Le 2 janv. 2016 à 9:25, Maury Markowitz via swift-evolution
<swift-evolution@swift.org> a écrit :
> I'm confused about 'defer'. Not the purpose, the chosen syntax.
>
> func confusing() {
> print("1")
> defer { print("2") }
> print("3")
> }
>
> Produces 1,3,2. Trying to describe what is happening here is non-trivial...
> it runs line 1, then we put line 2 on a stack, then line three runs, then we
> pop the stack... what?! And stepping through it in the debugger... ugh.
>
> Unless I missed something obvious, wouldn't placing "code that always has to
> run at the end" actually *at the end* not make more sense? Like this...
>
> func clear() {
> print("1")
> print("3")
>
> always { print("2") }
> }
>
> Not only is the code clearly expressing its intent, the actual execution is
> following the source.
With your proposal, code like this:
func test() throws -> T {
let a = try makeSomething()
defer { a.cleanup() }
let b = try makeSomething()
defer { b.cleanup() }
return try compute(a, b)
}
would have to be rewritten somewhat like this:
func test() throws -> T {
let a = try makeSomething()
do {
let b = try makeSomething()
do {
return try compute(a, b)
always { b.cleanup() }
}
always { a.cleanup() }
}
}
which is a bit inconvenient. One important thing with `defer` is that where you
put it in the function impacts at which point it gets pushed on the "cleanup
stack", something that gets lost with `always` and which I need to add back
through `do {}` blocks here.
I think what you are looking for is a `do {} finally {}` block, which you
should feel free to propose if you think it's worth it. I personally don't
think it makes the above example better, but it certainly has the benefit of
always running things in source order.
func test() throws -> T {
let a = try makeSomething()
do {
let b = try makeSomething()
do {
return try compute(a, b)
} finally {
b.cleanup()
}
} finally {
a.cleanup()
}
}
--
Michel Fortin
https://michelf.ca
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