Exceptions

    We will present the Green exception system using an example. In Java, we would use

try {
  makeJuice();
}
catch ( BananaException exc ) {
  ...
}
catch ( FruitException exc ) {
  ...
}

to catch exceptions thrown by makeJuice which should have been declared as
     void makeJuice()
          throws FruitException, BananaException;

In Green, this code is written as

catch = CatchFruitBananaException.new();
try(catch)
  makeJuice();
end

in which makeJuice is declared as

     proc makeJuice()
                   ( exception : CatchFruitBananaException )

and CatchFruitBananaException declared as

class CatchFruitBananaException subclassOf CatchUncheckedException
  public:
    proc throw( x : FruitException )
      begin
      ...
      end
    proc throw( x : BananaException )
      begin
      ...
      end
end

    The exception treatment should be put in the throw methods. If an exception is thrown inside makeJuice, a message throw is sent to object catch that treats the exception. An exception is thrown by sending a message to exception:
     exception.throw( FruitException.new() );

    The previous example might have been written as follows if the method bodies of CatchFruitBananaException were empty. That is, the exception treatment is put in the case statement.

try( catch = CatchFruitBananaException.new() )
  makeJuice();
end
case catch.getClassException() of
  FruitException :
    begin
    ...
    end
  BananaException :
    begin
    ...
    end
end { case }

The hierarchy of predefined Green exceptions is in the table that follows.
 
 

Hierarchy of predefined Green Exceptions

Exception   TypeErrorException   WrongParametersException   NotFoundException   PackedException   TooManyDimensionsException      MetaException     ClassNotInAllowedSetException     NoShellException     NoExtensionException        UncheckedException     StackOverflowException     IllegalArrayIndexException     OutOfMemoryException     InternalErrorException     MessageSendToNilException          NoReflectiveInfoException       NoReflectiveBodyInfoException       NoReflectiveCallInfoException            ArithmeticException       DivisionByZeroException       RealOverflowException       RealUnderflowException     AssertionException       AssertionAfterException       AssertionBeforeException       AssertionCastCharException       AssertionCastBooleanException       AssertionCastByteException       AssertionCastIntegerException       AssertionCastLongException       AssertionCastRealException       AssertionCastDoubleException

 

What is new ?

• catch objects may be passed as parameters and stored in variables;
• catch classes such as CatchFruitBananaException can be subclassed;
• the throwing of an exception is just a message send and treated as such by the type system. No exception but some predefined ones, such as IllegalIndexArrayException, may be thrown and not caught at run time.

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