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Tcl (originally from "Tool Command Language", but nonetheless conventionally rendered as "Tcl" rather than "TCL"; and pronounced like "tickle") is a scripting language created by John Ousterhout. Originally "born out of frustration" — 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, but powerful in competent hands. It is most commonly used for rapid prototyping, scripted applications, GUIs and testing. Tcl is also used for CGI scripting. The combination of Tcl and the Tk GUI toolkit is referred to as Tcl/Tk.
Features Tcl'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: Very simple and consistent syntax Tcl 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 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 the expr command.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 commands Assignments are made with the command set, no equality sign. set variable value '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. while If command if Commands may have no arguments pwd gives back the current working directory. With set wdir pwd you store the string describing the working directory in the variable wdir. A command may give back as a result a list glob aPattern gives back a list of file names in the working directory whose names match aPattern. Procedures Procedures are defined as follows proc nameOfProc Associative arrays The following code snippet creates and initializes an associative array which in other languages is called a map, dictionary, or hash table. set capital(France) Paris set capital(Italy) Rome set capital(Germany) Berlin set capital(Poland) Warsaw set capital(Russia) Moscow set capital(Spain) Madrid To query it and put the result on standard output use puts $capital(Italy) To get a list of all countries for which a capital is defined use array names capital 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 lsort array names capital Note however that arrays are collections of variables, not first-class objects, and cannot be freely passed around the way strings are. The commands set CapitalCopy $capital and DoSomethingWith $capital both produce the error message can't read "capital": variable is array In general, note that Tcl has a different concept of references than some people might expect. To refer to an array (to pass it by reference), give its variable name: proc demo _arr To pass the contents of an array by value, use array get resp. array set. For example, to copy one array to another: array set Newarr array get Oldarr For pure-value associative arrays, Tcl introduced the dict type in version 8.5. Variable scope For a more detailed explanation of variable scope in tcl, visit here. When referring to variables inside a procedure, the program will (by default) only recognize variables that were _also defined in that procedure_ . Another way of saying this is that the program will only recognize local variables by default. Here is an example program: proc nest1 nest1 puts "$lvar" On line 7, the command 'puts "$gvar"' will create an error because it is technically only referring to the variable with that name that is also within the local namespace. However, there is no variable "gvar" within that local namespace because gvar was not declared in the nest1 namespace. Also, on line 11, the command 'puts "$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: ... proc nest1 nest1 puts "$lvar" 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: ... proc nest1 ... 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 that relevent information at here. Extension packages The Tcl language has always supported extension packages, which provide additional functionality (such as a GUI, terminal-based application automation, database access, etc.) The 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. A number of database extensions are available: Hello, world! puts "Hello, world!" or puts Adding Numbers Method (A) - Adding using a 'foreach' loop set numbers set result 0 foreach number $numbers puts $result Method (B) - A much more elegant way of adding numbers using the 'join' command set numbers puts expr join $numbers + + 0 Echo server A simple working example, demonstrating event-based handling of a socket, follows.
Digital clock Another example using Tk (from A simple A/D clock) and timer events, a digital clock in three lines of code: proc every pack label .clock -textvar time every 1000
Explanation: the first line defines a command, "every", which re-schedules an action ('body') every 'ms' milliseconds; the second creates a label whose content is bound to the variable 'time'; the third 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 array In array tcl_platform, platform-specific properties are kept. A list of the names of the properties is obtained by array names tcl_platform The following snippet lists them together with their values foreach i array names tcl_platform If the properties should be sorted foreach i lsort array names tcl_platform This 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 proc parray that comes delivered with Tcl. Intersection of two sets The filter procedure returns those elements of the list where the script returns TRUE: proc filter The in procedure is shorthand for list inclusion: proc in Testing: % filter b c Factorial proc ! x This demonstrates that any string can be a command name, and the ?: operator as from C is available in Tcl expressions too. Also, recursion is easily possible, although Tcl has no tail call optimisation so the maximum depth of recursion is restricted. History The 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!) See also | |||||||||||||||||||||||||||
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