Exceptions - Tcl style

This is now an archive web site.
Some is still relevant as at February 2026 but some is purely of historic interest.

Lisa and I (Graham) are now fully retired from IT training.We have made many friends over 30 years of teaching about Python, Tcl, Perl, PHP, Lua, Java, C and C++ - and MySQL, Apache, Linux and Solaris/SunOS too. Our training notes are out of date, but with upward compatability some examples remain operational and relevant. You are welcome to make use of them "as seen", at your own risk. We now live in what was our training centre in Melksham - happy to meet with former delegates here - but do check ahead before coming round. We remain active, enjoying the times that we are retired but still healthy enough in mind and body to do things!

I am also active in many other area and still look after a lot of web sites - you can find an index ((here))

Exceptions - Tcl style (written 2011-05-12)

Checking for all possible errors isn't a "failsafe" way of coding. If you try and check for absolutely any error, you're still very likely to miss one or two possibilities, and it's fare better to add a block to your code to say (in essence) "if anything at all goes wrong, go here...". And that's commonly known as throwing and catching exceptions.

Tcl has a catch command:
catch {code block} varname
in which you can wrap a piece of code from which you want to catch exceptions. The result of running the block - either its return value or error message - will be saved into the named variable. Catch also return a true / false boolean so that you know whether or not an exception was thrown. So - put your catch into an if block ... and add exception handling code in the optional block. There's a full example [here] from the Tcl course I was teaching yesterday.

Tcl also supports an unknown command, which is run if you ask for a command that's not defined to be run. That's a further type of exception catching - also illustrated in the example that I've linked to above.
(written 2011-05-12)

Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles

T214 - Tcl/Tk - Other Facilities in Tcl
  [239] What and why for the epoch - (2005-03-08)
  [364] pu daily and p hourly - (2005-06-30)
  [366] Error handling in Tcl through catch - (2005-07-02)
  [461] Shortened interactive commands - (2005-10-11)
  [748] Getting rid of variables after you have finished with them - (2006-06-06)
  [782] Converting between Hex and Decimal in Tcl - (2006-06-28)
  [1277] AgtInvoke - a command to drive Agilent Tcl software extensions - (2007-07-26)
  [1334] Stable sorting - Tcl, Perl and others - (2007-09-06)
  [1338] Handling Binary data in Tcl (with a note on C) - (2007-09-09)
  [2467] Tcl - catching an error before your program crashes - (2009-10-22)
  [3570] Trapping errors in Tcl - the safety net that catch provides - (2012-01-06)
  [3583] Expanding a list of parameters in Tcl - {*} and eval - (2012-01-17)
  [4207] Exception handling in Tcl - (2013-11-14)
  [4523] Catching failed commands and not crashing the program in Tcl - (2015-10-10)
  [4525] What does Tcl do if you try to run a command that is not defined? - (2015-10-10)
  [4762] Coverage map in Tcl - how many times has each proc been called? - (2017-09-28)

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This is a page archived from The Horse's Mouth at http://www.wellho.net/horse/ - the diary and writings of Graham Ellis. Every attempt was made to provide current information at the time the page was written, but things do move forward in our business - new software releases, price changes, new techniques. So much so, and it's so long ago that we are retired

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