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catching exceptions - Tcl style
Other Facilities in Tcl example from a Well House Consultants training course
More on Other Facilities in Tcl [link]

This example is described in the following article(s):
   • Exceptions - Tcl style - [link]

Source code: goalie Module: T214

# catching exceptions - Tcl style

# The unknown command is run if you call a command which does not exist!

proc unknown {args} {
        puts "lost:-- $args"

# Set up some values for the demo

set val default
set debug 0

puts "How long? "
gets stdin len

puts "How wide? "
gets stdin wide

# variables len and wide MAY contain values that work in expr, or they may
# contain errors!

# catch will trap the error.

# - the message is put into a separate variabe that you MAY use rather than
# being sent to stderr

# - catch returns a true or false status to let you know whether or not there
# was and error

if {! [catch {
        set val [expr $len * $wide]
        } errmsg ]} {
   puts "Yes, that worked nicely"
} else {
   puts "Someone's done an oopsie"
   if $debug {puts "message is $errmsg"}

# An undefined command - that would normally crash a Tcl program

porridge milk salt sultanas marmite

puts "area is $val"
Learn about this subject
This module and example are covered on the following public courses:
 * Learning to program in Tcl
 * Tcl Programming
 * Tcl Programming
Also available on on site courses for larger groups

Books covering this topic
Yes. We have over 700 books in our library. Books covering Tcl, Tk and Expect are listed here and when you've selected a relevant book we'll link you on to Amazon to order.

Other Examples
This example comes from our "Other Facilities in Tcl" training module. You'll find a description of the topic and some other closely related examples on the "Other Facilities in Tcl" module index page.

Full description of the source code
You can learn more about this example on the training courses listed on this page, on which you'll be given a full set of training notes.

Many other training modules are available for download (for limited use) from our download centre under an Open Training Notes License.

Other resources
• Our Solutions centre provides a number of longer technical articles.
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The Horse's mouth provides a daily tip or thought.
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