> On May 3, 2016, at 8:53 PM, Chris Lattner via swift-evolution
> <[email protected]> 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
>
> "A @once parameter
>
> It was mentioned in the discussion that the "once" behavior and @noescape
> look orthogonal, and the "once" behavior could be useful on closures that
> escape. However, it is only possible to verify that a closure has been
> executed exactly once if it does not escape. Because of this, "once" and
> @noescape are better left together."
I dislike the proposed syntax.
noescape-ness and once-ness are semantically orthogonal. The compiler's ability
to verify once-ness depends on noescape and some other semantics all being
present, as noted in the above quote from the proposal. The fact that the
compiler is limited should not forgive the semantic transgression. Note that
semantically the once-ness is the more important part for the designed usage;
noescape-ness is only dragged in because of the desire to enforce once-ness at
compile time. The @noescape(once) syntax is therefore backwards: the feature is
not some small modification to or variation of plain @noescape.
The proposed behavior includes restrictions that are required for its designed
usage but are unrelated to noescape-ness and once-ness. ("A @noescape(once)
closure may only read from variables that were initialized before it was
formed.") That suggests the proposed functionality should use a new
higher-level name rather than being bolted on to @noescape.
--
Greg Parker [email protected] Runtime Wrangler
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