Some of these things do not quite work this way, but they demonstrate some interesting peculiarities! The eval should be expr, as you are trying to evaluate a mathematical expression, not a command. Similar to the common footgun:

    set a 1
    puts [expr $a + $a] # Wrong!
    puts [expr {$a + $a}] # Correct

Also $$b expands to $a, not 5, as dollar expansion stops at the next non-alphanumeric-or-underscore character. This would usually be written as [set $b] (equivalent to [set [set b]] ), utilising that the set command outputs the value of the variable with that name. Similarly, [set var[set $b]] for the last line. Also, # is only a special character at the beginning of a statement, so comments have to follow a newline or semicolon.

In the “everything is a string” department, my favourite are block comments:

    if 0 {
        This is a comment.
    }

You can write any kind of comment in there, even special characters like $undefined or [undefined] which would cause errors in normal code. Only wrongly matched {} would be a problem.