Raising a customized error/exception

Question

I'm playing around with Pharo, and would like to raise a customized error/exception/something, but I've got no idea how.
I've browsed the relevant (I think) chapters of "Pharo by Example" and "Deep into Pharo"; but couldn't make heads or tails about it - it seems way above the level that I need...

I've got a setter for a variable, and would like to raise an error/exception if the input isn't an Integer:

    MyClass >> someVariable: anInteger
       anInteger isInteger
          ifFalse: [ self SomehowProtestLoadly - the part I don't know how to do ].
       someVariable := anInteger

Just aborting or exiting would be sufficient... but if possible, I'd like to do it with a bit more "flare" - and in a more explainatory way...

Answer 1

The easiest way is to just signal a generic error:

someVariable: anInteger
  anInteger isInteger
    ifFalse: [self error: 'must be an Integer'].
  someVariable := anInteger

Now, if you want to signal a specific error, do the following:

1. Create a subclass of Error, say, NonIntegerArgument

2. Write your code like this

   someVariable: anInteger
     anInteger isInteger
       ifFalse: [NonIntegerArgument signal: 'must be an Integer'].
     someVariable := anInteger
   

3. For handling this exception do the following

   [myObject someVariable: self value]
     on: NonIntegerArgument
     do: [:ex | self handleException: ex]
   

Note that your exception could provide more information, say the actual argument that was sent. To do this add an instance variable to your NonIntegerArgument class, namely, argument. Add a getter and a setter for it. Then

NonIntegerArgument class >> #signal: aString argument: anObject
  ^self new
    argument: anObject;
    signal: aString

and use it this way

someVariable: anInteger
  anInteger isInteger
    ifFalse: [
      NonIntegerArgument
        signal: 'must be an Integer'
        argument: anInteger].
  someVariable := anInteger

Now, the ex variable will be able to respond with the argument of the offending message.