[swift-evolution] Proposal: Add scan, takeWhile, dropWhile, and iterate to the stdlib

[Kevin Ballard]

"suffixAfter" sounds like the equivalent of dropFirst(_:), i.e. it sounds like it should take a count of elements to skip. Similarly, actually trying an expression that takes a predicate looks weird:

seq.suffixAfter({ isspace($0) })

Even knowing what it's supposed to do, it's hard for me to read that
expression in any sensible fashion.

Also, I'm not sure the "noun phrase" convention really makes sense for
SequenceType methods. They're not technically mutating methods, but single-
pass collections are in fact destructively mutated by Array-returning
sequence methods (and the methods that return SubSequence also
destructively mutate upon any access to the returned subsequence). Which
is to say, despite not being marked as "mutating", they do in fact
behave like mutating methods for single-pass sequences. Which suggests
that verb phrases are perfectly fine.

-Kevin Ballard

On Wed, Jan 13, 2016, at 08:36 PM, David Smith via swift-evolution wrote:
>
> Rob Rix pointed out that "suffixAfter" would meet all my original
> criteria. Not sure if keeping the original "match the stuff to drop
> rather than the stuff to keep" semantics are critical, but this gives
> us an option for either way 😊
>
> David
>
> On Jan 13, 2016, at 6:40 PM, David Smith via swift-evolution <swift-
> evolution at swift.org> wrote:
>> Naming conventions would suggest that something returning a new
>> collection should be named with a noun phrase describing what it
>> returns. Unfortunately, most of the ones I can think of off the top
>> of my head are fairly clunky. "suffixFromFirstNonMatchingElement"
>> describes what it does, but I haven't thought of a non-painful way to
>> say that yet. "suffixExcluding" is almost right, but it incorrectly
>> implies (to my eye at least) that the returned collection excludes
>> all elements matching the predicate, rather than just matching
>> prefixes. Hm, what about flipping the predicate and getting a
>> "suffixFrom" overload that takes a predicate for the first matching
>> element to be included, rather than the last matching element to be
>> excluded?
>>
>> David
>>
>>> On Jan 13, 2016, at 5:54 PM, Dany St-Amant via swift-evolution <swift-
>>> evolution at swift.org> wrote:
>>>
>>> The dropWhile sounds weird to me, I usually would see such
>>> functionality as a dropUntil; I discard stuff until I see what I
>>> want. Your example below doesn’t use dropWhile, but skipWhile; which
>>> sounds a bit better that dropWhile as one skip what he doesn’t want.
>>>
>>> What do the other languages use? A dropWhile, skipWhile or dropUntil
>>> concept?
>>>
>>> Dany
>>>
>>>
>>>> Le 11 janv. 2016 à 01:20, Kevin Ballard via swift-evolution <swift-
>>>> evolution at swift.org> a écrit :
>>>>
>>>> Here's a few toy examples, if it helps:
>>>>
>>>> // list of all powers of 2 below some limit iterate(1, apply: { $0
>>>> * 2 }).takeWhile({ $0 < limit })
>>>>
>>>> // first "word" of a string, skipping whitespace let cs =
>>>> NSCharacterSet.whitespaceCharacterSet()
>>>> String(str.unicodeScalars.skipWhile({
>>>> cs.longCharacterIsMember($0.value) })
>>>> .takeWhile({ !cs.longCharacterIsMember($0.value) }))
>>>>
>>>> // running total of an array of numbers numbers.scan(, combine:
>>>> +).dropFirst()
>>>>
>>>> // infinite fibonacci sequence iterate((,1), apply: { ($1, $0+$1)
>>>> }).lazy.map({$1})
>>>>
>>>> -Kevin Ballard
>>>>
>>>>>>> On Mon, Dec 28, 2015, at 03:59 PM, Kevin Ballard wrote:
>>>>>>> >
>>>>>>> > ## Detailed design
>>>>>>> >
>>>>>>> > We add the following extension to SequenceType:
>>>>>>> >
>>>>>>> > extension SequenceType {     func scan<T>(initial: T,
>>>>>>> > @noescape combine: (T, Self.Generator.Element) throws -> T)
>>>>>>> > rethrows -> [T]     func dropWhile(@noescape dropElement:
>>>>>>> > (Self.Generator.Element) throws -> Bool) rethrows ->
>>>>>>> > [Self.Generator.Element]     func takeWhile(@noescape
>>>>>>> > takeElement: (Self.Generator.Element) throws -> Bool) rethrows
>>>>>>> > -> [Self.Generator.Element] }
>>>>>>> >
>>>
>>>
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