Tcl ships with the tcltest test suite / harness - and standard advice for coding is to write lots of tests and check with them every time you run your code.

There's a (new) first example [here] - loading in Tcltest and defining 3 tests:

  package require tcltest
  
  tcltest::test 001 {Hello test world} {
    expr 7 * 9
  } 63
  
  tcltest::test 002 {A test that will fail} {
    expr 7 * 10
  } 64
  
  tcltest::test 003 {Alternative formatting} \
    -body { expr 7 * 10 } \
    -result {70}

Other examples ...
Using setup and cleanup blocks - [here]
Running all the tests in a dirctory [here]
Running only if certain constraints are met [here]
(written 2015-03-11)

 Q456 - Object Orientation and General technical topics - Test Driven Development and Behaviour Driven Development
  [4326] Learning to program - comments, documentation and test code - (2014-11-22)
  [4336] Test Driven Development - a first example of principle in C - (2014-12-01)
  [4346] A behaviour driven example of writing a Java program - (2014-12-09)
  [4374] Test driven development, and class design, from first principles (using C++) - (2014-12-30)
  [4380] Behaviour Driven Development / Ruby and Cucumber - (2015-01-02)
  [4387] Regression Testing my website - Cucumber and Watir - (2015-01-07)
  [4542] The principle of mocking - and the Python Mock package - (2015-10-17)
  [4634] Regression testing - via a very short C testing framework - (2016-01-29)
  [4652] Testing new algorithms in PHP - (2016-02-20)

T249 - Tcl/Tk - The Tcltest Package

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Previous and next or Horse's mouth home

Forward to A new Tcl/tk example - a window to show system status

Reading from a URL, and reading Json, from your Tcl script
Using Object Oriented Tcl and the Tk toolkit together - real life example
Managing the window size (and layout) in Tcl/Tk
A new Tcl/tk example - a window to show system status
Test framework for TCL - Tcltest - some examples
Objects in Tcl - iTcl - updated first steps example
Working out distance between places, using OS grid references and a program in Tcl
Everything is a string - even a list
Tcl variable names - no real limits!
Binary data handling - Python and Perl