1 use crate::iter::{FusedIterator, TrustedLen};
4 /// Creates a new iterator that repeats elements of type `A` endlessly by
5 /// applying the provided closure, the repeater, `F: FnMut() -> A`.
7 /// The `repeat_with()` function calls the repeater over and over again.
9 /// Infinite iterators like `repeat_with()` are often used with adapters like
10 /// [`Iterator::take()`], in order to make them finite.
12 /// If the element type of the iterator you need implements [`Clone`], and
13 /// it is OK to keep the source element in memory, you should instead use
14 /// the [`repeat()`] function.
16 /// An iterator produced by `repeat_with()` is not a [`DoubleEndedIterator`].
17 /// If you need `repeat_with()` to return a [`DoubleEndedIterator`],
18 /// please open a GitHub issue explaining your use case.
20 /// [`repeat()`]: crate::iter::repeat
21 /// [`DoubleEndedIterator`]: crate::iter::DoubleEndedIterator
30 /// // let's assume we have some value of a type that is not `Clone`
31 /// // or which we don't want to have in memory just yet because it is expensive:
32 /// #[derive(PartialEq, Debug)]
35 /// // a particular value forever:
36 /// let mut things = iter::repeat_with(|| Expensive);
38 /// assert_eq!(Some(Expensive), things.next());
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());
45 /// Using mutation and going finite:
50 /// // From the zeroth to the third power of two:
52 /// let mut pow2 = iter::repeat_with(|| { let tmp = curr; curr *= 2; tmp })
55 /// assert_eq!(Some(1), pow2.next());
56 /// assert_eq!(Some(2), pow2.next());
57 /// assert_eq!(Some(4), pow2.next());
58 /// assert_eq!(Some(8), pow2.next());
60 /// // ... and now we're done
61 /// assert_eq!(None, pow2.next());
64 #[stable(feature = "iterator_repeat_with", since = "1.28.0")]
65 pub fn repeat_with<A, F: FnMut() -> A>(repeater: F) -> RepeatWith<F> {
66 RepeatWith { repeater }
69 /// An iterator that repeats elements of type `A` endlessly by
70 /// applying the provided closure `F: FnMut() -> A`.
72 /// This `struct` is created by the [`repeat_with()`] function.
73 /// See its documentation for more.
74 #[derive(Copy, Clone, Debug)]
75 #[stable(feature = "iterator_repeat_with", since = "1.28.0")]
76 pub struct RepeatWith<F> {
80 #[stable(feature = "iterator_repeat_with", since = "1.28.0")]
81 impl<A, F: FnMut() -> A> Iterator for RepeatWith<F> {
85 fn next(&mut self) -> Option<A> {
86 Some((self.repeater)())
90 fn size_hint(&self) -> (usize, Option<usize>) {
95 fn try_fold<Acc, Fold, R>(&mut self, mut init: Acc, mut fold: Fold) -> R
97 Fold: FnMut(Acc, Self::Item) -> R,
100 // This override isn't strictly needed, but avoids the need to optimize
101 // away the `next`-always-returns-`Some` and emphasizes that the `?`
102 // is the only way to exit the loop.
105 let item = (self.repeater)();
106 init = fold(init, item)?;
111 #[stable(feature = "iterator_repeat_with", since = "1.28.0")]
112 impl<A, F: FnMut() -> A> FusedIterator for RepeatWith<F> {}
114 #[unstable(feature = "trusted_len", issue = "37572")]
115 unsafe impl<A, F: FnMut() -> A> TrustedLen for RepeatWith<F> {}