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.
37 lines
822 B
Text
37 lines
822 B
Text
/-
|
||
Copyright (c) 2025 Lean FRO, LLC. All rights reserved.
|
||
Released under Apache 2.0 license as described in the file LICENSE.
|
||
Authors: Paul Reichert
|
||
-/
|
||
module
|
||
|
||
prelude
|
||
public import Init.Data.Iterators.Producers.Monadic.List
|
||
|
||
@[expose] public section
|
||
|
||
/-!
|
||
# List iterator
|
||
|
||
This module provides an iterator for lists that is accessible via `List.iter`.
|
||
-/
|
||
|
||
namespace Std.Iterators
|
||
|
||
/--
|
||
Returns a finite iterator for the given list.
|
||
The iterator yields the elements of the list in order and then terminates.
|
||
|
||
The monadic version of this iterator is `List.iterM`.
|
||
|
||
**Termination properties:**
|
||
|
||
* `Finite` instance: always
|
||
* `Productive` instance: always
|
||
-/
|
||
@[always_inline, inline]
|
||
def _root_.List.iter {α : Type w} (l : List α) :
|
||
Iter (α := ListIterator α) α :=
|
||
((l.iterM Id).toIter : Iter α)
|
||
|
||
end Std.Iterators
|