E07850: only a single Exception is supported

What went wrong

A catch declares more than one exception parameter. A try has exactly ONE catch, and that catch takes exactly one parameter (the '->' line under 'catch').

Worked examples

Compilable examples demonstrating this error and the correct fix:

• ek9 -q 932 (Can I catch multiple exception types in one catch bloc...)

The [EXAMPLE] section below shows a catch with two parameters, then the same catch taking a single Exception.

Rationale

One catch with one parameter means every exception leaving the try is handled in one place, by one name. There is no list of catches to fall through and no multi-catch whose variable has a type the reader has to work out.

How to fix it

1. Declare a single parameter, typed as the nearest super type of every exception the try can throw (Exception, or a base exception type of your own)
2. If the reaction must differ by exception type, put it in the exception types themselves (a method each one overrides) and call that method in the catch

Common causes

• Habit from Java 7+ multi-catch ('catch (IOException | SQLException e)')
• Expecting one catch block per exception type, as in Java or C# - EK9 has one catch

See also

• E07850: multiple exceptions in one catch (this error)
• E04030: type must extend Exception

Examples

Every example is compiled by the EK9 compiler: the first fails exactly as captioned, the others fix it.

Fails with E07850: only a single Exception is supported

#!ek9
defines module error.e07850.bad

  defines function

    exceptionFunction()
      -> count as Integer
      <- rtn as Integer: count
      if count < 0
        throw Exception("negative")

    tryOnce()
      stdout <- Stdout()
      try
        stdout.println($exceptionFunction(-1))
      catch
        //A catch takes one exception
        ->
          first as Exception
          second as Exception
        stdout.println(first.reason() + second.reason())

Fixed: Catch a single exception

#!ek9
defines module error.e07850.good

  defines function

    exceptionFunction()
      -> count as Integer
      <- rtn as Integer: count
      if count < 0
        throw Exception("negative")

    tryOnce()
      stdout <- Stdout()
      try
        stdout.println($exceptionFunction(-1))
      catch
        -> ex as Exception
        stdout.println(ex.reason())

Questions and answers

These Q&A entries show E07850 and its fix:

• Q134: How do I use try/catch to handle exceptions in EK9?
• Q135: How does try/catch/finally work in EK9?
• Q932: Can I catch multiple exception types in one catch block in EK9?