Tcl facts for kids

Quick facts for kids
Tcl

Paradigm	Multi-paradigm: event-driven, functional, imperative, object-oriented
Designed by	John Ousterhout
Developer	Tcl Core Team
First appeared	1988; 37 years ago (1988)
Stable release	9.0.0  / 26 September 2024; 10 months ago (26 September 2024)
Typing discipline	Dynamic typing, everything is a string
License	BSD-style
Filename extensions	.tcl, .tbc
Website
Major implementations
ActiveTcl Androwish
Dialects
Jim, Eagle
Influenced by
AWK, Lisp
Influenced
PHP, PowerShell, Tea, TH1

Tcl (pronounced "tickle") is a powerful and easy-to-use programming language. It was designed to be simple yet very strong. Tcl treats everything as a "command," even basic actions like setting a variable or creating a new function. It can be used in many different ways, including object-oriented programming (where you use "objects" to organize your code) and functional programming (where you use functions to do tasks).

Tcl is often used inside other computer programs, especially those written in C. It's great for quickly building new programs, creating graphical user interfaces (GUIs) like the windows and buttons you see on your screen, and for testing software. Tcl works on many different operating systems, so your Tcl code can run almost anywhere. Because it's a small language, it's also used in embedded systems, which are special computer systems built into other devices.

When Tcl is combined with something called the Tk extension, it's known as Tcl/Tk. This combination lets you build GUIs directly using Tcl. You might have even used Tcl/Tk without knowing it, as it's part of the standard Python installation (called Tkinter).

History of Tcl

The Tcl programming language was created in the spring of 1988 by a person named John Ousterhout. At the time, he was working at the University of California, Berkeley. John created Tcl because he was frustrated that many programmers were making their own small languages for different software tools. He wanted a simple, common language that could be used to extend many programs.

Later, Tcl became popular on its own. John Ousterhout even won an award called the ACM Software System Award in 1997 for creating Tcl/Tk. The name Tcl originally stood for Tool Command Language, but people usually write it as Tcl instead of TCL.

Here are some important moments in Tcl's history:

• January 1990: Tcl was first announced to a wider audience.
• January 1991: The Tk extension, which helps build GUIs, was announced.
• August 1997: Tcl 8.0 came out, which made Tcl programs run faster by using a "bytecode compiler."
• April 1999: Tcl 8.1 added full support for Unicode, which means it could handle many different characters and languages from around the world.
• August 2000: The Tcl Core Team was formed. This meant that many people from the community started working together to develop Tcl.
• December 2012: Tcl 8.6 was released. It added a new way to do object-oriented programming directly within Tcl, called TclOO.
• September 2024: Tcl 9.0 was released, improving how it handles large numbers and making it work better with modern computer systems.

Tcl conferences and workshops are held regularly in different countries. Many companies, like FlightAware, use Tcl in their products.

Key Features of Tcl

A Tcl file being edited in the Eclipse IDE

Tcl has many useful features that make it a great language for different tasks:

• Everything is a Command: In Tcl, all actions, even basic ones like adding numbers or making decisions, are treated as commands. You write them with the command name first, followed by its inputs.
• Flexible Arguments: Commands can often take a different number of inputs, making them very flexible.
• Dynamic and Changeable: You can change or redefine almost anything in Tcl while the program is running.
• Strings are Key: All types of data, like numbers, lists, or even the program's own code, can be treated as strings (text). Tcl automatically converts data types when needed.
• No Variable Declarations: You don't need to tell Tcl what type of data a variable will hold before using it. You just assign a value to it.
• Object-Oriented System: Tcl has a built-in system called TclOO for object-oriented programming, which helps organize complex code using "objects" and "classes."
• Event-Driven: Tcl can easily react to events, like when a network connection is made or a file is changed.
• Extensible: You can add new features to Tcl by writing code in other languages like C, C++, or Java.
• Interpreted Language: Tcl programs are run by an "interpreter" that reads and executes the code line by line, often after converting it into a faster "bytecode."
• Unicode Support: Tcl fully supports Unicode, so it can work with text in almost any language.
• Cross-Platform: Tcl works on many different operating systems, including Windows, Unix, Linux, and Macintosh.
• GUI Integration: It works very well with the Tk library to create graphical user interfaces (GUIs) that look good on different systems.
• Easy Distribution: You can easily share Tcl programs. They can be packaged into single files that run without needing a complex installation.
• Free and Open Source: Tcl's code is freely available under a BSD license, meaning anyone can use, change, and share it.

Safe-Tcl

Safe-Tcl is a special version of Tcl that has limited features. This means that Tcl programs running in Safe-Tcl cannot harm your computer or other applications. For example, they can't easily access your computer's files or run other system commands. It was designed to make sure that scripts from unknown sources are safe to run. This safety feature is now part of the regular Tcl/Tk software.

How Tcl Code Works

A Tcl program is made up of a series of "commands." Each command is a list of words separated by spaces. The first word is the command's name, and the words after it are its inputs (arguments).

Here's a simple example:

commandName argument1 argument2 ... argumentN

For instance, the `puts` command (short for "put string") displays text on your screen:

puts "Hello, World!"

This command will show "Hello, World!" on your screen.

