[swift-evolution] Optional Assignment Operator

[John McCall]

> 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 <mailto:swift-evolution at swift.org>> wrote:
> 
> > On Jan 25, 2017, at 8:40 AM, Nichi Shin via swift-evolution <swift-evolution at swift.org <mailto: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.
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> 
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