[Pharo-project] Improving Pharo's Exception Hierarchy

[Sven Van Caekenberghe]

Hi All,

I have this idea that I would like to discuss on the next Pharo sprint and 
maybe start to implement if there is some consensus on it.

It has been mentioned many times before on this list that Object>>#error: 
should be replaced by signalling specific Exceptions in as many cases as 
possible (or where it makes sense). Even today, there was 
http://code.google.com/p/pharo/issues/detail?id=3987 !

I think the core system should set the example here, by defining a number of 
general purpose exceptions and by using them. Going over #error: senders will 
be a lot of work but can be done in incremental/Pharo steps.

So I went over the current hierarchy, scanned quickly through their references 
and some #error: senders by fundamental classes and looked at some other 
Smalltalks. I came up with the following very rough draft of an extended 
hierarchy (** marks new exceptions). In between brackets are the instance 
variables. Of course, we have to discuss about naming, instance variables and 
opportunities for reuse and inheritance (could be fun). 

I am no expert on exceptions and would certainly like to learn more. I am 
probably overlooking some aspects. We have to keep this Smalltalk like, 
flexible, dynamic, not start mimicing other languages. 

Any step in this direction will be an improvement over the current situation. 
Really using them correctly throughout the system will take some time. Defining 
the hierarchy could be done rather quickly. Refactoring is always possible 
later on.

Exception (messageText)
  Abort
  Error
    ArithemticError
      ZeroDivide (dividend)     
      FloatingPointException
      RangeError ** (from, to)
      NaNError **
    FileStreamException (fileName)
      FileDoesNotExistException (readOnly)
      FileExistsException (fileClass)
      CannotDeleteFileException
      FileWriteError
      FileReadError **
      FileClosedException **
      CannotAccessFileException **
      ReadonlyFileException **
    MessageNotUnderstood (message, receiver)
    NonBooleanReceiver (object)
    OutOfMemory
    Halt
      AssertionFailure
      BreakPoint
    SyntaxError ** !exists! (input, position)
      NumberFormatException **
    HeadlessError **
    TimedOut ** (object, operation, timeout)
    VerificationException
    IllegalOperation ** (operation, object)
    SizeMismatch (objects)
    LowMemory ** (percentageFree)
    SubclassResponsibility ** (message, receiver)
    NotYetImplemented ** (message, receiver)
    CannotInstanciate ** (class)
    ReadOnlyObject ** (object)
    OutOfFreeSpace ** (object, size)
    InvalidArgument ** (message, receiver, argument)
    NotIndexable ** (object)    
    NoKeyedAccess ** (object)
    NonIntegerIndex ** (receiver, index)
    SubscriptOutOfBounds ** (receiver, index, from, to)
    NotFoundException ** (receiver, object)
      KeyNotFound **
      ValueNotFound **
      ElementNotFound **
    StreamException (stream)
      PositionError ** (index, from, to) 
      EndOfStream **
      BeginOfStream ** 

All feedback is welcome.

Regards,

Sven