>>> `compiled-prog-mode' that has generic commands for building program,
>>> `interpreted-prog-mode' that has generic commands for REPL
[...]
>    (define-prog-mode foo
>     :type 'compiled

<soapbox>
Let me point out that the idea that some languages are compiled and
others are interpreted is bogus.  This is a property of a language's
*implementation* and not of the language per se.
And of course, here we don't even really care about this facet of the
implementation: you're using those terms as a proxy for whether we use
a REPL or a batch-compiler.
</soapbox>

Many languages have both REPLs and batch compilers (like, say ELisp), so
a major mode needs to be able to offer access to both functionalities at
the same time.

Regarding the original suggestion to provide a uniform access to a REPL,
I started on this a long time ago, but never got it "finished" :-(
I attached what I still have of that effort.


        Stefan