[swift-evolution] Add a while clause to for loops

[Xiaodi Wu]

It may be workable if you can have only one or the other, but mixing and
matching them as proposed above would be a world of hurt:

```
for foo in bar where condition1 while condition2 { ... }
```

If condition1 and condition2 both evaluate to true, then whether you
continue or break would depend on the relative order of where and while;
for generality, you would want to allow both `for...in...where...while` and
`for...in...while...where`, and likely `for...in...while...where...while`,
etc. There is nothing in the meaning of those words that would suggest that
`while...where` behaves differently from `where...while`, etc. This is why
words like "break" and "continue" are IMO far superior.


On Tue, Jun 7, 2016 at 2:34 PM, Erica Sadun <erica at ericasadun.com> wrote:

>
> On Jun 7, 2016, at 1:16 PM, Tim Vermeulen via swift-evolution <
> swift-evolution at swift.org> wrote:
>
> The meaning of the proposed while is not at all a pair for where, since
> where clauses in while loops would do the same thing as while clauses in
> for loops. That's crazy.
>
>
> It sounds crazy, but it’s the nature of the while loop. A where clause in
> a while loop also has a different result than a where clause in a for loop.
>
>
> The where_clause appears in the for in statement
>
> for_in_statement : 'for' 'case'? pattern 'in' expression where_clause?
> code_block
>
> It's syntactic sugar because the expression can be already be limited
> through functional chaining of some sort or another. At the same time, it's
> nice and pleasant to have `where` and I'm not itching to throw it out. The
> same courtesy could be easily extend to `when` (because I don't really want
> to use the `while` keyword here, but I could easily be convinced otherwise
> because I don't have a strong stance either way):
>
> for_in_statement : 'for' 'case'? pattern 'in' expression (where_clause |
> when_clause)? code_block
> when_clause : 'when' expression
>
> and again it could be nice and pleasant to have, although not necessary.
> The question comes down to how much does the language benefit by this sugar.
>
> I'd say that in both cases, combining chaining and statements is
> marginally *less good* than either using standalone chaining or
> statements without chaining. But as I say this, I know as a fact, I fully
> intend to use `sequence(_:, next:).take(while:)` with for0in statements, so
> I'm starting from a hypocritical vantage point.
>
> To summarize, I'm more +0.01 than I am -0.01 on this.
>
> -- E
> p.s. Sorry, wux
>
>
>
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