In cabal we have a feature called "cabal repl" where it loads ghci with all the appropriate flags for the package or sandbox.

One of the nice things we want to do is extend the :reload (and related :add, :load) so that it calls out to cabal to run any pre-processors (e.g. hsc2hs, happy, alex etc).

Unfortunately the :def! command does not override builtin ghc commands. It looks like it works (:def reports the thing) but then the builtin command is still used, e.g.:

:def! reload (\_ -> print "yay" >> return "")
:def
the following macros are defined:
reload
:reload
Ok, modules loaded: none.

Related to this overriding, is that we also want a way to run the original (the builtin). Technically speaking it's possible using :undef, but then it's rather tricky to define again. You have to do something like define reload to call out to cabal, :undef reload, :reload, :def reload .... The tricky part is that last, we have to tie the knot, because that is the definition of :reload so it has to be able to access it's own code, tricky but not impossible (can load it from a file, but ugly).

So ideally we'd have something like :builtin [cmd] [args] that bypasses macros and executes the builtin command. Then we could define:

:def! reload :! cabal repl --reload ${other_stuff_here}
:builtin :reload

And similarly for the others :add, :load etc.