Re: Exceptions: Are they GOTOs?

[gauthier@unilim.fr (Michel Gauthier)]

In article <brad.837052481@ss1>, brad@cs.uwa.oz.au (Bradley Edelman) wrote:

>>  What do people think about exception handling?  I believe it to be a major
>>  strength of Ada but there is a school of thought that exceptions are just
>>  GOTO statements and should be avoided accordingly.
>>  
>>  I think raising an exception is a neater way of handling an error than, say,
>>  returning an error code or status.
>>  
>>  I'd appreciate thoughts either way.

You might find a somewhat long answer in my book
"Ada, a professional course", published by Macmillan.
May I abstract it here ?

The main difference between exception raise and goto is that :
- raising an exception is essentially declarative : I assert that...
- goto'ing (rather than 'going to') is imperative : do this...

Exceptions are not a concept. The concept is 'reporting' 
that the required service could not be obtained.
Exceptions can implement reports, as well as any kind
of status code. Most of the problems come from reporting,
not from its implementation.

The main problem is not in goto nor in raise but in the lable or 
in the handling. It is caused by the multipleness of program 
flows that converge. Hence :
- the assertion is a disjunct, therefore weakened,
- generally, the programmer has only a partial knowledge of
   this set of arrows, and in fact knows nothing useful.

Additional thougths later, if requested.

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