OP here.<p>This IDE started as a final project in Gerry Sussman's LISP class at MIT.<p>When programming and debugging, you often want to know what your code is going to do without the possibility of unintended side effects (state change) by actually running the code.<p>Towards this goal, we added a concept we call "preview execution." That is: you can preview the output of code in our IDE simply by highlighting a section of code. The IDE will show the output of the highlighted section, but when you de-select the section, there are no side effects: no <i>state</i> is changed in the Scheme instance.<p>It's downloadable as a Mac app, but also open source. Would love to hear people's thoughts on it!