TCL subst or eval command is not working in my case ..

Question

subst or eval command is not working in my case .. 

% proc sum {a b} {
    return [expr $a+$b]
}
%
% set a 1
1
% set b 2
2
% sum $a $b
3

%
% sum {$a} {$b}
can\'t use n         


        

Answer 1

You put the square braces [] to the wrong place (resp. you even don't need them in the eval case). In the way you wrote the commands the sum {$a} {$b}] is evaluated before the subst or eval command could evaluate the contents of $a and $b.

Correct is:

eval sum {$a} {$b}

or

sum [subst {$a}] [subst {$b}]