Envy opened on Exceptions

[Francesco Raymondi]

Hi all,

I have a problem with exceptions: when I want to handle an exception like in the example below:

[1/0 ]

 when: ExAll

 do: [:signal | Transcript show: 'Error' ]

the Transcript shows the string 'Error' correctly, but the ENVY also opens (I attached a picture with ENVY)

Why?

What I mistake?

My configuration is:

VA ST 6.0.2 

WidgetKit 6.0.2

WindowBuilder Pro 6.0.2

Thank you very much.

Regards,

Francesco

[Richard Sargent]

>> the Transcript shows the string 'Error' correctly, but the *debugger* also opens

The problem is that you have not specified what to do with the exception. The default is to resume. Look at the "ANSI-API (signaledException)" category of Signal. You may want to use #return or #return: to provide a replacement value for the outcome of the failed block.

By the way, catching all exception is a potential weakness in a program, unless it is in the outermost part of execution (e.g., in a read and dispatch loop). Even then, it means your handler will be responding to Warnings and Notifications, not just Errors. You almost certainly will want to respond differently to resumable exceptions versus non-resumable ones.

[Francesco Raymondi]

Richard, I tried to do your solution but I'm not sure to have understood ...

Instead, I wrote something like this and it works:

[1/0]

 when: ExAll

 do: [:signal|  

Transcript show: (signal argument) . 

signal exitWith: [error]].

I have no idea if this solution will generate some problems or not.

Thank you for help.

Regards

[Richard Sargent]

That is basically correct. The argument to #exitWith: (ANSI: #return:) should be consistent with what the guarded block should have returned. In your case, you have returned a Block evaluating some kind of variable named "error". Is that really what you wanted?

For example, the code that would have called 1/0 was expecting a number back, so typically the exception handler should return a number e.g. 0. Alternatively, but poorer, is to return nil or some other special value ... but that is a Java-like approach, so there are better ways along the lines of the #at:ifAbsent: methods.

Assuming the example you provided was in a method called #determineSomething, my preferred approach is to create a #determineSomethingOr: method which takes a Block for an argument. In the example you provided, you wouldn't tell the signal to exit, you would instead answer the value of the Block argument (e.g. ^aBlock value). This unwinds the stack, dropping the exception, etc.

Also, as I alluded before, always guard for the most specific exception rather than the most general, except in specific areas. In this example, the most specific exception would have been ZeroDivide.

[1/0]

   on: ZeroDivide

   do: [:signal | self logException: signal. signal return: 42 "or whatever :-) " ].

Note also that these days, #on:do: is preferred over #when:do:.

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