Add raise? to String::fold and StringView::fold

[Copilot]

String::fold and StringView::fold (and their rev_fold variants) lacked raise? annotations, preventing exceptions from propagating through the callback function. This was inconsistent with all other collection types (Array, FixedArray, ReadOnlyArray, Iter, Iterator) which support raise? in their fold operations.

Changes

• Added raise? to callback parameter type: (A, Char) -> A → (A, Char) -> A raise?
• Added raise? to return type: -> A → -> A raise?
• Applied to: StringView::fold, StringView::rev_fold, String::fold, String::rev_fold

Example

// Exception now propagates correctly
let result = try? "hello".fold(init=0, (acc, c) => {
  if c == 'l' {
    raise Failure("found l")
  }
  acc + 1
})
// result: Err(Failure("found l"))

Original prompt

---

This section details on the original issue you should resolve

<issue_title>[Consistency Review] add raise? to String::fold and StringView::fold</issue_title>
<issue_description>Current behaviors

Array, FixedArray, ReadOnlyArray, Iter, and Iterator have the following signatures for fold:

fn[A, B] Array::fold(Self[A], init~ : B, (B, A) -> B raise?) -> B raise?
fn[A, B] FixedArray::fold(Self[A], init~ : B, (B, A) -> B raise?) -> B raise?
fn[A, B] ReadOnlyArray::fold(Self[A], init~ : B, (B, A) -> B raise?) -> B raise?
fn[T, B] Iter::fold(Self[T], init~ : B, (B, T) -> B raise?) -> B raise?
fn[X, R] Iterator::fold(Self[X], init~ : R, (R, X) -> R raise?) -> R raise?

but String and StringView have these signatures:

fn[A] String::fold(String, init~ : A, (A, Char) -> A) -> A
fn[A] StringView::fold(Self, init~ : A, (A, Char) -> A) -> A

Expected behaviors

I expected String::fold and StringView::fold to have raise? so that exceptions raised by the callback are raised by fold.

Justification

Adding raise? would increase consistency with other implementations of fold and make the exception types more accurate.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes [Consistency Review] add raise? to String::fold and StringView::fold #3006

---

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.