From aef59e4fb87fe8c79d3c5d6e7164c02c84fe2b8b Mon Sep 17 00:00:00 2001
From: kit <kit@hastur.io>
Date: Sun, 11 Jul 2021 15:43:30 +1000
Subject: [PATCH] Add a `try_reduce` method to the Iterator trait

---
 library/core/src/iter/traits/iterator.rs   | 80 ++++++++++++++++++++++
 library/core/tests/iter/traits/iterator.rs | 28 ++++++++
 library/core/tests/lib.rs                  |  1 +
 3 files changed, 109 insertions(+)

diff --git a/library/core/src/iter/traits/iterator.rs b/library/core/src/iter/traits/iterator.rs
index 88e7623eba1..267fa406798 100644
--- a/library/core/src/iter/traits/iterator.rs
+++ b/library/core/src/iter/traits/iterator.rs
@@ -2216,6 +2216,86 @@ pub trait Iterator {
         Some(self.fold(first, f))
     }
 
+    /// Reduces the elements to a single one by repeatedly applying a reducing operation. If the
+    /// closure returns a failure, the failure is propagated back to the caller immediately.
+    ///
+    /// The return type of this method depends on the return type of the closure. If the closure
+    /// returns `Result<Self::Item, E>`, then this function will return `Result<Option<Self::Item>,
+    /// E>`. If the closure returns `Option<Self::Item>`, then this function will return
+    /// `Option<Option<Self::Item>>`.
+    ///
+    /// When called on an empty iterator, this function will return either `Some(None)` or
+    /// `Ok(None)` depending on the type of the provided closure.
+    ///
+    /// For iterators with at least one element, this is essentially the same as calling
+    /// [`try_fold()`] with the first element of the iterator as the initial accumulator value.
+    ///
+    /// [`try_fold()`]: Iterator::try_fold
+    ///
+    /// # Examples
+    ///
+    /// Safely calculate the sum of a series of numbers:
+    ///
+    /// ```
+    /// #![feature(iterator_try_reduce)]
+    ///
+    /// let numbers: Vec<usize> = vec![10, 20, 5, 23, 0];
+    /// let sum = numbers.into_iter().try_reduce(|x, y| x.checked_add(y));
+    /// assert_eq!(sum, Some(Some(58)));
+    /// ```
+    ///
+    /// Determine when a reduction short circuited:
+    ///
+    /// ```
+    /// #![feature(iterator_try_reduce)]
+    ///
+    /// let numbers = vec![1, 2, 3, usize::MAX, 4, 5];
+    /// let sum = numbers.into_iter().try_reduce(|x, y| x.checked_add(y));
+    /// assert_eq!(sum, None);
+    /// ```
+    ///
+    /// Determine when a reduction was not performed because there are no elements:
+    ///
+    /// ```
+    /// #![feature(iterator_try_reduce)]
+    ///
+    /// let numbers: Vec<usize> = Vec::new();
+    /// let sum = numbers.into_iter().try_reduce(|x, y| x.checked_add(y));
+    /// assert_eq!(sum, Some(None));
+    /// ```
+    ///
+    /// Use a [`Result`] instead of an [`Option`]:
+    ///
+    /// ```
+    /// #![feature(iterator_try_reduce)]
+    ///
+    /// let numbers = vec!["1", "2", "3", "4", "5"];
+    /// let max: Result<Option<_>, <usize as std::str::FromStr>::Err> =
+    ///     numbers.into_iter().try_reduce(|x, y| {
+    ///         if x.parse::<usize>()? > y.parse::<usize>()? { Ok(x) } else { Ok(y) }
+    ///     });
+    /// assert_eq!(max, Ok(Some("5")));
+    /// ```
+    #[inline]
+    #[unstable(feature = "iterator_try_reduce", reason = "new API", issue = "87053")]
+    fn try_reduce<F, R>(&mut self, f: F) -> ChangeOutputType<R, Option<R::Output>>
+    where
+        Self: Sized,
+        F: FnMut(Self::Item, Self::Item) -> R,
+        R: Try<Output = Self::Item>,
+        R::Residual: Residual<Option<Self::Item>>,
+    {
+        let first = match self.next() {
+            Some(i) => i,
+            None => return Try::from_output(None),
+        };
+
+        match self.try_fold(first, f).branch() {
+            ControlFlow::Break(r) => FromResidual::from_residual(r),
+            ControlFlow::Continue(i) => Try::from_output(Some(i)),
+        }
+    }
+
     /// Tests if every element of the iterator matches a predicate.
     ///
     /// `all()` takes a closure that returns `true` or `false`. It applies
diff --git a/library/core/tests/iter/traits/iterator.rs b/library/core/tests/iter/traits/iterator.rs
index 422e389e380..d38bca1e3b3 100644
--- a/library/core/tests/iter/traits/iterator.rs
+++ b/library/core/tests/iter/traits/iterator.rs
@@ -454,6 +454,34 @@ fn test_find_map() {
     }
 }
 
+#[test]
+fn test_try_reduce() {
+    let v: Vec<usize> = vec![1, 2, 3, 4, 5];
+    let sum = v.into_iter().try_reduce(|x, y| x.checked_add(y));
+    assert_eq!(sum, Some(Some(15)));
+
+    let v: Vec<usize> = vec![1, 2, 3, 4, 5, usize::MAX];
+    let sum = v.into_iter().try_reduce(|x, y| x.checked_add(y));
+    assert_eq!(sum, None);
+
+    let v: Vec<usize> = Vec::new();
+    let sum = v.into_iter().try_reduce(|x, y| x.checked_add(y));
+    assert_eq!(sum, Some(None));
+
+    let v = vec!["1", "2", "3", "4", "5"];
+    let max = v.into_iter().try_reduce(|x, y| {
+        if x.parse::<usize>().ok()? > y.parse::<usize>().ok()? { Some(x) } else { Some(y) }
+    });
+    assert_eq!(max, Some(Some("5")));
+
+    let v = vec!["1", "2", "3", "4", "5"];
+    let max: Result<Option<_>, <usize as std::str::FromStr>::Err> =
+        v.into_iter().try_reduce(|x, y| {
+            if x.parse::<usize>()? > y.parse::<usize>()? { Ok(x) } else { Ok(y) }
+        });
+    assert_eq!(max, Ok(Some("5")));
+}
+
 #[test]
 fn test_iterator_len() {
     let v: &[_] = &[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
diff --git a/library/core/tests/lib.rs b/library/core/tests/lib.rs
index d49c2552cfd..9ab98ba8886 100644
--- a/library/core/tests/lib.rs
+++ b/library/core/tests/lib.rs
@@ -56,6 +56,7 @@
 #![feature(iter_intersperse)]
 #![feature(iter_is_partitioned)]
 #![feature(iter_order_by)]
+#![feature(iterator_try_reduce)]
 #![feature(const_mut_refs)]
 #![feature(const_pin)]
 #![feature(const_slice_from_raw_parts)]