How to check the string value of an argument passed to expect script?

I'm writing a expect script that takes command line arguments. I would like to be able to detect whether the first argument is "--help" and print a Usage string then. Otherwise use the argument as a port number with a specific default (let's say 1818).

I tried this code that fails:

#!/usr/bin/expect 

if {[llength $argv] != 1} {
    puts "No Port number specified, defaulting to port 1818."
    set port 1818
} else {
    if {[lindex $argv 0] eq "--help"} {
        puts "Usage: testit [--help] [port]"
        exit
    } else {
        set port [lindex $argv 0]
    }
}

The error is:

invalid command name "--help"
    while executing
"--help"
    invoked from within
"if {[llength $argv] != 1} {
    puts "No Port number specified, defaulting to port 1818."
    set port 1818
} else {
    if {[lindex $argv 0] eq "--he..."

Obviously it is trying to interpret the content of the "--help" string while I'm trying to make the script compare the value of argument 0 to "--help".

What is wrong in the above logic or syntax?

I tried using other strings, like "help" instead of "--help" but the outcome is the same.

I'm not that familiar with expect and tcl, but I tried the expression in tclsh and the same thing happens there. So this issue has to do with invalid tcl code. The following tcsh session shows that the syntax if {$variable=="--help"} {...} is OK, but removing the white space in my string comparison attempt above does not solve the problem.

Here's the tcsh session:

% set v1 "--help"
--help
% if [v1 == "--help'] { puts "allo"}
extra characters after close-quote
% if [v1 == "--help"] { puts "allo"}
invalid command name "v1"
% if [$v1 == "--help"] { puts "allo"}
invalid command name "--help"
% if $v1 == help {puts "allo"}
invalid bareword "help"
in expression "--help";
should be "$help" or "{help}" or "help(...)" or ...
% if $v1 == "--help" {puts "allo"}
invalid bareword "help"
in expression "--help";
should be "$help" or "{help}" or "help(...)" or ...
% if {$v1=="--help"} {puts "allo"}
allo
% 

The problem is this line:

    puts "Usage: testit [--help] [port]"

And the problem with it is that [...] does command substitution in that situation. You need to add a couple of backslashes in there to prevent that, like this:

    puts "Usage: testit \[--help] \[port]"

Or you can enclose the string in braces to inhibit all substitutions:

    puts {Usage: testit [--help] [port]}

Either will work (and they'll get compiled to exactly the same thing so use whichever you prefer).