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For 2023 (and 2024 ...) - we are now fully retired from IT training.
We have made many, many friends over 25 years of teaching about Python, Tcl, Perl, PHP, Lua, Java, C and C++ - and MySQL, Linux and Solaris/SunOS too. Our training notes are now very much out of date, but due to upward compatability most of our examples remain operational and even relevant ad you are welcome to make us if them "as seen" and at your own risk.

Lisa and I (Graham) 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 are far from inactive - rather, enjoying the times that we are retired but still healthy enough in mind and body to be active!

I am also active in many other area and still look after a lot of web sites - you can find an index ((here))
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
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)
  [3287] Exceptions - Tcl style - (2011-05-12)
  [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)

T209 - Tcl/Tk - File and Directory Handling
  [779] The fragility of pancakes - and better structures - (2006-06-26)
  [785] Running external processes in Tcl and Tcl/Tk - (2006-06-29)
  [1407] Reading from another process in Tcl (pipes and sockets) - (2007-10-26)
  [1426] Buffering up in Tcl - the empty coke can comparison - (2007-11-10)
  [1467] stdout v stderr (Tcl, Perl, Shell) - (2007-12-10)
  [3192] Tcl - Some example of HOW TO in handling data files and formats - (2011-03-04)
  [3320] Reading the nth line from a file (Perl and Tcl examples) - (2011-06-09)
  [3429] Searching through all the files in or below a directory - Ruby, Tcl, Perl - (2011-09-09)
  [3617] The fileutil package and a list of file system commands in Tcl - (2012-02-18)
  [4461] Reading from a URL, and reading Json, from your Tcl script - (2015-03-12)
  [4524] Tcl - a new example for data reformatting - (2015-10-10)


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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. 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.

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