Merge pull request #7038 from dotty-staging/add-rescue

Add rescue to support streamline error handling

Author: nicolasstucki

tests/run/rescue.scala

@@ -0,0 +1,78 @@
+import scala.util.control.NonFatal
+import scala.util.control.NonLocalReturns._
+
+object lib {
+  inline def (op: => T) rescue[T] (fallback: => T) =
+    try op
+    catch {
+      case NonFatal(_) => fallback    // ReturnThrowable is fatal error, thus ignored
+    }
+
+  inline def (op: => T) rescue[T, E <: Throwable] (fallback: PartialFunction[E, T]) =
+    try op
+    catch {
+      // case ex: ReturnThrowable[_] => throw ex      // bug #7041
+      case ex: E =>
+        // user should never match `ReturnThrowable`, which breaks semantics of non-local return
+        if (fallback.isDefinedAt(ex) && !ex.isInstanceOf[ReturnThrowable[_]]) fallback(ex) else throw ex
+    }
+}
+
+import lib._
+
+@main def Test = {
+  assert((9 / 1 rescue 1) == 9)
+  assert((9 / 0 rescue 1) == 1)
+  assert(((9 / 0 rescue { case ex: NullPointerException => 5  }) rescue 10) == 10)
+  assert(((9 / 0 rescue { case ex: ArithmeticException => 5  }) rescue 10) == 5)
+
+  assert(
+    {
+      9 / 0 rescue {
+        case ex: NullPointerException => 4
+        case ex: ArithmeticException => 3
+      }
+    } == 3
+  )
+
+  (9 / 0) rescue { case ex: ArithmeticException => 4 }
+
+  assert(
+    {
+      {
+        val a = 9 / 0 rescue {
+          case ex: NullPointerException => 4
+        }
+        a * a
+      } rescue {
+        case ex: ArithmeticException => 3
+      }
+    } == 3
+  )
+
+  assert(foo(10) == 40)
+  assert(bar(10) == 40)
+
+  // should not catch fatal errors
+  assert(
+    try { { throw new OutOfMemoryError(); true } rescue false }
+    catch { case _: OutOfMemoryError => true }
+  )
+
+  // should catch any errors specified, including fatal errors
+  assert(
+    try { { throw new OutOfMemoryError(); true } rescue { case _: OutOfMemoryError => true } }
+    catch { case _: OutOfMemoryError => false }
+  )
+
+  // should not catch NonLocalReturns
+  def foo(x: Int): Int = returning[Int] {
+    { throwReturn[Int](4 * x) : Int } rescue 10
+  }
+
+  // should catch specified exceptions, but not NonLocalReturn
+  def bar(x: Int): Int = returning[Int] {
+    { throwReturn[Int](4 * x) : Int } rescue { case _ => 10 }
+  }
+
+}