[SOLVED] Variable substitution in a list in Tcl

Variable substitution in a list in Tcl

I am trying to substitute a variable in a list:

% set a 1  
1  
% set b {1 $a}  
1 $a  
%  
% set a 1  
1  
% set b [list 1 $a]  
1 1

When I use the curly braces {}, to define the list the variable 'a' doesn't get substituted with its value but in case of using list it does.

How do I substitute a variable's value when using a list defined with {}?

Solution

% set a 1
1
% subst {1 $a}
1 1
% set b [subst {1 $a}]
1 1

But most Tcl:ers actually prefer to use list when creating a list value with substitutions, like in your second try:

% set b [list 1 $a]

One reason for this is that subst won't preserve list structure in the value:

% set a {a b}
a b
% set b [subst {1 $a}]
1 a b
% set b [list 1 $a]
1 {a b}

Another possibility is to use double quotes instead of braces; this also won't preserve list structure.

% set b "1 $a"
1 a b