This proposal runs counter to the spirit of what defer seems to be meant for in
the first place. It’s to save you forgetting to do something (like closing a
stream) before you exit a block of code. If you push it down in the code to
after the return, then you might as well have just put the code in *before* the
return without the defer.
public func doMyThing() {
let theStream = openStream()
defer {
closeStream()
}
…
…
…
<enough code that it’s off the bottom of the screen>
return
}
In this case, putting closeStream() after the return would defeat the purpose.
-Kenny
> On Jun 6, 2016, at 12:50 PM, donny wals via swift-evolution
> <[email protected]> wrote:
>
> Hi,
>
> When we’re using defer we write some code that we want to execute the moment
> a scope exits.
> This leads to code that could read like:
>
> let fibonacci = sequence(state: (0, 1)) { (pair: inout (Int, Int)) -> Int in
> defer { pair = (pair.1, pair.0 + pair.1) }
> return pair.0
> }
>
> What I find strange about this is that we have to write the code that we want
> to execute after the return before the return.
>
> I’d like to propose a change to defer that would allow the above code to be
> written as:
>
> let fibonacci = sequence(state: (0, 1)) { (pair: inout (Int, Int)) -> Int in
> return pair.0
> defer { pair = (pair.1, pair.0 + pair.1) }
> }
>
> This would make the intent of the code more clear (return first, then mutate
> state). Not all cases can benefit from this change, but anytime you exit a
> scope using a return I think it might be more clear to define the defer after
> the return. The code would more closely mirror the intent of the code.
>
> A rule of thumb I’ve come up with for this is that whenever you’re using
> return to exit a scope, any defer in that same scope should be executed
> regardless of it’s position in that same scope. This proposal would
> supplement the way defer currently works.
>
> What do you all think?
> _______________________________________________
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