It is certainly not new. There are some things that Racket doesn't allow, but they are very much not "compile new code". For example, you can't replace the mean

It is certainly not new. There are some things that Racket doesn't allow, but they are very much not "compile new code". For example, you can't replace the meaning of existing modules at runtime, nor can you modify the implementation of functions that are on the stack already (both things that some runtime systems for some languages allow). But `eval` has been there since the beginning.