Re: thoughts about unwind_protect

[dbateman]

Robert T. Short wrote:
> 
> Sounds more like an argument against global variables than an argument 
> for unwind_protect.
> 
> I suppose I should use this feature a bit just to understand.  Probably 
> won't though.
> 
> Bob
> 
> 
> David Bateman wrote:
>> Robert T. Short wrote:
>>> I have a general question about unwind_protect.
>>>
>>> Would someone take the time to explain the advantages of 
>>> unwind_protect over try/catch?
>>>
>> From the FAQ
>>
>> Octave has a lisp like unwind_protect block that allows blocks of
>> code that terminate in an error to ensure that the variables that
>> are touched are restored. You can do something similar with
>> @code{try}/@code{catch} combined with @samp{rethrow (lasterror ())} in
>> @sc{Matlab}, however rethrow and lasterror are only available in 
>> Octave 2.9.10 and later.
>>
>> Note that using @code{try}/@code{catch} combined with @samp{rethrow
>> (lasterror ())} can not guarantee that global variables will be
>> correctly reset, as it won't catch user interrupts with Ctrl-C. For
>> example
>>
>> @example
>> @group
>>  global a
>>  a = 1;
>>  try
>>    _a = a;
>>    a = 2
>>    while true
>>    end
>>  catch
>>    fprintf ('caught interrupt\n');
>>    a = _a;
>>    rethrow (lasterror());
>>  end
>> @end group
>> @end example
>>
>> @noindent
>> compared to
>>
>> @example
>> @group
>>  global a
>>  a = 1;
>>  unwind_protect
>>    _a = a;
>>    a = 2
>>    while true
>>    end
>>  unwind_protect_cleanup
>>    fprintf ('caught interrupt\n');
>>    a = _a;
>>  end
>> @end group
>> @end example
>>
>> Typing Ctrl-C in the first case returns the user directly to the
>> prompt, and the variable "a" is not reset to the saved value. In the
>> second case the variable "a" is reset correctly.  Therefore @sc{Matlab}
>> gives no save way of temporarily changing global variables.
>>
>>
> 
> 
> 

Ok then consider

saved_path = pwd ();
unwind_protect
  saved_path = pwd ();
  cd ('some_other_directory')
  a_function_that_does_something ();
unwind_protect_cleanup
   cd (saved_path);
end

Wouldn't you prefer that the user comes back to the same directory after
hitting Ctrl-C?

D.



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