In my case, only part of the program is recompiled and re-evaluated. The rest is located in a "committed" frozen area. Users can try new changes and throw them freely. The editor performs an evaluation/rollback on every keystroke, ensuring no accumulated or unintended changes to the stated were made during editing. When the user is satisfied and hit run, a long-term snapshot is created and the source code snippet is moving to the frozen area. Thats critical because the edit also rollback the file positions and streams.