Re: Exception style (was: Tcl/Tk 8.5a4 released)

On Fri, 28 Apr 2006 22:07:53 +0200, Cameron Laird <claird@lairds.us> wrote:

> In article <4bf4ouF11e870U1@news.dfncis.de>,
> Michael Schlenker <schlenk@uni-oldenburg.de> wrote:
> .
> [little sympathy for cer-
> tain legacy maintainers]
> .
> .
>> One reason to really check what you want to know and not some side
>> effects of another command. [info exists BOB($x)] would be safer.
> .
> .
> .
> There's a real question here whose answer I certainly don't know.
> For decades, I, too, have not only said, "well, you can check what
> you *really* want ...", but have advocated doing so as the clearer
> solution. On alternate days, though, I'm sympathetic with the
> argument that what one really wants in this situation is to
> increment the variable, and that anything else *deserves* to be
> coded as an exception.

I agree. One other condition is where someone accidentally mistypes a
variable. Currently he/she would get an error and would realize his/her
mistake.

The new non-error solution is error prone, because someone may not realize
that this variable does not exist or is incorrectly referenced - and I can
imagine [upvar]/[uplevel] related situations where this may not be easily
traceable.

> So: I'm uncertain whether your "info exists ..." is truly safer.

I agree. Although Tcl itself is singlethreaded in context of each
variable, I can imagine variables being linked to an SQL database, where
[info exists] may execute some SQL and [incr] will definitely execute read
traces and write traces. Sure, it is a complex scenario, but tcldb's
config does that and I personally use it quite often for counters.

I am for having a -nocomplain (or any other name) switch :)

-- 
WK