Le 10 avr. 2016 à 6:17, Brent Royal-Gordon via swift-evolution
<[email protected]> a écrit :
> Remember, conditional branches are relatively slow, and we want to avoid them
> where we can. If this is, for instance, the test of a loop, the extra branch
> is not a good thing.
Perhaps it's a bit extreme, but my idea for stride is that it should only have
one branch in the loop condition and absolutely no other branch. The primitive
stride could be defined like this:
stride(from: 0, compare: <, to: 10, by: 2)
and on top you could add some convenience functions for the <, >, <=, and >=
cases:
stride(from: 0, to: 10, by: 2) // stride(from: 0, compare: <, to: 10,
by: 2)
stride(from: 10, downTo: 0, by: -2) // stride(from: 10, compare: >, to:
0, by -2)
stride(from: 0, through: 10, by: 2) // stride(from: 10, compare: <=,
to: 10, by 2)
stride(from: 10, downThrough: 0, by: -2) // stride(from: 10, compare:
>=, to: 0, by -2)
None of these should try to prevent you from making an infinite sequence, so no
trap for `by: 0` or one that moves the value in the wrong direction. And
certainly no extra loop branch based on the sign of the stride (that's the most
important part).
Based on that, you could add some convenience for ranges:
(0 ..< 10).striding(by: 2) // shortcut for: stride(from: 0, to: 10, by
2)
(0 ..< 10).stridingDown(by: -2) // shortcut for: stride(from: 10, to:
0, by -2)
I guess in this case it'd be fine to add a precondition to check that `by` is
not in the reverse direction so we can preserve the semantics of iterating over
a collection. But because of this precondition and the other one in Range, I
would actually prefer the non-range forms if I was using a `stride` in a nested
loop.
--
Michel Fortin
https://michelf.ca
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