If a hash character ("#") appears at a point where Tcl is expecting the first character of the first word of a command, then the hash character and the characters that follow it, up through the next newline, are treated as a comment and ignored. The comment character only has significance when it appears at the beginning of a command. (From: TclHelp).
Tcl students frequently ask "Why can I not place unmatched braces in Tcl comments?"
Tcl comments are almost like null-operation commands. You could write one yourself like this:
proc -- args {}
-- This is a call to the new proc that does nothing, like a comment
-- but this here still does an [exit]!! So it's not like
# [exit] [exit] [exit] ... and still won't die ;-)Note the "if 0 ..." idiom:
if 0 {
Any Tcl code to be commented out (with matching braces, of course!)
or any other kind of text, will be ignored - and even [exit] won't fire
because it's in braces, so left unevaluated!} and a fancy sugar for that: "#" is special only if first character of a command name. Nobody hinders you to write a
proc {#} args {}where the braces are only visual markup - the parser strips them off, the proc's name will be just "#". But to call it, you have to escape the "#" sign - with a backslash, or with my favorite sugar:
{#} {
This is a comment in braces
}where the name of the command is pretty self-documenting: "comment in braces"... Much more intuitive than e.g. Ruby's "=begin ... =end", and available with almost zero implementation! (RS)
In the Tcl chatroom, MS pointed out: "the bytecode compiler recognizes "proc whatever args {}" and compiles no invocation for that; the cost is (almost) nothing. For other empty procs, an invocation will be compiled, so that you have a call/return overhead", and that "if 0 {...}" may raise errors on some contents (up to tcl8.3). Any word corresponding to a bcc'ed command at the start of a line is risky ...:
% if 0 {set a b c}
% if 0 {if you want to write this, you can't}
% if 0 {while you read this, tcl errs out}Therefore,
{#} {set any number of words; if you so wish} is more robust than "if 0 {...}", as long as you DO NOT forget the braces, and before 8.4. But
{#} do not [[exit]]*will* exit ...
AB Forgive me for an easy question, but is there a way to comment out a whole block of code in TCL? For instance, I know in C++ you can do so by typing "/*", and then everything that follows is commented out until the compiler comes to "*/". Thanks!
Use
if {0} {
... code to comment ...
}MG Tcl doesn't have -real- block-commenting like that, though. The if {0} { ... } method won't work if you have mismatched braces inside, or anything like that - the first mismatched } will end the if, and the code after it will start running.
(noname) 18-Aug-05 Try it! Use
# First, define an block comment procedure, as:
proc /* {args} {}
# Then,you can use it as:
/* { ...
.. any comments,(with carefull { } pairs).
} and more args in this line will be ignored.RS 2006-07-13: In the Tcl chatroom someone asked how to have comments in lists. First answer is, lists are data, and comments apply only where commands are parsed; but the second answer is: of course! Lists in a source file come as strings (often in {} braces), and that Tcl is good at doing things with strings is well-known :^) One regsub is enough to strip substrings between # and the next newline:
set data {
# This is an example of comments in data
test {
4711 # an integer
all # a word
}
date today # one more test
}
% regsub -all {#.*?\n} $data \n
test {
4711
all
}
date today
RLH Wouldn't it be nifty:
#- Everything in here is a comment or something similiar. #-