Practical Programming in Tcl & Tk, Third Edition By Brent B. Welch
Table of Contents
Chapter 1. Tcl Fundamentals
A common error is to forget a space between arguments when grouping with braces or quotes. This is because white space is used as the separator, while the braces or quotes only provide grouping. If you forget the space, you will get syntax errors about unexpected characters after the closing brace or quote. The following is an error because of the missing space between } and {:
if {$x > 1} {puts "x = $x"}
A double quote is only used for grouping when it comes after white space. This means you can include a double quote in the middle of a group without quoting it with a backslash. This requires that curly braces or white space delimit the group. I do not recommend using this obscure feature, but this is what it looks like:
set silly a"b
When double quotes are used for grouping, the special effect of curly braces is turned off. Substitutions occur everywhere inside a group formed with double quotes. In the next command, the variables are still substituted:
set x xvalue set y "foo {$x}bar" => foo {xvalue}bar
When double quotes are used for grouping and a nested command is encountered, the nested command can use double quotes for grouping, too.
puts "results [format "%f %f" $x $y]"
Spaces are not required around the square brackets used for command substitution. For the purposes of grouping, the interpreter considers everything between the square brackets as part of the current group. The following sets x to the concatenation of two command results because there is no space between ] and [.
set x [cmd1][cmd2]
Newlines and semicolons are ignored when grouping with braces or double quotes. They get included in the group of characters just like all the others. The following sets x to a string that contains newlines:
set x "This is line one. This is line two. This is line three."
During command substitution, newlines and semicolons are significant as command terminators. If you have a long command that is nested in square brackets, put a backslash before the newline if you want to continue the command on another line. This was illustrated in Example 1-9 on page 8.
A dollar sign followed by something other than a letter, digit, underscore, or left parenthesis is treated as a literal dollar sign. The following sets x to the single character $.
