Do you agree that the logic responsible for asking users for a better input belongs to the user interface part and it is not a part of the business logic? I.e. business logic layer have to callback user interface part when it tries to do the error recovery. So you still have to pass control to the callee to do the error recovery that business logic part cannot do on its own as if you were using exception or return codes style of error handling. To me it looks that the error recovery mechanism is essantially the same in both cases.

The only difference is added complexity of callback approach. Let's plot a couple of diagrams. Traditional approach (exceptions or return codes) for case when the business logic part bails with error:

UserInterface                   BusinessLogic
     |        
 user input   
     |                               
     |------------------------------>|
                                     |
                               do some action
                                     |
     |<----------failure-------------|
     |                               
  handle the error
     |                               
     |
[download]

If at the last point the user interface part can handle the error it can either ask the business logic part to redo the action or if this error is unrecoverable print diagnostic error or do something else.

Now callback approach:

UserInterface                   BusinessLogic                 UserInte
+rface
     |
 user input
     |
     |------------------------------>|
                                     |
                               do some action
                                     |
                                     |----------failure------------->|
                                                                     |
                                                             handle th
+e error
                                                                     |
                                                                     |
[download]

This diagram clearly show two problems:

1. This design adds additional requirement on reentrability for at least the user interface and probably for other parts of the system if the error recovery callback calls them.
2. If error recovery callback can handle the error then program flow is clear. It returns the control to the business logic part which in its turn returns the control to the callee, i.e. back to the user interface part. But what about another case when it cannot handle the error? You still have to return the control back using either exceptions or return codes! Why then bother with callbacks at all?

--
Ilya Martynov, ilya@iponweb.net
CTO IPonWEB (UK) Ltd
Quality Perl Programming and Unix Support UK managed @ offshore prices - http://www.iponweb.net
Personal website - http://martynov.org

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