[swift-evolution] [proposal] Allow "let" for computed properties which only reference immutable data

[Alexander Momchilov]

Yeah, there will need to be a facility to annotating function purity

On Wed, May 11, 2016 at 1:00 PM, Joe Groff <jgroff at apple.com> wrote:

>
> > On May 11, 2016, at 8:34 AM, Matthew Johnson via swift-evolution <
> swift-evolution at swift.org> wrote:
> >
> >
> >
> > Sent from my iPad
> >
> >> On May 11, 2016, at 10:26 AM, Sean Heber via swift-evolution <
> swift-evolution at swift.org> wrote:
> >>
> >> I kind of agree with the logic of that, but imagine the following:
> >>
> >> class Test {
> >> let hello = “hello”
> >> var subject = “world”
> >> var phrase: String { return hello + “, “ + subject }
> >> }
> >>
> >> In this scenario, “subject” can be changed.. and that changes the
> result of “phrase”. Things like this are why computed properties are “var”.
> >
> > This example would still be required to be var because it accesses a var
> in its implementation.
> >
> > I like the idea of allowing this feature, but only in cases where the
> compiler can verify immutable semantics.  Having it may help drive other
> features that could expand the cases the compiler can verify.  That would
> be great as this is a direction I would like to see Swift take in the
> future.
>
> Yeah, this is why we don't currently allow computed "let" properties. We'd
> only want to do so once we have the language facilities to ensure a
> computed 'let' property is immutable and has no observable side effects.
>
> -Joe
>
> >>
> >> l8r
> >> Sean
> >>
> >>
> >>> On May 11, 2016, at 8:25 AM, Alexander Momchilov via swift-evolution <
> swift-evolution at swift.org> wrote:
> >>>
> >>> I came an interesting SO question which pointed out a strange quirk: a
> computed property must always use the "var" keyword, even if it's
> read-only, and only referencing other immutable data.
> >>>
> >>> class Test {
> >>>
> >>>
> >>> let hello = "hello"
> >>>
> >>>
> >>> let world = "world"
> >>>
> >>>
> >>> var phrase: String { //why must this be 'var'?
> >>>
> >>>
> >>> return self.hello + self.
> >>> world
> >>>
> >>> }
> >>> }
> >>> It would be more appropriate for such a read-only, immutable property,
> to use the "let" syntax, so that its immutability is correctly expressed.
> >>>
> >>> Thoughts?
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