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Tcl
Tcl (originally from "Tool Command Language", but nonetheless conventionally rendered as "Tcl" rather than "TCL"; pronounced as "tickle" or "tee-cee-ell"[1]) is a scripting language created by John Ousterhout. Originally "born out of frustration"[2]?according to the author?with programmers devising their own (poor quality) languages intended to be embedded into applications, Tcl quickly gained wide acceptance on its own and is generally thought to be easy to learn[3], but powerful in competent hands. It is most commonly used for rapid prototyping, scripted applications, GUIs and testing. Tcl is used extensively on embedded systems platforms, both in its full form and in several other small-footprinted versions. Tcl is also used for CGI scripting and as the scripting language for eggdrop bots. The combination of Tcl and the Tk GUI toolkit is referred to as Tcl/Tk. FeaturesTcl's features include:
Tcl did not originally support object oriented syntax, being a functional language, but recent versions do support extensions which provide OO functionality, such as the XOTcl extension to Tcl. Other OO extensions also exist, such as incr Tcl, Snit, and STOOOP (simple tcl-only object-oriented programming). Functional programming can easily be done in Tcl, as higher-order functions or functional abstractions are built into the language, though it is not widely used for this purpose. As an example, consider the ease with which two functions can be composed: SyntaxVery simple and consistent syntaxTcl has a very simple syntax which is applied in a consistent way. A Tcl script consists of several commands. A command is a list of words separated by whitespace. word0 word1 word2 ... wordN The first word is the name of a command, which is not built into the language, but which is in the library. The following words are arguments. So we have: commandName argument1 argument2 ... argumentN Practical example, using the puts command which outputs a string, adding a trailing newline, by default to the stdout channel: Any argument may be replaced by another command in square brackets. The subcommand is evaluated first and the result is substituted as the argument. Alternatively, any argument placed in curly braces will not be evaluated, but rather will be handed directly back to the original command as an argument. To summarize: there is one basic construct (the command) and a set of simple substitution rules, and only the square brackets, the curly braces, quotes, and the backslash have special meaning. The single equality sign (=) for example is not used at all, and the double equality sign (==) is the test for equality, and even then only in expression contexts such as the All commands have the same structure: a keyword which is followed by several parameters. A command is terminated by a newline or a semicolon. Even comments are just commands which happen to do nothing. Tcl is not statically typed: each variable may contain integers, floats, strings, lists or any other value. Symbols with a special meaning
Some examples of commandsAssignments are made with the command set, i.e., the equal sign ('=') is not used for assignments. 'While' loops are implemented by the command while which takes two arguments. The first argument is a Tcl expression (expressions are written in what is essentially a miniature language for doing C-style math and comparison expressions). The second argument is the script to run on every iteration. They are generally put in curly braces to avoid immediate execution. If command Switch command Commands may have no arguments gives back the current working directory. With you store the string describing the working directory in the variable wdir. A command may give back as a result a list gives back a list of file names in the working directory whose names match aPattern. ProceduresProcedures are defined as follows Associative arraysThe following code snippet creates and initializes an associative array which in other languages is called a map, dictionary, or hash table. To query it and put the result on standard output use To get a list of all countries for which a capital is defined use The result is unsorted because Tcl arrays are based on hash tables. Poland Spain Russia Germany Italy France If you like to have it sorted use Note however that arrays are collections of variables, not first-class objects, and cannot be freely passed around the way strings are. The commands And both produce the error message can't read "capital": variable is array In general, note that Tcl has a different concept of references than some people might expect. To refer to an array (to pass it by reference), give its variable name: To pass the contents of an array by value, use array get or array set. For example, to copy one array to another: For pure-value associative arrays, Tcl introduced the dict type in version 8.5. Variable scopeFor a more detailed explanation of variable scope in tcl, visit here. When referring to variables inside a procedure, the program will (by default) only recognize variables that were also defined in that procedure. Another way of saying this is that the program will only recognize local variables by default. Here is an example program: On line 6, the command 'puts "$gvar"' will create an error because it is technically only referring to the variable with that name that is also within the local namespace. However, there is no variable "gvar" within that local namespace because gvar was not declared in the nest1 namespace. Also, on line 9, the command 'puts "$lvar"' will create an error because it is looking for a variable called lvar within the current namespace (which happens to be the global namespace). Since there is no lvar that was globally declared, an error will occur. This will fix the first problem: The global call specifically told the program to look for the subsequent variables in the global namespace. But this still doesn't fix the second problem. If you want to be able to recall the variable lvar from global scope, you need to declare it as being within the global scope. Here is how you could accomplish that: By adding the namespace path separator '::' without specifying a sub-namespace (for example '::IAmASubNamespace'), you have made it clear to the program that you are actually declaring and defining a variable inside the global namespace. If you are doing something where speed really matters, you can find the relevant information here. Interfacing with Other LanguagesTcl interfaces natively with the C language. C++ Interoperability
