Add cursors rfc

text/0000-linked-list-cursors.md

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+- Feature Name: linked_list_cursors
+- Start Date: 2018-10-14
+- RFC PR: (leave this empty)
+- Rust Issue: (leave this empty)
+
+# Summary
+[summary]: #summary
+
+Many of the benefits of linked lists rely on the fact that most operations
+(insert, remove, split, splice etc.) can be performed in constant time once one
+reaches the desired element. To take advantage of this, a `Cursor` interface
+can be created to efficiently edit linked lists. Furthermore, unstable
+extensions like the `IterMut` changes will be removed.
+
+# Motivation
+[motivation]: #motivation
+
+From Programming Rust:
+> As of Rust 1.12, Rust’s LinkedList type has no methods for removing a range of
+> elements from a list or inserting elements at specific locations in a list.
+> The API seems incomplete.
+
+Both of these issues have been fixed, but in different and incompatible ways.
+Removing a range of elements is possible though the unstable `drain_filter` API,
+and inserting elements in at specific locations in a list is possible through
+the `linked_list_extras` extensions to `IterMut`.
+
+This motivates the need for a standard interface for insertion and deletion of
+elements in a linked list. An efficient way to implement this is through the use
+of "cursors". A cursor represents a position in a collection that can be moved
+back and forth, somewhat like a `DoubleEndedIterator`. However, mutable cursors
+can also edit the collection at their position.
+
+A mutable cursor would allow for constant time insertion and deletion of
+elements and insertion and splitting of lists at its position. This would allow
+for simplification of the `IterMut` API and a complete LinkedList
+implementation.
+
+# Guide-level explanation
+[guide-level-explanation]: #guide-level-explanation
+
+The cursor interface would provides two new types: `Cursor` and `CursorMut`.
+These are created in the same way as iterators.
+
+With a `Cursor` one can seek back and forth through a list and get the current
+element. With a `CursorMut` One can seek back and forth and get mutable
+references to elements, and it can insert and delete elements before and behind
+the current element (along with performing several list operations such as
+splitting and splicing).
+
+Lets look at where these might be useful.
+
+## Examples
+
+This interface is helpful most times insertion and deletion are used together.
+
+For example, consider you had a linked list and wanted to remove all elements
+which satisfy a certain predicate, and replace them with another element. With
+the old interface, one would have to insert and delete separately, or split the
+list many times. With the cursor interface, one can do the following:
+
+``` rust
+fn remove_replace<T, P, F>(list: &mut LinkedList<T>, p: P, f: F)
+	where P: Fn(&T) -> bool, F: Fn(T) -> T
+{
+	let mut cursor = list.cursor_mut();
+	// move to the first element, if it exists
+	loop {
+		let should_replace = match cursor.peek() {
+			Some(element) => p(element),
+			None => break,
+		};
+		if should_replace {
+			let old_element = cursor.pop().unwrap();
+			cursor.insert(f(old_element));
+		} 
+		cursor.move_next();
+	}
+}
+```
+
+This could also be done using iterators. One could transform the list into an
+iterator, perform operations on it and collect. This is easier, however it still
+requires much needless allocation.
+
+For another example, consider code that was previously using `IterMut`
+extensions. 
+``` rust
+fn main() {
+	let mut list: LinkedList<_> = (0..10).collect();
+	let mut iter = list.iter_mut();
+	while let Some(x) = iter.next() {
+		if x >= 5 {
+			break;
+		}
+	}
+	iter.insert_next(12);
+}
+```
+This can be changed almost verbatim to `CursorMut`:
+``` rust
+fn main() {
+	let mut list: LinkedList<_> = (0..10).collect();
+	let mut cursor = list.cursor_mut() {
+	while let Some(x) = cursor.peek_next() {
+		if x >= 5 {
+			break;
+		}
+		cursor.move_next();
+	}
+	cursor.insert(12);
+}
+```
+In general, the cursor interface is not the easiest way to do something.
+However, it provides a basic API that can be built on to perform more
+complicated tasks.
+
+# Reference-level explanation
+[reference-level-explanation]: #reference-level-explanation
+
+One gets a cursor the exact same way as one would get an iterator. The
+returned cursor would point to the "empty" element, i.e. if you got an element
+and called `current` you would receive `None`.
+``` rust
+// Provides a cursor to the first element of the list
+pub fn cursor(&self) -> Cursor<T>;
+
+/// Provides a cursor with mutable references and access to the list
+pub fn cursor_mut(&mut self) -> CursorMut<T>;
+```
+
+These would provide the following interface:
+
+``` rust
+impl<T> Cursor<T> {
+	/// Move to the subsequent element of the list if it exists or the empty
+	/// element
+	pub fn move_next(&mut self);
+	/// Move to the previous element of the list
+	pub fn move_prev(&mut self);
+
+	/// Get the current element
+	pub fn current(&self) -> Option<&T>;
+	/// Get the following element
+	pub fn peek(&self) -> Option<&T>;
+	/// Get the previous element
+	pub fn peek_before(&self) -> Option<&T>;
+}
+
+impl<T> CursorMut<T> {
+	/// Move to the subsequent element of the list if it exists or the empty
+	/// element
+	pub fn move_next(&mut self);
+	/// Move to the previous element of the list
+	pub fn move_prev(&mut self);
+
+	/// Get the current element
+	pub fn current(&mut self) -> Option<&mut T>;
+	/// Get the next element
+	pub fn peek(&mut self) -> Option<&mut T>;
+	/// Get the previous element
+	pub fn peek_before(&mut self) -> Option<&mut T>;
+
+	/// Get an immutable cursor at the current element
+	pub fn as_cursor(&self) -> Cursor<T>;
+
+	// Now the list editing operations
+
+	/// Insert `item` after the cursor
+	pub fn insert(&mut self, item: T);
+	/// Insert `item` before the cursor
+	pub fn insert_before(&mut self, item: T);
+
+	/// Remove and return the item following the cursor
+	pub fn pop(&mut self) -> Option<T>;
+	/// Remove and return the item before the cursor
+	pub fn pop_before(&mut self) -> Option<T>;
+
+	/// Insert `list` between the current element and the next
+	pub fn insert_list(&mut self, list: LinkedList<T>);
+	/// Insert `list` between the previous element and current
+	pub fn insert_list_before(&mut self, list: LinkedList<T>);
+
+	/// Split the list in two after the current element
+	/// The returned list consists of all elements following the current one.
