[swift-evolution] [Review] SE-0007 Remove C-style for-loops with conditions and incrementers

[Brent Royal-Gordon]

>> As for c-style-for vs while, the two are mechanically convertible.
> 
> This is provably false and has been demonstrated, but here is an example of it again: 
> 
>     var sum = 0
>     for var i = 10 /* expr1 */; i > 0 /* expr2 */; i -= 1 /* expr3 */ {
>         if i % 2 == 0 { continue } // statement
>         sum += 1                   // statement
>     }
>     print(sum)
> 
> 
>     var sum = 0
>     var i = 10 // expr1
>     while i > 0 /* expr2 */ {
>         if i % 2 == 0 { continue } // statement
>         sum += 1                   // statement
>         
>         i -= 1 // expr3
>     }
>     print(sum)
> 
> It’s not just a mechanical conversion; consideration for early loop-exits and continuations need to be made as well. The rote conversion for the while version above is an infinite loop. Not only that, expr1 now leaks variables into a scope that is no longer contained within the loop. 

You know, Perl *makes* it mechanically convertible by adding a continue {} block after the while {} loop. The continue {} block is run between the loop body and the condition, so it’s skipped by `break` (well, `last`) and other early constructs that leave the loop, but not `continue` (spelled `next` there). Maybe something along those lines would help us here by making the loss of `for` more palatable. (Although that does kind of defeat the purpose of removing this syntax…)

-- 
Brent Royal-Gordon
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