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Tool Command Language
TCL/TKs (Tool Command Language/Tool Kit) strength is that it is a small, interpreted, language that can be embedded, access graphics and system features, and is extensible. It also is multiplatform and has very wide support. It can be downloaded on the NET, and it runs TCL scripts for backend processing. TCL is a language, called the Tool Command Language. TK is a set of extensions or "widgets"(the actual official designation) that happened to be written by TCL's author to add graphic capability to his TCL interpreter they include things like list boxes, entry boxes, radio buttons, check boxes, etc.... In fact, prior to TCL/TK 8.0, TCL was at version 7.3, and TK was at version 4, as I recall. In a way, TCL's relationship to TK is kind of like VBs relationship to activex(which ALSO has controls as mentioned above, and are sold as one package). In fact, a VB user can program for a long time without realizing that he or she is using activex components all over the place.
Tcl was originally intended to be a reusable command language. Its developers had been creating a number of interactive tools, each requiring its own command language. Since they were more interested in the tools themselves than the command languages they would employ, these command languages were constructed quickly, without regard to proper design.
After implementing several such "quick-and-dirty" command languages and experiencing problems with each one, they decided to concentrate on implementing a general-purpose, robust command language that could easily be integrated into new applications. Thus Tcl (Tool Command Language) was born
Tool Command Language
The Tcl scripting language grew out on design tools for integrated circuits at the University of California at Berkeley in the early 1980's.
The notion of embeddability is one of the most unique aspects of Tcl, and it led to the following three overall goals for the language:
- The language must be extensible : it must be very easy for each application to add its own features to the basic features of the language, and the application-specific features should appear natural, as if they had been designed into the language from the start.
- The language must be very simple and generic, so that it can work easily with many different applications and so that it doesn't restrict the features that applications can provide.
- Since most of the interesting functionality will come from the application, the primary purpose of the language is to integrate or "glue together" the extensions. Thus the language must have good facilities for integration.
The conclusion that to reduce the resource requirements by building large systems out of reusable components. If most of the complexity of a system was in the components, and it could carry the components forward from system to system, perhaps it could build large powerful systems .
The additional resources provided by Sun make major improvements to Tcl and Tk. Scott Stanton and Ray Johnson ported Tcl and Tk to Windows and the Macintosh, so that Tcl became an outstanding cross-platform development environment; today, more than two-thirds of Tcl downloads are for Windows
Grouping and Substitution
The Tcl syntax is used to guide the Tcl parser through three steps: argument grouping, result substitution, and command dispatch.
- Argument grouping. Tcl needs to determine how to organize the arguments to the commands. In the simplest case, white space separates arguments. As stated earlier, the quotation marks and braces syntax is used to group multiple words into one argument. In the previous example, double quotation marks are used to group a single argument to the puts command.
- Result substitution. After the arguments are grouped, Tcl performs string substitutions. Put simply, it replaces $foo with the value of the variable foo , and it replaces bracketed commands with their result. That substitutions are done after grouping is crucial. This sequence ensures that unusual values do not complicate the structure of commands.
- Command dispatch. After substitution, Tcl uses the command name as a key into a dispatch table. It calls the C procedure identified in the table, and the C procedure implements the command. You also can write command procedures in Tcl. There are simple conventions about argument passing and handling errors
Tcl was not designed to be a serious programming language. It was designed to be a "scripting language", on the assumption that a "scripting language" need not try to be a real programming language. So Tcl doesn't have the capabilities of one. It lacks arrays; it lacks structures from which you can make linked lists. It fakes having numbers, which works, but has to be slow. Tcl is ok for writing small programs, but when you push it beyond that, it becomes insufficient
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