[swift-evolution] Using "where" to filter an array

[Jacob Bandes-Storch]

A weak -1.

I'm not certain that my argument holds up very well; I'm just pointing out
how I see this as consistent with the rest of the language. My point is
that "where" is a keyword which adds an extra condition onto a braced
block; it doesn't play a role in producing values.

"for x in y where x > 2" means:

    var g = y.generate()
    while let x = g.next() {
        if x > 2 { // added by "where" clause
            // contents of for-in loop here
        }
    }

Similarly, "if let x = y where y > 2" means roughly:

    if y != nil {
        let x = y!
        if x > 2 { // added by "where" clause
            // contents of if-statement here
        }
    }

The purpose of "where" is merely to add an extra "if" without requiring
another level of nesting. A plain for-in loop doesn't produce a new array
of values, so using "in" and/or "where" by themselves to produce a new
array isn't possible.

Jacob Bandes-Storch

On Sat, Dec 19, 2015 at 10:10 PM, Charles Constant <charles at charlesism.com>
wrote:

> Hi Jacob,
>
> I take it that's a -1 ?
>
> I don't quite understand your example, because y in a "for" loop is an
> Array. So if I modify your example (I'll rename "y" to "numbers")...
>
>     for x in numbers where x > 2 { }
>
> ...my intuition immediately barks "it's a filter!"
>
>     let valid_numbers = x in numbers where x > 2
>
> I realize it's good to have a closure-based solution for
> map/filter/reduce, so fair enough that we let the filter method exist.
> Maybe if I understood Swift better (is this related to sequences?) this
> would all make sense... but with what grasp of Swift I do have, it makes
> the language feel baroque to have this "where" clause that looks like it
> ought to instantiate an array, but can't.
>
>
>
>
>
>
>
>
>
> On Sat, Dec 19, 2015 at 9:45 PM, Jacob Bandes-Storch <jtbandes at gmail.com>
> wrote:
>
>> It doesn't bother me, because I read it as an additional constraint on
>> the `for` loop rather than an operation being done on the enumerated
>> sequence. That is,
>>
>>     for x in y where x > 2 { }
>>
>> is fairly consistent with
>>
>>     if let x = y where x > 2 { }
>>
>> And you wouldn't expect to be able to do "let x = y where x > 2".
>>
>> Jacob Bandes-Storch
>>
>> On Sat, Dec 19, 2015 at 8:22 PM, Charles Constant via swift-evolution <
>> swift-evolution at swift.org> wrote:
>>
>>> Thanks Charles ,
>>>
>>> I know it's possible to do the same thing with filter. The syntax I drew
>>> attention to would be an alternate way.
>>>
>>> My only rationale is that, if it works in one place, it ought to work
>>> everywhere. The redundancy bothers me too, but since the concept exists, we
>>> may as well allow it everywhere. Otherwise, it's still redundant in a "for"
>>> loop (maybe performance is better, but that could be optimized?), but also
>>> inconsistent because it seems intuitively like it ought to create a subset
>>> whereas in reality it is illegal.
>>>
>>> Does it bother anyone else, or am I nitpicking?
>>>
>>> On Sat, Dec 19, 2015 at 7:52 PM, Charles Srstka <
>>> cocoadev at charlessoft.com> wrote:
>>>
>>>> On Dec 19, 2015, at 9:43 PM, Charles Constant via swift-evolution <
>>>> swift-evolution at swift.org> wrote:
>>>>
>>>>
>>>> I noticed several weeks ago that the following is illegal:
>>>>
>>>>     let new_arr = el in arr where el.is_foo // illegal
>>>>
>>>> I assumed it would work because it's consistent with the filtering that
>>>> exists in a "for" loop, i.e.:
>>>>
>>>>     for el in arr where el.is_foo // legal
>>>>
>>>> Is this "new proposal" material? I thought I would check in case it's
>>>> already on the roadmap, or inherently wrong.
>>>>
>>>>
>>>> I believe what you want is already possible via the “filter” method.
>>>>
>>>> Charles
>>>>
>>>>
>>>
>>> _______________________________________________
>>> swift-evolution mailing list
>>> swift-evolution at swift.org
>>> https://lists.swift.org/mailman/listinfo/swift-evolution
>>>
>>>
>>
>
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