I had this though in my head for a while now. I built an IDE for a DSL, and somewhere toward the end of the first revision I understood something very interesting. My IDE wasn't actually using the textual representation of the language. The scripts that the user was editing were actually live instances, and they were fully capable of validating and saving themselves.
The IDE worked with those instances, used them to do its operations, and allowed to edit them on the fly. It was quite challenging to do, I must say, and I kept thinking about the image model of smalltalk, where everything is a live instance.
This brings to mind Greenspan's tenth rule, which state: Any sufficiently complicated C or Fortran program contains an ad hoc, informally-specified, bug-ridden, slow implementation of half of Common Lisp.