Java Interoperability
Extension packagesThe Tcl language has always supported extension packages, which provide additional functionality (such as a GUI, terminal-based application automation, database access, etc.) TkThe most popular Tcl extension is the Tk toolkit, which provides a graphical user interface library for a variety of operating systems. Each GUI consists of one or more frames. Each frame has a layout manager. Tile/TtkTile/Ttk is a styles and theming widget collection which can replace most of the widgets in Tk with variants which are truly platform native through calls to an operating system's API. Themes covered in this way are Windows XP, Windows Classic, Qt (which hooks into the X11 KDE environment libraries) and Aqua (Mac OS X). A theme can also be constructed without these calls using widget definitions supplemented with image pixmaps. Themes created this way include Classic Tk, Step, Alt/Revitalized, Plastik and Keramik. Under Tcl 8.4, this package is known as Tile, while in Tcl 8.5 it is included in the core distribution as Ttk. Itcl/IncrTclItcl is an object system for Tcl, and is normally named as [incr Tcl] (that being the way to increment in Tcl, similar in fashion to the name C++). TcllibTcllib is a set of scripted packages for Tcl that can be used with no compilation steps. DatabasesA number of database extensions are available:
and many, many others - see the Tcl/Tk Wiki, specifically the Tcl/Tk Wiki Database Category. ExpectThe Expect package provides tools for automating interactive non-GUI programs, and is built on top of Tcl. ExamplesThe following are simple code snippets that can be pasted directly into any Tcl shell. Adding NumbersMethod (A) - Adding using a 'foreach' loop Method (B) - A much more elegant way of adding numbers using the 'join' command Method (C) - Use the addition command in conjunction with list expansion syntax Echo serverA simple working example, demonstrating event-based handling of a socket, follows. Digital clockAnother example using Tk (from A simple A/D clock) and timer events, a digital clock in four lines of code: Explanation: the first line loads the Tk package, the second (which actually continues over 4 lines) defines a command, "every", which re-schedules an action ('body') every 'ms' milliseconds; the third creates a label whose content is bound to the variable 'time' and arranges for it to be displayed on the screen; the fourth line arranges so that the variable 'time' is updated to formatted local time every second. Note that the "every" command treats its first parameter as a number and its second parameter as a script, resulting in correct calls being those programs that pass in arguments that look like those pieces of syntax. However, the Tcl interpreter assigns no special syntactic interpretation to either argument, meaning that the code is free to interpret those arguments as desired. (By contrast, a language using a conventional BNF description could only achieve the equivalent functionality by defining "every" as a keyword, adding extra syntax to mark the executable argument, or by making the curly braces deeply magical indeed, possibly requiring a more complex declaration.) List of content of associative arrayThe array tcl_platform contains a set of properties describing the platform that the Tcl interpreter is executing on. A list of the names of the properties is obtained by The following snippet lists them together with their values. If the properties should be sorted by name, it is best to do that first, like this. This also demonstrates how commands may be nested. In fact they may be nested to any depth. If you want it fancier (keys padded with blanks so that the equal signs align) just use the parray procedure, that comes delivered with Tcl. Finally, if the contents of the array is just for machine consumption, there is a separate subcommand of array to do that: Intersection of two setsThe filter procedure returns those elements of the list where the script returns TRUE: The in procedure is shorthand for list inclusion: Testing: % filter {a b c} {in {b c d}} b c FactorialThis demonstrates that any string (in this case, "!") can be a command name, and the ?: operator as from C is available in Tcl expressions too. Also, recursion (let the function call itself again) is easily possible, although Tcl has no tail call optimization so the maximum depth of recursion is restricted. For comparison, here is the iterative variant which, together with the big integers available from Tcl 8.5, allows very large factorials: HistoryThe Tcl programming language was created in the spring of 1988 by John Ousterhout while working at the University of California, Berkeley.
(Need rest of potted history!) Tcl conferences and workshops are held in both the United States and Europe.
References
Notes
See also
External linksMain Tcl developer sites
Tcl distributions
Tcl tutorials, books, etc.
bg:Tcl ca:Tcl cs:Tcl de:Tcl es:Tcl eo:Tcl fr:Tool Command Language gl:TCL ko:Tcl is:Tcl it:Tcl la:Tcl ms:Tcl nl:TCL (programmeertaal) ja:Tcl/Tk no:Tcl pl:Tcl pt:Tcl ro:Tcl/TK ru:Tcl fi:TCL sv:Tcl vi:Tcl tg:Tcl tr:Tcl bug:Tcl uk:Tcl zh:Tcl Source: Wikipedia | The above article is available under the GNU FDL. | Edit this article
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