[swift-evolution] About the PermutationGenerator
plx
plxswift at icloud.com
Wed Jan 6 12:42:11 CST 2016
> On Jan 6, 2016, at 11:37 AM, Jordan Rose via swift-evolution <swift-evolution at swift.org> wrote:
>
> I've bounced this idea off of Dave and Dmitri internally, so might as well put it out publicly:
>
> In Magic DWIM Swift, there would only be two types that you'd ever conform to: a destructive iteration type (today's "Generator"), and a multi-pass indexed type (today's "Collection"). Some operations can meaningfully use either one (like forEach or maxElement); these operations go on a general "traversable" type (today's "Sequence").
>
> In this world, both GeneratorType and CollectionType are refinements of SequenceType (i.e. any GeneratorType "is-a" SequenceType), including the necessary default implementations. Maybe we rename some of the protocols in the process. Again, no concrete type would ever conform to SequenceType; it's just something you can use as a generic constraint.
>
> We can't actually do this today because it creates a circularity between SequenceType and GeneratorType that the compiler can't handle. I'm pretty sure it's possible to change the compiler's protocol checking logic to allow this, though.
>
> Anyway, that's that idea. At the very least it helped me clear up my thoughts about Sequence, Collection, and Generator back when I was first learning them.
>
> Jordan
>
> P.S. This idea falls apart if someone comes up with a model (concrete type) for SequenceType that isn't a Collection or Generator. I wasn't able to think of one back when I was originally thinking about this, but of course that doesn't mean there isn't one. (Infinite collections are interesting as discussed on the "cycle" thread, but it's not the sequence/generator distinction that's really meaningful there.)
It’s not clear what you mean by a `SequenceType` that isn’t either a `Collection` or a `Generator`, but if you mean a *concrete* sequence that:
- can be re-iterated (thus not a `Generator`)
- has no meaningful index (!) (thus not a `Collection`)
…then I can provide you with examples of such. The (!) is b/c you can of course always use `Int` as an index, in the sense that “the value at index `n` is obtained by iterating `n` steps from the start of the sequence”; I’ll assume this doesn’t “count” as an index for purposes of this discussion.
Given the above, I will provide two examples.
Here is one that is stable, re-iterable, infinite, and has no “non-trivial" index:
// Modest generalization of a seedable PRNG.
// We assume that identically-seeded sources generate
// identical elements (via `randomElement`), in identical order.
protocol RandomElementSourceType {
typealias Element
typealias Seed: Equatable
// ^ `:Equatable` isn't actually necessary, but it's nice
init(seed: Seed)
mutating func randomElement() -> Element
}
struct RandomElementGenerator<R:RandomElementSourceType> : GeneratorType {
typealias Element = R.Element
private var source: R
mutating func next() -> Element? {
return source.randomElement() // <- never stops!
}
}
struct RandomElementSequence<R:RandomElementSourceType> : SequenceType {
typealias Generator = RandomElementGenerator<R>
private let seed: R.Seed
func generate() -> Generator {
return Generator(source: R(seed: seed))
// ^ as per assumptions, each iteration will be identical b/c
// because each iteration uses the same seed
}
}
…and here is one that is not-necessarily-stable, reiteration-capable, finite (one hopes!), and with no “non-trivial” index:
struct SuperviewGenerator : GeneratorType {
typealias Element = UIView
private var currentView: UIView? // or NSView on OS X, etc.
private init(initialView: UIView?) {
currentView = initialView
}
mutating func next() -> Element? {
guard let here = currentView else {
return nil
}
currentView = here.superview
return here
}
}
// also e.g. analogous constructs for `CALayer`,
// `UIViewController`, `UIResponder`, “folders/directories”, and so on...
struct SuperviewSequence : SequenceType {
typealias Generator = SuperviewGenerator
private let initialView: UIView?
init(initialView: UIView?) {
self.initialView = initialView
}
func generate() -> Generator {
return Generator(initialView: initialView)
}
func underestimateCount() -> Int {
return initialView != nil ? 1 : 0
}
}
// Useful extensions:
extension UIView {
// Enumerates the view hierarchy upward from `self`, including `self`.
func inclusiveSuperviewSequence() -> SuperviewSequence {
return SuperviewSequence(initialView: self)
}
// Enumerates the view hierarchy upward from `self`, omitting `self`.
func exclusiveSuperviewSequence() -> SuperviewSequence {
return SuperviewSequence(initialView: self.superview)
}
}
>
>
>> On Dec 31, 2015, at 9:52, Erica Sadun via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>
>> I'm trying to work them out, so it's still muddled.
>>
>> Right now, I think SequenceType is better described as CollectionWalkType but that's kind of (1) a mouthful and (2) not entirely accurate.
>>
>> Moving back a step: SequenceType is defined as: "A type that can be iterated with a `for`...`in` loop." But it says nothing about whether that loop ever terminates and many stdlib sequence functions currently don't make sense (at least if they're not lazy) with respect to infinite sequences, which should probably be "StreamType" not sequences. A couple of examples:
>> Here's my fib: http://swiftstub.com/189513594/ <http://swiftstub.com/189513594/>
>> And here's Oisin's user-input sequence: https://gist.github.com/oisdk/2c7ac33bf2188528842a <https://gist.github.com/oisdk/2c7ac33bf2188528842a>
>> Both of these are theoretically filterable, but they aren't dropLast-able, suffix-able, properly split-able, etc.
>>
>> Hopefully that's enough of a starting point to indicate where my thinking is at and what I'm trying to think through when it comes to this. -- E
>>
>>
>>> On Dec 31, 2015, at 10:09 AM, Dave Abrahams <dabrahams at apple.com <mailto:dabrahams at apple.com>> wrote:
>>>
>>>
>>>> On Dec 31, 2015, at 9:05 AM, Erica Sadun via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>>>
>>>> It does seem that in Swift the concepts of collection, sequence, permutation, stream, etc are a bit muddled.
>>>
>>> This is a pretty vague critique. Do you have specifics, and suggestions that address them?
>>>
>>>>
>>>> -- E
>>>>
>>>>
>>>>> On Dec 31, 2015, at 6:51 AM, Tino Heth via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>>>>
>>>>>> Those are collections. Collections can be iterated over multiple times.
>>>>> Speaking of the Fibonacci-numbers:
>>>>> Sure we can write an algorithm that iterates over them several times — it just won't ever finish the first iteration ;-)
>>>>> (only nitpicking — I just couldn't resist)
>>>>>
>>>>> Happy new year!
>>>>> _______________________________________________
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>>>>> swift-evolution at swift.org <mailto:swift-evolution at swift.org>
>>>>> https://lists.swift.org/mailman/listinfo/swift-evolution <https://lists.swift.org/mailman/listinfo/swift-evolution>
>>>>
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>>>
>>> -Dave
>>>
>>
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