| use crate::iter::{FusedIterator, TrustedLen}; |
| use crate::ops::Try; |
| |
| /// Creates a new iterator that repeats elements of type `A` endlessly by |
| /// applying the provided closure, the repeater, `F: FnMut() -> A`. |
| /// |
| /// The `repeat_with()` function calls the repeater over and over again. |
| /// |
| /// Infinite iterators like `repeat_with()` are often used with adapters like |
| /// [`Iterator::take()`], in order to make them finite. |
| /// |
| /// If the element type of the iterator you need implements [`Clone`], and |
| /// it is OK to keep the source element in memory, you should instead use |
| /// the [`repeat()`] function. |
| /// |
| /// An iterator produced by `repeat_with()` is not a [`DoubleEndedIterator`]. |
| /// If you need `repeat_with()` to return a [`DoubleEndedIterator`], |
| /// please open a GitHub issue explaining your use case. |
| /// |
| /// [`repeat()`]: crate::iter::repeat |
| /// [`DoubleEndedIterator`]: crate::iter::DoubleEndedIterator |
| /// |
| /// # Examples |
| /// |
| /// Basic usage: |
| /// |
| /// ``` |
| /// use std::iter; |
| /// |
| /// // let's assume we have some value of a type that is not `Clone` |
| /// // or which we don't want to have in memory just yet because it is expensive: |
| /// #[derive(PartialEq, Debug)] |
| /// struct Expensive; |
| /// |
| /// // a particular value forever: |
| /// let mut things = iter::repeat_with(|| Expensive); |
| /// |
| /// assert_eq!(Some(Expensive), things.next()); |
| /// assert_eq!(Some(Expensive), things.next()); |
| /// assert_eq!(Some(Expensive), things.next()); |
| /// assert_eq!(Some(Expensive), things.next()); |
| /// assert_eq!(Some(Expensive), things.next()); |
| /// ``` |
| /// |
| /// Using mutation and going finite: |
| /// |
| /// ```rust |
| /// use std::iter; |
| /// |
| /// // From the zeroth to the third power of two: |
| /// let mut curr = 1; |
| /// let mut pow2 = iter::repeat_with(|| { let tmp = curr; curr *= 2; tmp }) |
| /// .take(4); |
| /// |
| /// assert_eq!(Some(1), pow2.next()); |
| /// assert_eq!(Some(2), pow2.next()); |
| /// assert_eq!(Some(4), pow2.next()); |
| /// assert_eq!(Some(8), pow2.next()); |
| /// |
| /// // ... and now we're done |
| /// assert_eq!(None, pow2.next()); |
| /// ``` |
| #[inline] |
| #[stable(feature = "iterator_repeat_with", since = "1.28.0")] |
| pub fn repeat_with<A, F: FnMut() -> A>(repeater: F) -> RepeatWith<F> { |
| RepeatWith { repeater } |
| } |
| |
| /// An iterator that repeats elements of type `A` endlessly by |
| /// applying the provided closure `F: FnMut() -> A`. |
| /// |
| /// This `struct` is created by the [`repeat_with()`] function. |
| /// See its documentation for more. |
| #[derive(Copy, Clone, Debug)] |
| #[stable(feature = "iterator_repeat_with", since = "1.28.0")] |
| pub struct RepeatWith<F> { |
| repeater: F, |
| } |
| |
| #[stable(feature = "iterator_repeat_with", since = "1.28.0")] |
| impl<A, F: FnMut() -> A> Iterator for RepeatWith<F> { |
| type Item = A; |
| |
| #[inline] |
| fn next(&mut self) -> Option<A> { |
| Some((self.repeater)()) |
| } |
| |
| #[inline] |
| fn size_hint(&self) -> (usize, Option<usize>) { |
| (usize::MAX, None) |
| } |
| |
| #[inline] |
| fn try_fold<Acc, Fold, R>(&mut self, mut init: Acc, mut fold: Fold) -> R |
| where |
| Fold: FnMut(Acc, Self::Item) -> R, |
| R: Try<Output = Acc>, |
| { |
| // This override isn't strictly needed, but avoids the need to optimize |
| // away the `next`-always-returns-`Some` and emphasizes that the `?` |
| // is the only way to exit the loop. |
| |
| loop { |
| let item = (self.repeater)(); |
| init = fold(init, item)?; |
| } |
| } |
| } |
| |
| #[stable(feature = "iterator_repeat_with", since = "1.28.0")] |
| impl<A, F: FnMut() -> A> FusedIterator for RepeatWith<F> {} |
| |
| #[unstable(feature = "trusted_len", issue = "37572")] |
| unsafe impl<A, F: FnMut() -> A> TrustedLen for RepeatWith<F> {} |