TCL (pronounced Tickle) is a simple interpretive programming language. Tk (pronounced tee-kay) is a graphics toolkit callable from TCL. TCL/Tk has been developed at the University of California at Berkeley (UCB) funded by NASA, DARPA and by Digital in 1990. It has been placed in the public domain and add-on developments by third parties encouraged. A commercial company (Neosoft) is able to provide support for customers who need it.
Although the Tk toolkit is presently X11 specific there are plans to fund a port to Windows and to the MAC. If this happens then the availabilty of a single toolkit portable across these 3 platforms and with an existing reasonably sized (5000 developers approx) user group will ensure it's future. There are no licensing problems in using TCL/TK in commercial applications.
TCL is designed to be embedded in applications written in C. If the low level workhorse routines of the application are written in C and the higher level controlling routines in TCL, then the application may be readily re-configured or extended, much like Emacs is controlled by a built in LISP interpreter. The ISLE development already used a Librarian and object store written in C++ so it was felt that there was little need to embed TCL into C programmed structure tools. Instead the structuring applications were written in entirely in TCL/Tk and a communications extension to TCL: TCL-TCP was used to communicate with the librarian/object store. The Windowing shell ``Wish'' which reads TCL commands and executes them was therefore used as the tool to both develop and run the tools.
TCL is an interpretative language, therefore debugging of applications was found to be much easier as elemens of code could be tried out within the interpreter before being included in the actual TCL program. All variables were visible via the interpreter and any patches required to the state of the application could be made to allow execution and debugging to continue.