[swift-evolution] Cancelable named defer statements

[Derrick Ho]

Its probably better to use a Boolean variable close_file to "cancel" it,
rather than change the entire language.

func openFile(kind: String) -> UnsafeMutablePointer<FILE>? {

    var file = fopen("MyFile.txt", "r")
    var close_file = true
    defer { if close_file { fclose(file) }} // <--

    if fileIsNotValid(file) { return nil }

    if fileDoesNotContainsData(file, kind) { return nil }

    if fileDataOutOfDate(file) { return nil }

    // Prevent the deferred handler from closing the file
   close_file = false // <--

    return file
}

On Sat, Jan 7, 2017 at 1:20 PM Rien via swift-evolution <
swift-evolution at swift.org> wrote:

> Is there any interest in a proposal to introduce a named defer statement
> that can be cancelled?
>
> Lately I find myself writing this kind of code:
>
> func openFile(kind: String) -> UnsafeMutablePointer<FILE>? {
>
>
>     var file = fopen("MyFile.txt", "r")
>
>
>     var closeFile = true
>
>     defer { if closeFile { fclose(file) } }
>
>
>     if fileIsNotValid(file) { return nil }
>
>     if fileDoesNotContainsData(file, kind) { return nil }
>
>
>     if fileDataOutOfDate(file) { return nil }
>
>
>     // Prevent the deferred handler from closing the file
>     closeFile = false
>
>     return file
> }
>
> Which imo would be much cleaner if we were able to write:
>
> func openFile(kind: String) -> UnsafeMutablePointer<FILE>? {
>
>     var file = fopen("MyFile.txt", "r")
>
>     CLOSE_FILE: defer { fclose(file) }
>
>     if fileIsNotValid(file) { return nil }
>
>     if fileDoesNotContainsData(file, kind) { return nil }
>
>     if fileDataOutOfDate(file) { return nil }
>
>     // Prevent the deferred handler from closing the file
>     cancel CLOSE_FILE
>
>     return file
> }
>
> Regards,
> Rien
>
> Site: http://balancingrock.nl
> Blog: http://swiftrien.blogspot.com
> Github: http://github.com/Swiftrien
> Project: http://swiftfire.nl
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>
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