1. Stop the system.

2. Modify the source code containing the function.

2a. Save the file.

3. Copy function definition to the debugger’s repl, and evaluate it.

4. Resume the system from frame that calls the function.

But yes, you must be diligent about making sure the source files contain the code that is actually running, but that’s not much different than artifact tracking.

Usually one wants to make the change persistent in the sources.

Step a): One just evaluates from the sources, while changing them. Once done -> save sources.

Step b): Create a patch file, which changes the image. Such a patch file can be loaded while starting an image, until one decides to save a new image with the changes already loaded.

For example there is a new release for the commercial LispWorks IDE once every one or two years. Users usually get only patch files for bug fixes during the one or two years maintenance. Thus I have a directory for patches to LispWorks, which are loaded when I start the base image.

One way would be to make the change in the source file and then recompile/reload that one function. This is pretty easy to do with SLIME; you can highlight the snippet of code you want to evaluate, and then send that to your program to be evaluated on-the-fly.