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Implement std::iter::once/empty (RFC 771) #25817

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May 30, 2015
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94 changes: 94 additions & 0 deletions src/libcore/iter.rs
Original file line number Diff line number Diff line change
Expand Up @@ -3030,6 +3030,100 @@ pub fn repeat<T: Clone>(elt: T) -> Repeat<T> {
Repeat{element: elt}
}

/// An iterator that yields nothing.
#[unstable(feature="iter_empty", reason = "new addition")]
pub struct Empty<T>(marker::PhantomData<T>);

#[unstable(feature="iter_empty", reason = "new addition")]
impl<T> Iterator for Empty<T> {
type Item = T;

fn next(&mut self) -> Option<T> {
None
}

fn size_hint(&self) -> (usize, Option<usize>){
(0, Some(0))
}
}

#[unstable(feature="iter_empty", reason = "new addition")]
impl<T> DoubleEndedIterator for Empty<T> {
fn next_back(&mut self) -> Option<T> {
None
}
}

#[unstable(feature="iter_empty", reason = "new addition")]
impl<T> ExactSizeIterator for Empty<T> {
fn len(&self) -> usize {
0
}
}

// not #[derive] because that adds a Clone bound on T,
// which isn't necessary.
#[unstable(feature="iter_empty", reason = "new addition")]
impl<T> Clone for Empty<T> {
fn clone(&self) -> Empty<T> {
Empty(marker::PhantomData)
}
}

// not #[derive] because that adds a Default bound on T,
// which isn't necessary.
#[unstable(feature="iter_empty", reason = "new addition")]
impl<T> Default for Empty<T> {
fn default() -> Empty<T> {
Empty(marker::PhantomData)
}
}

/// Creates an iterator that yields nothing.
#[unstable(feature="iter_empty", reason = "new addition")]
pub fn empty<T>() -> Empty<T> {
Empty(marker::PhantomData)
}

/// An iterator that yields an element exactly once.
#[unstable(feature="iter_once", reason = "new addition")]
pub struct Once<T> {
inner: ::option::IntoIter<T>
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I guess a use option at the beginning would be nicer than the global path here.

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::option::IntoIter<T> isn't really necessary. You could just use Option + take().

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I would recommend using IntoIter (or some other iterator adapator) as it allows proxying the next, next_back, and size_hint functions (primarily size_hint)

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That's three levels of indirection (IntoIter -> Item -> Option) but I guess they'll just get optimized away anyways.

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I guess a use option at the beginning would be nicer than the global path here.

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I'd do that if I used it twice, but adding an import to a 3000+ line file to save two colons on one line seemed excessive.

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It's unusual to use a global path, though.

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Alternatively just store Option and return self.inner.take() on next?

}

#[unstable(feature="iter_once", reason = "new addition")]
impl<T> Iterator for Once<T> {
type Item = T;

fn next(&mut self) -> Option<T> {
self.inner.next()
}

fn size_hint(&self) -> (usize, Option<usize>) {
self.inner.size_hint()
}
}

#[unstable(feature="iter_once", reason = "new addition")]
impl<T> DoubleEndedIterator for Once<T> {
fn next_back(&mut self) -> Option<T> {
self.inner.next_back()
}
}

#[unstable(feature="iter_once", reason = "new addition")]
impl<T> ExactSizeIterator for Once<T> {
fn len(&self) -> usize {
self.inner.len()
}
}

/// Creates an iterator that yields an element exactly once.
#[unstable(feature="iter_once", reason = "new addition")]
pub fn once<T>(value: T) -> Once<T> {
Once { inner: Some(value).into_iter() }
}

/// Functions for lexicographical ordering of sequences.
///
/// Lexicographical ordering through `<`, `<=`, `>=`, `>` requires
Expand Down
13 changes: 13 additions & 0 deletions src/libcoretest/iter.rs
Original file line number Diff line number Diff line change
Expand Up @@ -1096,6 +1096,19 @@ fn test_fuse_count() {
// Can't check len now because count consumes.
}

#[test]
fn test_once() {
let mut it = once(42);
assert_eq!(it.next(), Some(42));
assert_eq!(it.next(), None);
}

#[test]
fn test_empty() {
let mut it = empty::<i32>();
assert_eq!(it.next(), None);
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Maybe add another assert_eq to check that the iterators are fused? And above for once.

}

#[bench]
fn bench_rposition(b: &mut Bencher) {
let it: Vec<usize> = (0..300).collect();
Expand Down
2 changes: 2 additions & 0 deletions src/libcoretest/lib.rs
Original file line number Diff line number Diff line change
Expand Up @@ -25,6 +25,8 @@
#![feature(slice_patterns)]
#![feature(float_from_str_radix)]
#![feature(cell_extras)]
#![feature(iter_empty)]
#![feature(iter_once)]

extern crate core;
extern crate test;
Expand Down