Deprecated: Nonlocal Returns

Returning from nested anonymous functions has been deprecated.

Nonlocal returns are implemented by throwing and catching scala.runtime.NonLocalReturnException-s. This is rarely what is intended by the programmer. It can be problematic because of the hidden performance cost of throwing and catching exceptions. Furthermore, it is a leaky implementation: a catch-all exception handler can intercept a NonLocalReturnException.

A drop-in library replacement is provided in scala.util.control.NonLocalReturns. Example:

import scala.util.control.NonLocalReturns._

extension [T](xs: List[T])
   def has(elem: T): Boolean = returning {
      for x <- xs do
         if x == elem then throwReturn(true)

@main def test =
   val xs = List(1, 2, 3, 4, 5)
   assert(xs.has(2) == xs.contains(2))