Catch an exception in an inner try block so the outer catch does not fire.

Inner catch consumes the exception:

  try
    try
      ex <- Exception("inner")
      throw ex
    catch
      -> e as Exception
      stdout.println("Inner handled")
    stdout.println("Outer continues")
  catch
    -> e as Exception
    stdout.println("Outer NOT reached")

Outer catch only fires if the exception is not caught by an inner block.

See Q1154 for basic try/catch. See Q1155 for finally.

Example

defines module qa.controlflow.nestedtrycatch

  defines program

    NestedTryCatchDemo()
      stdout <- Stdout()

      try
        stdout.println("Outer try: start")
        try
          stdout.println("Inner try: about to throw")
          ex <- Exception("Inner exception")
          throw ex
        catch
          -> e as Exception
          stdout.println("Inner catch: handled")
        stdout.println("Outer try: continues after inner block")
      catch
        -> e as Exception
        stdout.println("Outer catch: should not execute")

      stdout.println("Done")

Common mistakes

E01010 — EK9 has no 'new' keyword. Create the exception first, then throw it: ex <- Exception("msg") / throw ex.

Incorrect:

      throw new Exception("msg")

Correct:

      throw ex

Other ways to ask this

I need nested error handling where the inner block consumes the exception
In Java nested try-catch handles exceptions at the closest level. Show the EK9 pattern
Given inner and outer try blocks, verify the inner catch prevents outer catch from running
Handle an exception locally in a nested try without propagating to the outer block

Coming from another language?

Java: nested try-catch — same semantics. Python: nested try/except. EK9: nested try/catch — inner catch consumes the exception.

Keywords: outer, inner, catch, try, propagate, exception, nested