[swift-evolution] C-style For Loops

[ilya]

I think this is indeed an (anti-)feature of C-style loop.

`for var i = 0; i < 5; i += 1` is supposed to actually create a variable i;
in particular you can change this variable yourself or pass the reference
to it, so the implementation must work as follows:

var i = 0;
while( ...

Naturally, the closures are able to capture and modify i as well.

Or, in other words, by the definition of what C-style loop is, `for let i =
0; i < 5; i += 1` is impossible.

This magic behavior is in fact the best argument for the removal of C-style
loop, imho.



On Sun, Dec 6, 2015 at 10:14 PM, Erica Sadun via swift-evolution <
swift-evolution at swift.org> wrote:

> That is very odd indeed.
>
> If you add explicit capture, the issue goes away:
>
> var handlers: [() -> Void] = []
>
> for i in 0..<5 {
>     handlers.append {[i] in print(i, terminator:",") }
> }
>
> for handler in handlers {
>     handler()  // "0 1 2 3 4"
> }
>
> handlers = []
>
> for var i = 0; i < 5; i += 1 {
>     handlers.append {[i] in print(i, terminator:",") }
> }
>
> print("")
> for handler in handlers {
>     handler()  // was "5 5 5 5 5", now 1, 2, 3, 4, 5
> }
>
> It really seems like a bug to me rather than a feature of the for-loop
>
> -- Erica
>
>
> On Dec 6, 2015, at 12:03 PM, Kelly Gerber via swift-evolution <
> swift-evolution at swift.org> wrote:
>
> I think that the C-style *for* loop should be removed from Swift. The
> scope rules for this *for* loop are wrong. Every loop sees the same
> scope. This is a source of bugs if you export the loop variable name
> outside the scope of the *for* statement, for example in a closure. The
> following code illustrates the problem:
>
> var handlers: [() -> ()] = []
>
> for i in 0..<5 {
>     handlers.append { print(i) }
> }
>
> for handler in handlers {
>     handler()  // "0 1 2 3 4"
> }
>
> handlers = []
>
> for var i = 0; i < 5; i += 1 {
>     handlers.append { print(i) }
> }
>
> for handler in handlers {
>     handler()  // "5 5 5 5 5"
> }
>
> The Swift *for-in* loop does the right thing naturally. The C-style *for* loop
> does the wrong thing naturally. Removing the C-style *for* loop from
> Swift will eliminate one more class of possible errors from the language.
> _______________________________________________
> 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
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.swift.org/pipermail/swift-evolution/attachments/20151206/a6f87478/attachment.html>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: open.gif
Type: image/gif
Size: 43 bytes
Desc: not available
URL: <https://lists.swift.org/pipermail/swift-evolution/attachments/20151206/a6f87478/attachment.gif>