Comments

Tcl uses the pound character, #, for comments. Unlike in many other languages, the # must occur at the beginning of a command. A # that occurs elsewhere is not treated specially. An easy trick to append a comment to the end of a command is to precede the # with a semicolon to terminate the previous command:

# Here are some parameters
set rate 7.0    ;# The interest rate
set months 60   ;# The loan term

One subtle effect to watch for is that a backslash effectively continues a comment line onto the next line of the script. In addition, a semicolon inside a comment is not significant. Only a newline terminates comments:

# Here is the start of a Tcl comment \
and some more of it; still in the comment

The behavior of a backslash in comments is pretty obscure, but it can be exploited as shown in Example 2-3 on page 27.

A surprising property of Tcl comments is that curly braces inside comments are still counted for the purposes of finding matching brackets. The motivation for this odd feature was to keep the original Tcl parser simpler. However, it means that the following will not work as expected to comment out an alternate version of an if expression:

# if {boolean expression1} {
if {boolean expression2} {
    some commands
}

The previous sequence results in an extra left curly brace, and probably a complaint about a missing close brace at the end of your script! A technique I use to comment out large chunks of code is to put the code inside an if block that will never execute:

if {0} {
unused code here
}