Find a substring

find and rfind return the first or last match as Option[i64]. Unlike Python’s -1 sentinel, a missing match is Nothing. Positions count Unicode scalars:

def main() -> Result[(), IoError]:
    text = "é🙂é🙂"
    print(text.find("🙂"))?
    print(text.rfind("🙂"))?
    print(text.find("missing"))?
    print(text.rfind(""))?
    Ok(())

Option[i64].Some(1)
Option[i64].Some(3)
Option[i64].Nothing
Option[i64].Some(4)

Both operations borrow their inputs and allocate nothing. Use match or ? in an Option-returning function. An empty needle matches at the beginning for find and at the end for rfind. Each takes one literal pattern; optional bounds and regexes are not supported.