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 don't understand Tcl correctly.

subst takes a string and substitues all variables in it, right.
But you pass the result of sum {$a} {$b} into it, which fails.

So you could either subst each parameter before you call sum:

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

Or modify the sum to do the evaluation for you:

proc sum {a b} {
    uplevel 1 [list expr $a + $b]
}

expr does an own round of evaluation, this is why you usually pass a litteral string (enclosed with {}) to it, but in this case we actually use this fact. uplevel executes the command in the context of the caller.

If you call a command, Tcl will replace the variables before the actuall call is made, so maybe this snippet could help you to understand Tcl a little bit better:

set a pu
set b ts
$a$b "Hello World!"