Friday, 3 July 2009

The Twouble with Tcl

From time to time I do a bit of tcl. Mostly as maintenance for existing tcl programs. I haven't made up my mind entirely about it yet - I've seen some very powerful programs in tcl, and yet occasionally I'm still "surprised" by a feature.

I spent a couple of hours today trying to figure out why a tcl program of mine wasn't running, and so I've made some notes:

Real languages don't have reserved words

In tcl, you can redifine parts of the language in tcl itself. For example if you wanted to redefine if, just write a new function:
proc if {cond expres} {
puts "cond is $cond"
puts "expr is $expres"

set a 1
if {$a == 1} {
puts hello
Sounds neat. In fact proc itself is just a command that takes 3 arguements (name, arguements, and body). However, don't start using simple names in your tcl languages like this:
proc open {} {
set ::alarm_socket [open_socket $::options(-alarm_host)]

foreach host $::options(-hosts) {
verbose "open $host"
set ::sockets($host) [open_socket $host]
# set up an event handler for when data is readable on this socket:
fileevent $::sockets($host) readable [list process $host]
# initialise socket timeout with open time
set ::socket_t($host) [clock seconds]
Because open is what you might call a reserved word. (I should have known with open, but should I really have to remember all the reserved words?)

The error looked something like this:
$ ./test.tcl
invalid command name "::tcl::tm::UnknownHandler"
while executing
"::tcl::tm::UnknownHandler ::tclPkgUnknown msgcat 1.4"
("package unknown" script)
invoked from within
"package require msgcat 1.4"
("uplevel" body line 2)
invoked from within
"uplevel \#0 {
package require msgcat 1.4
if { $::tcl_platform(platform) eq {windows} } {
if { [catch { package require registry 1.1 }] } {
(file "/usr/lib/tcl8.5/clock.tcl" line 23)
invoked from within
"source -encoding utf-8 [file join $TclLibDir clock.tcl]"
(procedure "::tcl::clock::format" line 3)
invoked from within
"clock format [clock seconds]"
(procedure "alarm_timeouts" line 3)
invoked from within
... and so on. Yeuch! And it was encountered with this one line:
   puts "$::argv0: [clock format [clock seconds]]"
The problem? I redefined open, which was used internally by clock.

Real languages don't have types:

You can do lots of nice things in tcl without types, in a similar (but different) way to perl.
set a world
puts "Hello $a" ;# prints 'Hello world'
set a 1
incr $a
puts "$a + 2" ;# prints '2 + 2'
This looks normal to someone used to perl and tcl. Noticed how I don't need format specifiers or concat functions. You can also do this:

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

Which prints:
Hello World

Real languages don't have comments

They have a comment command of course! And the comment command is a command that takes arguments (the comment itself) that aren't evaluated. Except that because of this, you can't have an unmatched brace in a comment:
# if ($sometest) {

The tcl solution?
if (0)
blocks or the like...


So if you haven't yet learnt tcl, I encourage you to find a tcl hackers tcl program, and delve into it to see just how it works.

If you try to learn tcl only by writing it and not by reading others' code, then you'll learn tcl with the habits you're used to, and you will possibly miss some of the powerful features.

After all, C programmers can program C in just about any language!

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