Excerpts from Neil Mitchell's message of 2015-09-14 14:07:14 -0700:
1) Is there any way to do the two compilations sharing some cached state, e.g. loaded packages/.hi files, so each compilation goes faster.
You can, using withTempSession in the GhcMonad. The external package state will be preserved across calls here, but things put in the HPT will get thrown out.
So as far as I can tell, you are suggesting I basically do getSession in one session, grab the cache bits of the HscEnv, and inject them into the start of the next session with setSession? (withTempSession and all the other session functions just seem to be some variation on that pattern). I tried that, but even storing just hsc_EPS between sessions (which seemed like it should both be something that never changes), causes weird compile failures. Is moving things like hsc_EPS between sessions supported? Or were you suggesting I do something else?
No, something a bit different: I'm suggesting that you use this functionality to "fork" a session: so you do some work, getting a session, snapshot the session, do some more work, and then use the snapshot to rollback before the work. I haven't actually tested this, however. Edward