Introduction to Tcl and Tk

Outline– The Tcl Language

– The Tk Toolkit

– Tk Applications

– Composing Applications

– Status and Conclusions

Goal– Understand basics of Tcl and Tk

– Understand Tk/Tcl philosophy

Reading– Ch. 5-9, Practical Programming in Tcl and Tk

Overview

• What– Tcl: embeddable scripting language

– Tk: X11/Windows/Mac toolkit and widgets based on Tcl

• Principle– single interpretive language controls all aspects of all

interactive applications

» function

» interface

» composition of pieces

» communication between applications

• Results– higher- level graphics programming - simpler, 10X faster

– greater power - more programmable, programs work together

Tcl: Tool Command Language

• Problem– interactive programs need command languages

– traditionally redone for each application

– result: weak, quirky

– Emacs and csh nice, but cannot reuse

• Solution: Tcl– command language = embeddable C library

– powerful features: procedures, variables, lists, expressions, loops, etc.

– extensible by applications

Language Philosophy

• Language classes– large application implementation (structure,

performance important)

– scripting, extensions

– interactive commands (structure bad, performance not critical)

• One language can’t meet all three needs?

• Tcl goals– simple syntax (for humans)

– programmable

– easy to interpret

– simple interface to C/C++ procedures

C

Tcl

Tcl Syntax

• Basic syntax like shells– words separated by spaces:

cmd arg arg arg . . .

– commands separated by newlines, semicolons

– commands return string results

• Simple substitution rules– variables:

set a $b

– command results:

set a [expr $b+2]

– complex arguments:

if $a<0 {

puts stdout “a is negative”

}

More on the Tcl Language

• Rich set of built-in commands– variables, associative arrays, lists

– arithmetic expressions

– conditionals, looping

– procedures

– access to files, system commands

• Only data type is string– easy access from C/C++

– programs and data are interchangeable

Example: Factorial

proc fac x {

if {$x<=1} {

return 1

}

expr {$x*[fac [expr $x-1]]}

}

fac 4

Returns: 24

Embedding Tcl in Applications

Parser

Init

Command Loop

• Application generates Tcl scripts

• Tcl parses scripts, calls command procedures with argc, argv

• Application extends built-in command set– define new object types in C

– implement primitive operations on objects as new Tcl commands

– build complex features with Tcl scripts

Tcl Application

Built-In Commands Application Commands

The Tk Toolkit

• Problem– too hard to build applications with nice user interfaces

• Wrong Solution– C++, object-oriented toolkits

– only 10-20% improvement, must still program at low level

• Right Solution– raise the level of GUI programming

– create interfaces by writing Tcl scripts

Creating Interfaces with Tk

• Widgets/windows have path names

.dlg.quit

• Create widget with command named after class

button .dlg.quit -text Quit \

-foreground red -command exit

• Tell geometry manager where to display widget

place .dlg.quit -x 0 -y 0

pack .dlg.quit -side bottom

Other Tk Features

• Manipulate widgets with widget commands

.dlg.quit flash

.dlg.quit configure -relief sunken

• Use Tcl for interconnection– buttons, menu entries invoke Tcl commands

– scrollbars and listboxes communicate with Tcl

– can define new event bindings in Tcl

– selection, focus accessible via Tcl

• Tk also provides C interfaces– create new widget classes

– create new geometry managers

What’s a Tk-Based Application?

1. The Tcl interpreter

2. The Tk toolkit

3. Application-specific C code (primitives!)– new object types

– new widgets

4. Tcl scripts (compose primitives)– build GUI

– respond to events

Tcl commands

Wish: Simplest Tk Application

• Wish - windowing shell

• No C code except command-line reader

• Can build many apps as wish scripts– Hello, world:

label .hello \

-text “Hello, world”

.pack .hello

– simple directory browser

» 30 lines of Tk/Tcl

Browser Wish Script

listbox .list -yscroll “.scroll set” \-relief raised -width 20 -height 20

pack .list -side leftscrollbar .scroll -command “.list yview”pack .scroll -side right -fill y

if {$argc > 0} {set dir [lindex $argv 0]

} else {set dir “.”

}foreach i [exec ls -a $dir] {

.list insert end $i}bind .list <Double-Button-1> {

browse $dir [selection get]}bind .list <Control-c> {destroy .}focus .list

Browse Script Cont.

proc browse {dir file} {global envif {$dir != “.”} {set file $dir/$file}if [file isdirectory $file] {

exec browse $file &} else {

if [file isfile $file] {exec $env(EDITOR) $file &

} else {puts stdout “can’t browse $file”

}}

}

• Browse is also name of wish script

TkSol - Tk-Based Solitaire Program

TkSol Statistics

• Code– 1635 lines of Tk/Tcl code

– 4 card down bitmaps

– 52 card up bitmaps (4 suits of 13 cards each)

– almost all time in bitmap creation

• Comparison to MS Solitaire– simpler end game

– has auto-play mode

– slower startup

– slower deal

– similar interactive speed

TkMines Statistics

• Code– 1178 lines of Tk/Tcl code

– uses a few signal and time commands from TclX extension, are not really needed

– several small bitmaps for squares

• Comparison to MS Mines– simpler score file

– same look-and-feel

– slower startup

– similar interactive performance

Composing Applications

• Problem– only communication between applications is via selection

» OLE on MS Windows provides object selection

– result: monolithic programs

• Solution - send command– send appName command

– any Tk application can invoke anything in any other Tk application: interface or actions

– result: powerful communication

– result: big security hole

Composing Applications, cont.

• Examples– debugger - sends command to editor to highlight line of

execution

– UI editor - sends commands to modify interface of live application

– multimedia - send record, play commands to audio and video applications

– spreadsheets - cell sends commands to database to fetch current value

• Revolutionary results– build complex systems as collections of specialized but

reusable hypertools

– easy to create active objects: embeddable Tcl commands. Hypertext, hypermedia is easy.

Status

• Source and documentation freely available– developed at UC Berkeley

– now maintained and developed at Sun Labs

– code redistributable in commercial applications

– runs on UNIX, Windows, Mac

• Large user/developer community– 50,000+ worldwide

– comp.lang.tcl newsgroup

– many commercial products

– several books

• Many extensions available– Tk is just one extension

Drawbacks

• Must learn new language– substitution can be confusing

• Interpreted language has performance limits– want a compiler for some applications

– want to avoid C programming

– dynamic compiler will provide at least 10x speedup

• Competition– Visual Basic

– AppleScript

– Java/JavaScript

Conclusions

• High-level programming– less to learn

– easy to learn

– build applications very quickly

• One interpretive language for everything– extend and modify applications at run-time

– make many things work together

• Web programming/agent language?– platform independent

– interpreted

– easily embedded

– secure