[swift-evolution] [Review] SE-0073: Marking closures as executing exactly once

[Andrew Bennett]

Replies inline:

On Sat, May 7, 2016 at 12:37 PM, Dave Abrahams via swift-evolution <
swift-evolution at swift.org> wrote:

>
> on Fri May 06 2016, Andrew Bennett <swift-evolution at swift.org> wrote:
>
> > Hi Dave,
> >
> > Sorry, Dave, sending a second time as I forgot to Reply-All.
> >
> > I agree, this proposal doesn't allow multiple closures where only one of
> them
> > should be run, and it should only be run once. I personally don't think
> lacking
> > that functionality is worth blocking this proposal for, another proposal
> can be
> > built on top of this if it is desired.
> >
> > These cases can also be handled by a more meaningful if/switch
> statement, using
> > @noescape(once), for example:
> > let x: Int
> > functionThatCallsAClosure(someTest()) { x = $0 ? 1 : 2 }
>
> Why is this better than
>
>     let x = functionThatCallsAClosure(someTest()) { $0 ? 1 : 2 }
>
> ?
>
>
I'm not saying it's better, neither is the proposal. I do think both are
better than this though:

    functionThatActsLikeIf( someTest(), then: { x = 1 }, else: { x = 2} )

My opinion is that cases where *the proposal* are limited by multiple
closures seem to be cases where you would be better off with a single
closure and more explicit control-flow. I'd be interested if there are
other cases, but it currently seems like a straw-man argument to me.

--

It may be useful if Swift allowed things like this:

let x = switch { ... }
let x = if { ... } else { ... }
etc.

I think that's a much larger change/discussion, with no clear victor.

However until Swift has that support it's necessary to consider
separated initialization
and declaration. Likewise until all Swift is pure functional.

Even if this proposal didn't let you assign to let statements outside the
closure it still has value:

   - It lets the type system reduce programmer error
   - It allows protocol declarations to have a more explicit requirement
   - The user can guarantee that their code, and its side-effects, will be
   executed


IMO separating initialization from declaration is *very* rarely needed
> and very much better avoided altogether, because it leads to code that's
> less clear.  Just because we *can* do this doesn't mean we should.
>
> > On Sat, May 7, 2016 at 6:24 AM, Dave Abrahams via swift-evolution
> > <swift-evolution at swift.org> wrote:
> >
> >     on Tue May 03 2016, Chris Lattner
> >     <swift-evolution at swift.org> wrote:
> >
> >     > Hello Swift community,
> >     >
> >     > The review of "SE-0073: Marking closures as executing exactly once"
> >     > begins now and runs through May 9. The proposal is available here:
> >     >
> >     >
> >
> https://github.com/apple/swift-evolution/blob/master/proposals/0073-noescape-once.md
> >
> >     >
> >     > Reviews are an important part of the Swift evolution process. All
> reviews
> >     should be sent to the swift-evolution mailing list at
> >     >
> >     > https://lists.swift.org/mailman/listinfo/swift-evolution
> >     >
> >     > or, if you would like to keep your feedback private, directly to
> the
> >     review manager.
> >     >
> >     > What goes into a review?
> >     >
> >     > The goal of the review process is to improve the proposal under
> review
> >     > through constructive criticism and contribute to the direction of
> >     > Swift. When writing your review, here are some questions you might
> >     > want to answer in your review:
> >     >
> >     > * What is your evaluation of the proposal?
> >
> >     I think it's of questionable importance and doesn't generalize well.
> >     For example, you can't use this to construct something like
> >
> >     var x: Int
> >     functionThatActsLikeIf( someTest(), then: { x = 1 }, else: { x = 2} )
> >
> >     If you need to initialize something in an outer scope with something
> >     computed by a closure, it's much better to arrange something like
> this:
> >
> >     var x = functionThatActsLikeIf( someTest(), then: { 1 }, else: { 2 }
> )
> >
> >     --
> >     Dave
> >
> >     _______________________________________________
> >     swift-evolution mailing list
> >     swift-evolution at swift.org
> >     https://lists.swift.org/mailman/listinfo/swift-evolution
> >
> > _______________________________________________
> > swift-evolution mailing list
> > swift-evolution at swift.org
> > https://lists.swift.org/mailman/listinfo/swift-evolution
>
> --
> Dave
>
> _______________________________________________
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>
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