+	// note: consuming the cursor is not necessary here, but it makes sense
+	// given the interface
+	pub fn split(self) -> LinkedList<T>;
+	/// Split the list in two before the current element
+	pub fn split_before(self) -> LinkedList<T>;
+}
+```
+One question that arises from this interface is what happens if `move_next` is
+called when a cursor is on the last element of the list, or is empty (or
+`move_prev` and the beginning). A simple way to solve this is to make cursors
+wrap around this list back to the empty element. One could complicate the
+interface by having move return a `bool`, however this is unnecessary since
+`current` is sufficient to know whether the iterator is at the end of the list.
+
+A large consequence of this new interface is that it is a complete superset of
+the already existing `Iter` and `IterMut` API. Therefore, the following two
+methods added to `IterMut` in the `linked_list_extras` features should be
+removed or depreciated:
+- `IterMut::insert_next`
+- `IterMut::peek_next`
+The rest of the iterator methods are stable and should probably stay untouched
+(but see below for comments).
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+The cursor interface is rather clunky, and while it allows for efficient code,
+it is probably not useful outside of many use-cases.
+
+One of the largest issues with the cursor interface is that it exposes the exact
+same interface of iterators (and more), which leads to unnecessary code
+duplication.
+However, the purpose of iterators seems to be simple, abstract and easy to use
+rather than efficient mutation, so cursors and iterators should be used
+in different places.
+
+# Rationale and alternatives
+[rationale-and-alternatives]: #rationale-and-alternatives
+
+There are several alternatives to this:
+
+1. Implement cursors as a trait extending `Iterator` (see the cursors
+pseudo-rfc below)
+
+Since the cursors are just an extension of iterators, it makes some sense to
+create them as a trait. However, I see several reasons why this is not the best.
+
+First, cursors work differently than the existing `Iterator` extensions like
+`DoubleEndedIterator`. In a `DoubleEndedIterator`, if one calls `next_back` and
+then `next`, it should not return the same value, so unlike a cursor, a
+`DoubleEndedIterator` does not move back and forth throughout a collection.
+
+Furthermore, while `Iterator` is a general interface for many collections,
+`Cursor` is very much specific to linked lists. In other collections such as
+`Vec` a cursor does not make sense. So it makes little sense to make a trait
+when it will only be used in one place.
+
+2. Using the `IterMut` linked list extensions
+
+Insertion was added to `IterMut` in the `linked_list_extras` feature. Many of
+these features could be added to it just as well. But, this overcrowds `IterMut`
+with many methods that have nothing to do with iteration (such as deletion,
+splitting etc.)
+It makes sense to put these explicitly in their own type, and this can be
+`CursorMut`.
+
+3. Do not create cursors at all
+
+Everything that cursors do can already be done, albeit in sometimes a less
+efficient way. Efficient code can be written by splitting linked lists often,
+and while this is a complicated way to do things, the rarity of the use case may
+justify keeping things how they are.
+
+# Prior art
+[prior-art]: #prior-art
+
+- [cursors pseudo-rfc](https://internals.rust-lang.org/t/pseudo-rfc-cursors-reversible-iterators/386/18)
+
+This rust internals post describes an early attempt at making cursors. The
+language was in a different state when it was written (pre-1.0), so details have
+changed since then. But this describes several different approaches to making
+cursors and where they led.
+
+- Java-style iterators
+
+Java (and other languages) tried to fix this by adding a `remove` function to
+their iterators. However, I feel this method would not be the best choice for
+Rust (even for specific `IterMut`s like those in LinkedList) since it diverges
+from the expected behaviour of iterators.
+
+- [linked list extras issue](https://github.com/rust-lang/rust/issues/27794)
+
+Discussion on the issue tracker about how this is currently managed with
+modifications to `IterMut`. The consensus seems to be that it is incomplete, and
+it is suggested to create a new `Cursor` and `CursorMut` types.
+
+# Unresolved questions
+[unresolved-questions]: #unresolved-questions
+
+- How will this interface interact with iterators?
+
+Will we keep both `Iter` and `Cursor` types? Implement one with another? I feel
+like they should be different things, but there is reason to consolidate them.
+
+- Only for linked lists?
+
+Should we implement this for more collections? It could make sense for other
+collections, such as trees and arrays, but the design would have to be reworked.