In Swift, what is a 'progression'?
control-flow, iteration, swift
Solution
The first mention I see of "progression" besides the for-in documentation is in the comments of the swift framework where the stride methods are defined.
func stride<T : Strideable>(from start: T, to end: T, by stride: T.Stride) -> StrideTo<T>
Return the sequence of values (`start`, `start + stride`, `start + stride + stride`, ... last) where last is the last value in the progression that is less than `end`.
func stride<T : Strideable>(from start: T, through end: T, by stride: T.Stride) -> StrideThrough<T>
Return the sequence of values (`start`, `start + stride`, `start + stride + stride`, ... last) where last is the last value in the progression less than or equal to `end`. Note:: There is no guarantee that `end` is an element of the sequence.
So in short, "progression" refers to the `Strideable` protocol similar to how "collection" refers to the `CollectionType` protocol and the classes and structs that conform to it.
Numeric types (Int, Double, Float, UnsafePointer, Bit, etc...) tend to conform to this protocol so they may be incremented/decremented for `for in` loops. Full inheritance graph for the `Strideable` protocol found here.
Problem
According to the Control Flow section in the Swift Language Guide, The `for-in` loop performs a set of statements for each item in a range, sequence, collection, or progression. I'm pretty sure I know what three of these are: - range: something defined with the range operators, `...` or `..<` - sequence: something that conforms to the `SequenceType` protocol (documentation not anywhere obvious, but various people have reverse-engineered it) - collection: either of the Swift collection types, i.e. `Array` and `Dictionary` (I note #3 is probably redundant as `Array` and `Dictionary` both seem to conform to `SequenceType`.) But what's a "progression"? Is it some fourth thing, or was the writer just being wordy? ETA: I see there's a `CollectionType` protocol as well, so that explains #3.