WithPosition::fold

src/with_position.rs

@@ -81,6 +81,33 @@ impl<I: Iterator> Iterator for WithPosition<I> {
     fn size_hint(&self) -> (usize, Option<usize>) {
         self.peekable.size_hint()
     }
+
+    fn fold<B, F>(mut self, mut init: B, mut f: F) -> B
+    where
+        F: FnMut(B, Self::Item) -> B,
+    {
+        if !self.handled_first {
+            // Haven't seen the first item yet, and there might be one to give.
+            self.handled_first = true;
+            if let Some(first) = self.peekable.next() {
+                // Peek to see if this is also the last item,
+                // in which case tag it as `Only`.
+                match self.peekable.peek() {
+                    Some(_) => init = f(init, (Position::First, first)),
+                    None => return f(init, (Position::Only, first)),
+                }
+            }
+        }
+        if let Some(mut head) = self.peekable.next() {
+            // Have seen the first item, and there's something left.
+            (init, head) = self.peekable.fold((init, head), |(acc, old), new| {
+                (f(acc, (Position::Middle, old)), new)
+            });
+            // The "head" is now the last item.
+            init = f(init, (Position::Last, head));
+        }
+        init
+    }
 }
 
 impl<I> ExactSizeIterator for WithPosition<I> where I: ExactSizeIterator {}