Programmer's Guide: Introduction

Lens was designed with the realization that it is not possible to write a simulator that can satisfy the needs of all users right out of the box. The ability to access the C data structures within scripts, and to assemble group types from components, makes it possible to implement many customizations without altering the code. However, more low-level additions are often necessary and one of the main goals of Lens was to make it relatively easy for more advanced users to get in and change the code to fit their needs. This guide is here to explain how that's done.

Lens is primarily written in C (about 25K lines in 27 modules), with the graphical displays written in Tcl/Tk (about 5K lines in 11 modules). Basically, the Tcl code is used for drawing windows, creating bindings between window events (like button presses) and commands, and controlling the behavior of the shell. Everything else, including all of the time-sensitive calculations, is done in C.

Essentially, Lens is a C program whose main activity is running a Tcl interpreter. The interpreter handles events and evaluates Tcl commands. Most of the Tcl commands end up calling a C command to do the work. These are implemented as special C functions that are registered with the Tcl interpreter to create Tcl commands that serve as handles to the C code.

Because Lens is still a work in progress, it is likely that the modifications to the main code will be made frequently. The philosophy at this point in development is that nothing is guaranteed to remain the same, though any changes will be backward compatible as much as possible.

This is bound to create some conflict with users who have their own versions of the code. Therefore, when changing the source code, it is important to restrict any changes to as few modules as possible. That way, if the generic version of those modules are changed, reimplementing the additions is as painless as possible. As explained in later sections, as long as you contain your code changes to the extension.c file, very little must be done to keep up-to-date with the latest version.

It is now possible to compile Lens as a library and embed it in another application. This allows you to integrate a neural network with your other software.

Douglas Rohde
Last modified: Mon Nov 13 13:57:53 EST 2000