Tcl - Quoted braces?!

Author

Quoted braces?!

Tim C

2005-07-24, 8:59 pm

I'm sure this has been asked before, but I've not been able to find it.

I have always treated braces such that their contents are not evaluated.
However, someone recently posed this question to me, and I can't figure
out why it works! I'm using TCL 8.4.5 Linux and ActiveTCL 8.5a2, the
other person's using ActiveTCL 8.4.10 Windows, though I suspect it's
something more general that I've avoided learning so far.

Given the brief code:
set var1 1
set var2 2
puts "{$var1 $var2}"
=====
{1 2}

I expected to see {$var1 $var2}, but instead it seems to be evaluated
to {1 2}. If I put the braces around the whole, of course, it's not
evaluated (i.e. {"{$var1 $var2}"} => "{$var1 $var2}", as I expected).

I've also tried this in a proc, and it's not (as was my first
impression) that puts is evaluating $var1 and $var2:
proc x {} {
set var1 1
set var2 2
return "{$var1 $var2}"
}
set var1 4
set var2 5
puts [x]
=====
{1 2}

Am I just missing something obvious, or is this some aspect of quoted
braces that I don't quite get yet? ;)

Thanks in advance,
-TC
--
swap triangle and cannedmeat to reply

Bryan Oakley

2005-07-24, 8:59 pm

Tim C wrote:
>
> Given the brief code:
> set var1 1
> set var2 2
> puts "{$var1 $var2}"
> =====
> {1 2}
>
> I expected to see {$var1 $var2}, but instead it seems to be evaluated
> to {1 2}. If I put the braces around the whole, of course, it's not
> evaluated (i.e. {"{$var1 $var2}"} => "{$var1 $var2}", as I expected).

Braces only have special significance if used for quoting. If they are
part of some data they have no special meaning. Thank goodness!

In your example, {}'s are not used for quoting, they are part of the
data that is quoted with double quotes. Since they aren't used for
quoting they have no special behavior.

SM Ryan

2005-07-24, 8:59 pm

# I've also tried this in a proc, and it's not (as was my first
# impression) that puts is evaluating $var1 and $var2:
# proc x {} {
# set var1 1
# set var2 2
# return "{$var1 $var2}"
# }
# set var1 4
# set var2 5
# puts [x]
# =====
# {1 2}
#
# Am I just missing something obvious, or is this some aspect of quoted
# braces that I don't quite get yet? ;)

Your entire proc body is a {-}-quoted string. If the proc body is a
"-quoted string or a [list ....] or something else, you'll get different
results.

% proc x {} {
set var1 1
set var2 2
return "{$var1 $var2}"
}
% set var1 4
4
% set var2 5
5
% puts [x]
{1 2}
% proc y {} "
set var1 1
set var2 2
return \"{$var1 $var2}\"
"
% puts [y]
{4 5}

--
SM Ryan http://www.rawbw.com/~wyrmwif/
Don't say anything. Especially you.

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