Re: eval-last-sexp -> syntax error -> deletes rest of buffer!

[David Reitter]

On 4 Jun 2005, at 02:41, Luc Teirlinck wrote:

> You can get rid of the behavior you described by setting
> eval-expression-debug-on-error to nil.  Unlike what I thought, the
> default value of this variable is t, even in released versions.  I do
> not know whether this is a very good idea.

The longer the expression that is evaluated, the greater the need is  
for eval-expression-debug-on-error, since you want to see where  
exactly the error occurred. (Alternatively, it would be nice to just  
show the error message, but set the point to the position of the  
error in the code.)

What's confusing is not the debug output. What's confusing is the  
narrowing. It occurs inconsistently, depending on some implementation  
specifics, and not directed by user needs. Some very specific  
expressions / syntax errors cause narrowing to happen.

> Because the error occurred while the buffer was narrowed.  That only
> applies to a minority of errors.  The narrowing was inside a
> `save-excursion', which widens again _after_ an error.

OK. Then what should happen from a user perspective is that you  
either a) unroll changes before bringing up the debugger when they  
were not caused by the code being evaluated or b) just don't narrow  
the buffer when evaluating expressions. a) might be too complex to  
implement (safely at this time), but b) could be a little easier to do.

D