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

[Tim Vermeulen]

That’s why I said “potentially less elegant”, some people might prefer `where` over `guard`. This proposal would give them the choice (in very specific situations) to use `where` rather than `guard` if they don’t want to sacrifice performance.

> On Wed, Jun 8, 2016 at 1:35 PM, Tim Vermeulen via swift-evolution<swift-evolution at swift.org(mailto:swift-evolution at swift.org)>wrote:
> > This is a really strong argument in my opinion. If we don’t add a `while` to for loops, then in some situations we will have to rewrite a `where` clause to something potentially less elegant, given that we don’t want to give up performance.
> I disagree. I argue that what you call "less elegant", namely if (or guard) inside the loop, is the most elegant solution.
> 
> > 
> > >IMO `.prefix` is just not the equal alternative for as proposed `while` :
> > >in case of 'while' expression `number<4_000_000` will be calculated
> > >*only* for those who `number % 2 == 0`. In case of `prefix` - the
> > >expression will be processed for each `number` and only after this filtered
> > >by `number % 2`. Let's assume we need to check for some
> > >veryExpensiveTest(number):
> > >
> > >for number in fibonacci where number % 2 == 0 while
> > >veryExpensiveTest(number) {}
> > >
> > >let numbers = fibonacci.prefix { veryExpensiveTest($0) }
> > >for number in numbers where number % 2 == 0 {}
> > >
> > >So, `while` for `for` loops just can't be always replaced with `prefix`
> > >
> > >On 08.06.2016 2:02, Xiaodi Wu via swift-evolution wrote:
> > >>On Tue, Jun 7, 2016 at 5:11 PM, Tim Vermeulen<tvermeulen at me.com(mailto:tvermeulen at me.com)
> > >><mailto:tvermeulen at me.com>>wrote:
> > >>
> > >>I’ve been thinking about this for a bit now, and I think it would make
> > >>most sense to evaluate these clauses from left to right. However, cases
> > >>where the order matters are very uncommon, and I would rather have the
> > >>power to choose which clause is evaluated first than to have a forced
> > >>default order. Either way I don’t see this as a reason not to allow
> > >>combining the two clauses because IMO it can lead to some very clean
> > >>code. For instance, say we want to loop through all even fibonacci
> > >>numbers below 4 million (see problem #2 from project euler), we could
> > >>do this:
> > >>
> > >>`for number in fibonacci where number % 2 == 0 while number<4_000_000
> > >>{ }`
> > >>
> > >>
> > >>This statement looks like spaghetti to me. I would not at all support
> > >>extending the language to permit it. Do you really think it's more readable
> > >>than going step-by-step?
> > >>
> > >>```
> > >>let numbers = fibonacci.prefix { $0<4_000_000 }
> > >>for number in numbers where number % 2 == 0 {
> > >>// ...
> > >>}
> > >>```
> > >>
> > >>or just:
> > >>
> > >>```
> > >>let numbers = fibonacci.prefix { $0<4_000_000 }
> > >>let evens = numbers.filter { $0 % 2 == 0 }
> > >>for number in evens {
> > >>// ...
> > >>}
> > >>```
> > >>
> > >>
> > >>I could have ordered the two clauses in any way I want. If combining
> > >>the clauses weren’t allowed, I’d have to put (at least) one of them
> > >>inside the block, which would be a (minor) pain.
> > >>
> > >>I don’t currently have a very strong opinion about the order of
> > >>evaluation, so I might be convinced otherwise. But combining the two
> > >>clauses is so powerful that I don’t think it’s worth to get rid of just
> > >>because of an edge case.
> > >>
> > >>>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(mailto:erica at ericasadun.com)
> > >><mailto:erica at ericasadun.com>(mailto:erica at ericasadun.com
> > >><mailto:erica at ericasadun.com>)>wrote:
> > >>>>
> > >>>>>On Jun 7, 2016, at 1:16 PM, Tim Vermeulen via swift-evolution<swift-evolution at swift.org(mailto:swift-evolution at swift.org)
> > >><mailto:swift-evolution at swift.org>(mailto:swift-evolution at swift.org
> > >><mailto: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
> > >>marginallyless goodthan 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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> > >
> > >
> > >
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