Home Accessibility Courses Twitter The Mouth Facebook Resources Site Map About Us Contact
 
Python and Tcl - public course schedule [here]
Private courses on your site - see [here]
Please ask about maintenance training for Perl, PHP, Lua, etc
 
Tcl - catching an error before your program crashes

There are times when a Tcl command can fail because of the data being passed in to it ... and when it fails, it can do so with a spectacular crash! For example, the glob command which matches files to a pattern (Tcl's ls or dir if you like to think of it that way) can go 'belly up' if there are no files at all that match the pattern:

no files matched glob pattern "*.lua"
   while executing
"glob *.lua"
   invoked from within
"set file [glob *.lua]"
   (file "tcl/falls" line 15)


OOooops!

If you want your Tcl program to continue, even on an error, you should use the catch command and run the command that may fail within a deferred block that's passed in to it ... something like:

catch {glob *.lua} yikes.

It works like this:
• If the command WORKS, the result is put back into the yikes variable, and catch returns a true value, but
• if the command FAILS, an error message is put into the yikes variable, and catch returns a false value.

There's a code example here that illustrates catching (on glob) with testihng the conditionals. And further examples here (failing) and here (caught). There's also a further explanation on the blog here, and a demonstration of how catch can handle a failure to open a file here.

When you are first writing a piece of code - writing what's known as a 'proof of concept' or a 'spike solution', you may not be thinking about error handling too much. But it reallity such code testing / defensive coding against errors is VITAL. So we make certain that we cover catch and loads of associated subjects on our Tcl Programming courses.
(written 2009-10-22)

 
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
T209 - Tcl/Tk - File and Directory Handling
  [4524] Tcl - a new example for data reformatting - (2015-10-10)
  [4523] Catching failed commands and not crashing the program in Tcl - (2015-10-10)
  [4461] Reading from a URL, and reading Json, from your Tcl script - (2015-03-12)
  [3617] The fileutil package and a list of file system commands in Tcl - (2012-02-18)
  [3429] Searching through all the files in or below a directory - Ruby, Tcl, Perl - (2011-09-09)
  [3320] Reading the nth line from a file (Perl and Tcl examples) - (2011-06-09)
  [3192] Tcl - Some example of HOW TO in handling data files and formats - (2011-03-04)
  [1467] stdout v stderr (Tcl, Perl, Shell) - (2007-12-10)
  [1426] Buffering up in Tcl - the empty coke can comparison - (2007-11-10)
  [1407] Reading from another process in Tcl (pipes and sockets) - (2007-10-26)
  [785] Running external processes in Tcl and Tcl/Tk - (2006-06-29)
  [779] The fragility of pancakes - and better structures - (2006-06-26)

T214 - Tcl/Tk - Other Facilities in Tcl
  [4762] Coverage map in Tcl - how many times has each proc been called? - (2017-09-28)
  [4525] What does Tcl do if you try to run a command that is not defined? - (2015-10-10)
  [4207] Exception handling in Tcl - (2013-11-14)
  [3583] Expanding a list of parameters in Tcl - {*} and eval - (2012-01-17)
  [3570] Trapping errors in Tcl - the safety net that catch provides - (2012-01-06)
  [3287] Exceptions - Tcl style - (2011-05-12)
  [1338] Handling Binary data in Tcl (with a note on C) - (2007-09-09)
  [1334] Stable sorting - Tcl, Perl and others - (2007-09-06)
  [1277] AgtInvoke - a command to drive Agilent Tcl software extensions - (2007-07-26)
  [782] Converting between Hex and Decimal in Tcl - (2006-06-28)
  [748] Getting rid of variables after you have finished with them - (2006-06-06)
  [461] Shortened interactive commands - (2005-10-11)
  [366] Error handling in Tcl through catch - (2005-07-02)
  [364] pu daily and p hourly - (2005-06-30)
  [239] What and why for the epoch - (2005-03-08)


Back to
Tcl - passing arrays and strings in and back out of procs
Previous and next
or
Horse's mouth home
Forward to
What are Tcl lists?
Some other Articles
A short form of if ... then ... else
Windows 7 and Open Source Programming
Beyond the Pale
What are Tcl lists?
Tcl - catching an error before your program crashes
Tcl - passing arrays and strings in and back out of procs
Melksham Town - asleep or awake?
Beauty in pictures
Tcl - a true interpretive, command based language
Python - how it saves on compile time
4759 posts, page by page
Link to page ... 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96 at 50 posts per page


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. Please check back via our main site for current courses, prices, versions, etc - any mention of a price in "The Horse's Mouth" cannot be taken as an offer to supply at that price.

Link to Ezine home page (for reading).
Link to Blogging home page (to add comments).

You can Add a comment or ranking to this page

© WELL HOUSE CONSULTANTS LTD., 2019: 404 The Spa • Melksham, Wiltshire • United Kingdom • SN12 6QL
PH: 01225 708225 • EMAIL: info@wellho.net • WEB: http://www.wellho.net • SKYPE: wellho

PAGE: http://www.wellho.net/mouth/2467_Tcl ... ashes.html • PAGE BUILT: Sat May 27 16:49:10 2017 • BUILD SYSTEM: WomanWithCat