You can also use variables and the results of other commands inside strings. The `set` command gives a value to a variable, and `expr` calculates a math expression:

# expr evaluates text string as an expression
set sum [expr 1+2+3+4+5]
puts "The sum of the numbers 1..5 is $sum."

In this example, `set sum [expr 1+2+3+4+5]` calculates the sum (which is 15) and stores it in a variable called `sum`. Then, `puts "The sum of the numbers 1..5 is $sum."` prints a message that includes the value of `sum`.

A `#` character means the rest of the line is a comment, which the computer ignores. Comments are for humans to understand the code.

Basic Commands

Here are some fundamental commands in Tcl:

• `set`: This command gives a value to a variable. If you use it with only one input, it shows you the current value of that variable.
• `proc`: This command lets you create your own new commands (also called "procedures" or "functions").
• `if`: This command runs a block of code only if a certain condition is true. You can also add `elseif` for other conditions or `else` for what to do if none of the conditions are true.
• `while`: This command keeps running a block of code repeatedly as long as a condition remains true.
• `foreach`: This command runs a block of code for each item in a list.
• `for`: This command is a shortcut for loops that need to set up a variable, check a condition, and then update the variable after each run.

You can control loops with these commands:

• `break`: Stops the loop immediately.
• `continue`: Stops the current run of the loop but continues to the next item or check.
• `return`: Stops the current command (or procedure) and gives a value back to where it was called from.

Advanced Commands

Some more advanced commands include:

• `expr`: Evaluates a math or logical expression.
• `list`: Creates a list of items.
• `array`: Works with special variables called "arrays" that can store many values.
• `dict`: Works with "dictionaries" (like a list of key-value pairs).
• `regexp`: Checks if a text string matches a specific pattern.
• `try`: Helps handle errors that might happen in your code.

Object-Oriented Programming in Tcl

Object-oriented programming (OOP) is a way of organizing code using "objects." These objects can have their own data (properties) and actions (methods). Tcl 8.6, released in 2012, added a built-in object system called TclOO.

Here's a simple example of how you might create objects in Tcl:

oo::class create fruit {
    method eat {} {
        puts "yummy!"
    }
}
oo::class create banana {
    superclass fruit
    constructor {} {
        my variable peeled
        set peeled 0
    }
    method peel {} {
        my variable peeled
        set peeled 1
        puts "skin now off"
    }
    method edible? {} {
        my variable peeled
        return $peeled
    }
    method eat {} {
        if {![my edible?]} {
            my peel
        }
        next
    }
}
set b [banana new]
$b eat               → prints "skin now off" and "yummy!"
fruit destroy
$b eat               → error "unknown command"

This code creates a `fruit` class and a `banana` class (which is a type of `fruit`). It shows how a `banana` object can be "peeled" and "eaten."

Before TclOO, programmers used other extensions like incr Tcl or XOTcl to do object-oriented programming. Now, many of these older extensions have been updated to use TclOO as their base.

Tcl for Web Applications

Tcl can also be used to build websites and web applications:

• Tcl Web Server: This is a web server written entirely in Tcl. It can serve web pages and handle requests from web browsers.
• Apache Rivet: This is a system that lets you use Tcl as a scripting language for creating dynamic web pages with the Apache HTTP Server. It's similar to how PHP or JSP work. Rivet can connect to databases like MySQL or PostgreSQL and use many other Tcl features to build powerful web applications.

Connecting Tcl with Other Languages

Tcl was designed to work well with the C language. This is because Tcl was originally created to be a simple way to control programs written in C. All of Tcl's commands are actually implemented as C functions.

Tools exist that can automatically help Tcl programs talk to C functions, and vice versa. Tcl is also used in programs that simulate digital logic, like those used to design computer chips.

Tcl Extension Packages

Tcl has many "extension packages" that add more features to the language. These packages can provide things like GUI tools, ways to automate tasks, or access to databases.

Some popular Tcl extensions include:

• Tk: This is the most famous Tcl extension. It provides a library for building graphical user interfaces (GUIs) with windows, buttons, and menus that work on many different operating systems.
• Expect: This extension is used to automate tasks that usually require a person to type commands, like logging into a server or running tests. It's very popular in the Unix world for scripting repetitive jobs.
• Tile/Ttk: This package makes Tk GUIs look more like the native applications on different operating systems (like Windows, macOS, or Linux).
• Tix: This is another set of user interface components that adds more advanced widgets to Tcl/Tk applications.
• Itcl/IncrTcl: This is an object system for Tcl, often called "[incr Tcl]". It helps organize code using object-oriented principles.
• Tcllib: This is a large collection of useful Tcl scripts and packages that you can use without needing to compile anything.
• Tklib: This is a collection of helpful tools specifically for Tk, similar to Tcllib.
• tDOM: This extension helps Tcl programs read and understand XML files.
• TclTLS: This provides secure communication features for Tcl, like those used for secure websites.
• TclUDP: This extension helps Tcl programs send and receive data using the User Datagram Protocol (UDP) over networks.
• TWAPI: This extension lets Tcl programs use many features of the Windows operating system.
• Databases: Tcl has ways to connect to many different databases, like MySQL, PostgreSQL, and SQLite, allowing Tcl programs to store and retrieve information.

See also

In Spanish: Tcl para niños