:l shell.hs
:def shell (\s -> do shell s; return "")

:unset +s
:unset +t

:shell cp A1.hs A.hs
:load B

-- eval something that isn't exported from B
z

:shell echo >> A.hs
:reload

-- A should be recompiled, B will be skipped.  We should still have
-- the full top-level scope of B available, though.  This was
-- nearly broken in GHC 6.4
z