if i change anything in filters.scm then i (load "filters.scm") instead of just sending a snippet to the interpreter to eval. i have this happen when i save the file. same thing, except i never have to worry about sending code or a mis-match between editor and repl. it also forces you to be more organized with code so that you don't override global state. i just code, save, code, save. you can have hooks run things after every save etc ...
my inkling is that the more special forms the more difficult it is to repl a language. if everything is an expression not only can you eval parts of it, but that is enough to support quite a bit of extensibility sans macro.
> In emacs, for instance, you often use `eval-last-sexp`, by default bound to C-x C-e. This lets you move your cursor to a particular point in the file, often deep in a function, and get the results of just the form(s) you're pointing at.
but that depends on how the environment is looking at the time.
during development, in scheme, i just have a function (reset) that reloads all the files of an app. sometimes i create a thread and poll the filesystem and reload everything that changed in the last second. that way i never have to C-x anything. if i need to eval a subexp i just jump on the repl and do it.