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

[Tim Vermeulen]

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.

> 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>>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>)>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>)>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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> 
>