AI Code

tcl Tcl Language Constructs


  • # This is a valid comment
  • # This is a valid { comment }

Placing Comments

Comments in Tcl are best thought of as another command.
A comment consists of a # followed by any number of characters up to the next newline. A comment can appear wherever a command can be placed.

# this is a valid comment
proc hello { } {
  # the next comment needs the ; before it to indicate a new command is
  # being started.
  puts "hello world" ; # this is valid
  puts "dlrow olleh" # this is not a valid comment

  # the comment below appears in the middle of a string.
  # is is not valid.
  set hw {
      hello ; # this is not a valid comment 

  gets stdin inputfromuser
  switch inputfromuser {
     # this is not a valid comment. 
     # switch expects a word to be here.
     go {
       # this is valid.  The switch on 'go' contains a list of commands
     stop {

Braces in comments

Due to the way the Tcl language parser works, braces in the code must be properly matched. This includes the braces in comments.

proc hw {} { 
   # this { code will fail
   puts {hello world}

A missing close-brace: possible unbalanced brace in comment error will be thrown.

proc hw {} {
  # this { comment } has matching braces.
  puts {hello world}

This will work as the braces are paired up properly.


In the Tcl language in many cases, no special quoting is needed.

These are valid strings:


The Tcl language splits words on whitespace, so any literals or strings with whitespace should be quoted. There are two ways to quote strings. With braces and with quotation marks.

{hello world}
"hello world"

When quoting with braces, no substitutions are performed. Embedded braces may be escaped with a backslash, but note that the backslash is part of the string.

% puts {\{ \}}
\{ \}
% puts [string length {\{ \}}]
% puts {hello [world]}
hello [world]
% set alpha abc123
% puts {$alpha}

When quoting with double quotes, command, backslash and variable substitutions are processed.

% puts "hello [world]"
invalid command name "world"
% proc world {} { return my-world }
% puts "hello [world]"
hello my-world
% puts "hello\tworld"
hello   world
% set alpha abc123
% puts "$alpha"
% puts "\{ \}"
{ }

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