Lean 4 fork for HoTT-compatible kernel extensions (Path types, transport, HITs). Maintained against upstream leanprover/lean4.
b5b34ee054
This PR introduces slices of lists that are available via slice notation (e.g., `xs[1...5]`). * Moved the `take` combinator and the `List` iterator producer to `Init`. * Introduced a `toTake` combinator: `it.toTake` behaves like `it`, but it has the same type as `it.take n`. There is a constant cost per iteration compared to `it` itself. * Introduced `List` slices. Their iterators are defined as `suffixList.iter.take n` for upper-bounded slices and `suffixList.iter.toTake` for unbounded ones. Performance characteristics of using the slice `list[a...b]`: * when creating it: `O(a)` * every iterator step: `O(1)` * `toList`: `O(b - a + 1)` (given that a <= b) Because the slice only stores a suffix of `xs` internally, two slices can be equal even though the underlying lists differ in an irrelevant prefix. Because the `stop` field is allowed to be beyond the list's upper bound, the slices `[1][0...1]` and `[1][0...2]` are not equal, even though they effectively cover the same range of the same list. Improving this would require us to call `List.length` when building the slice, which would iterate through the whole list. |
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