularly useful operation in idiomatic Swift,
> especially with simple assignment (=), and we could consider just
> disallowing it.
>
> It also comes up with optional properties, I think, which is something we
> weren't always certain we were going to ban in native Swift (as opposed to
> imported ObjC code, where they're a fact of life).
>
> John.
>
>
> Jacob
>
> On Wed, Jan 25, 2017 at 10:28 AM, John McCall <rjmccall at apple.com> wrote:
>
> On Jan 25, 2017, at 12:47 PM, Jacob Bandes-Storch via swift-evolution <
> swift-evolution at swift.org> wrote:
> Really? My observation from a quick test is that "a? = b" assigns b to a
> if a already has a value, or does nothing if it's nil. This is sort of the
> opposite of what's being proposed, which is that "a ?= b" should assign to
> a only if it does NOT have a value.
>
>
> Right.  On the other hand, this does seem like a poor spelling for the
> operator, given the ease of confusion.
>
> Also, I'm finding it hard to imagine a use for this where the equivalent
> ?? invocation wouldn't be *much* clearer.  It just feels like you must be
> doing something backwards — "I've filled in a default value for this
> variable, now overwrite it if this other value exists".  Wouldn't the
> reverse generally be better?
>
> John.
>
> On Wed, Jan 25, 2017 at 9:33 AM Joe Groff via swift-evolution <
> swift-evolution at swift.org> wrote:
>
>
> > On Jan 25, 2017, at 8:40 AM, Nichi Shin via swift-evolution <
> swift-evolution at swift.org> wrote:
> >
> > I’d like to propose a new operator for optional assignment in Swift.
> >
> > The idea is that by using this operator (e.g. by doing a ?= b), the
> optional on the right would be assigned to the variable on the left only
> when it has something to assign (i.e. when it's not nil).
>
> `a? = b` already does this. Maybe we need a fixit to make that more
> apparent, though.
>
> -Joe
>
> >
> > The implementation could be something as follows:
> >
> > /// Optional Assignment Operator
> > infix operator ?=: AssignmentPrecedence
> >
> > func ?=<T>(left: inout T, right: T?) {
> >     if right != nil {
> >         left = right!
> >     }
> > }
> >
> > func ?=<T>(left: inout T?, right: T?) {
> >     if right != nil {
> >         left = right
> >     }
> > }
> >
> > I hope you will consider adding this on a future release of this great
> programming language.
> >
> > Kind regards,
> > N. S.
> > _______________________________________________
> > swift-evolution mailing list
> > swift-evolution at swift.org
> > https://lists.swift.org/mailman/listinfo/swift-evolution
>
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