Can I catch multiple exception types in one catch block in EK9?

← Control Flow · Ref: Q932

Each EK9 catch block handles exactly ONE exception type. There is no multi-catch syntax.

SINGLE CATCH

  try
    riskyWork()
  catch
    -> ex as Exception
    stderr.println(`Failed: ${ex}`)

The catch uses -> on its own line to declare the incoming exception. This follows EK9's data-flow convention: -> means data flowing IN.

WHY ONE TYPE PER CATCH

Multi-catch (Java's catch (A | B e)) creates ambiguity about the exception's actual type inside the handler. EK9 enforces clarity: you know the exact type you are handling.

HANDLING DIFFERENT TYPES

Use separate try/catch blocks or catch the base Exception type:

  try
    riskyWork()
  catch
    -> ex as Exception
    stderr.println(`Caught: ${ex}`)

Since all EK9 exceptions descend from Exception, catching Exception handles everything.

See Q902 for try/catch basics. See Q934 for try/finally. See Q130 for control flow overview.

Example

defines module qa.controlflow.singlecatch

  defines function

    safeDivision() as pure
      ->
        numerator as Integer
        denominator as Integer
      <-
        rtn as Float: #^ numerator / #^ denominator

  defines program

    SingleCatchDemo()
      stdout <- Stdout()
      stderr <- Stderr()

      //Single exception type per catch
      try
        outcome <- safeDivision(42, 7)
        stdout.println(`42 / 7 = ${outcome}`)
      catch
        -> ex as Exception
        stderr.println(`Division failed: ${ex}`)

      //Catching the base Exception covers all exception types
      try
        second <- safeDivision(100, 5)
        stdout.println(`100 / 5 = ${second}`)
      catch
        -> ex as Exception
        stderr.println(`Error: ${ex}`)
      finally
        stdout.println("Division attempt complete")

Common mistakes

E07850 — An EK9 catch block's arrow may declare only ONE exception variable (E07850). Declaring two (ex1, ex2) in a single catch is rejected. Use one catch block, catching Exception to handle all types. See ek9 -h E07850 for details.

Incorrect:

      catch
        ->
          ex1 as Exception
          ex2 as Exception
        stderr.println(`Failed: ${ex1}`)

Correct:

      catch
        -> ex as Exception
        stderr.println(`Division failed: ${ex}`)

Coming from another language?

Java: catch (IOException | SQLException e) multi-catch since Java 7. Python: except (TypeError, ValueError) as e. C#: catch (Exception e) when — filter pattern. EK9: one type per catch block, no multi-catch.

Keywords: multi-catch, exception, type, catch, try, handler, E07850, single