Small rant ahead. You have been warned. My favorite system that supports hot-swapping is Linux. It works with every language, there is an excellent scheduler,

Small rant ahead. You have been warned. My favorite system that supports hot-swapping is Linux. It works with every language, there is an excellent scheduler, processes are protected from each other, there are all kinds of interprocess communication techniques available, you can choose between files and databases for persistence. I think the lame workflows people have while working within a conventional OS are a failure of imagination and a lack of understanding about how operating systems work. In particular, I suspect that a lot of people overestimate the cost of exec and IPC. Or, for example, do you really appreciate the fact that the operating system keeps file system data in RAM even if you exit the program and start up another one accessing the same file a microsecond later? Image-based persistence is not necessary for responsiveness. Use exec(), use a database, create the editors that make you productive. I've used Lisp and Smalltalk and I've created a miniature VM/OS running in the browser with processes and persistence and all that. These days I'm more excited about old fashioned Unix techniques.