2 use crate::iter::{FusedIterator, TrustedLen};
5 /// Creates a new iterator that repeats elements of type `A` endlessly by
6 /// applying the provided closure, the repeater, `F: FnMut() -> A`.
8 /// The `repeat_with()` function calls the repeater over and over again.
10 /// Infinite iterators like `repeat_with()` are often used with adapters like
11 /// [`Iterator::take()`], in order to make them finite.
13 /// If the element type of the iterator you need implements [`Clone`], and
14 /// it is OK to keep the source element in memory, you should instead use
15 /// the [`repeat()`] function.
17 /// An iterator produced by `repeat_with()` is not a [`DoubleEndedIterator`].
18 /// If you need `repeat_with()` to return a [`DoubleEndedIterator`],
19 /// please open a GitHub issue explaining your use case.
21 /// [`repeat()`]: crate::iter::repeat
22 /// [`DoubleEndedIterator`]: crate::iter::DoubleEndedIterator
31 /// // let's assume we have some value of a type that is not `Clone`
32 /// // or which we don't want to have in memory just yet because it is expensive:
33 /// #[derive(PartialEq, Debug)]
36 /// // a particular value forever:
37 /// let mut things = iter::repeat_with(|| Expensive);
39 /// assert_eq!(Some(Expensive), things.next());
40 /// assert_eq!(Some(Expensive), things.next());
41 /// assert_eq!(Some(Expensive), things.next());
42 /// assert_eq!(Some(Expensive), things.next());
43 /// assert_eq!(Some(Expensive), things.next());
46 /// Using mutation and going finite:
51 /// // From the zeroth to the third power of two:
53 /// let mut pow2 = iter::repeat_with(|| { let tmp = curr; curr *= 2; tmp })
56 /// assert_eq!(Some(1), pow2.next());
57 /// assert_eq!(Some(2), pow2.next());
58 /// assert_eq!(Some(4), pow2.next());
59 /// assert_eq!(Some(8), pow2.next());
61 /// // ... and now we're done
62 /// assert_eq!(None, pow2.next());
65 #[stable(feature = "iterator_repeat_with", since = "1.28.0")]
66 pub fn repeat_with<A, F: FnMut() -> A>(repeater: F) -> RepeatWith<F> {
67 RepeatWith { repeater }
70 /// An iterator that repeats elements of type `A` endlessly by
71 /// applying the provided closure `F: FnMut() -> A`.
73 /// This `struct` is created by the [`repeat_with()`] function.
74 /// See its documentation for more.
75 #[derive(Copy, Clone)]
76 #[stable(feature = "iterator_repeat_with", since = "1.28.0")]
77 pub struct RepeatWith<F> {
81 #[stable(feature = "iterator_repeat_with_debug", since = "1.68.0")]
82 impl<F> fmt::Debug for RepeatWith<F> {
83 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
84 f.debug_struct("RepeatWith").finish_non_exhaustive()
88 #[stable(feature = "iterator_repeat_with", since = "1.28.0")]
89 impl<A, F: FnMut() -> A> Iterator for RepeatWith<F> {
93 fn next(&mut self) -> Option<A> {
94 Some((self.repeater)())
98 fn size_hint(&self) -> (usize, Option<usize>) {
103 fn try_fold<Acc, Fold, R>(&mut self, mut init: Acc, mut fold: Fold) -> R
105 Fold: FnMut(Acc, Self::Item) -> R,
106 R: Try<Output = Acc>,
108 // This override isn't strictly needed, but avoids the need to optimize
109 // away the `next`-always-returns-`Some` and emphasizes that the `?`
110 // is the only way to exit the loop.
113 let item = (self.repeater)();
114 init = fold(init, item)?;
119 #[stable(feature = "iterator_repeat_with", since = "1.28.0")]
120 impl<A, F: FnMut() -> A> FusedIterator for RepeatWith<F> {}
122 #[unstable(feature = "trusted_len", issue = "37572")]
123 unsafe impl<A, F: FnMut() -> A> TrustedLen for RepeatWith<F> {}