Alan & Kim Zimmerman
At the moment there are issues with having multiple GHC API sessions in a single executable, which boil down to GHC having global variables.
A quick grep over the GHC sources shows the following instances
compiler/ghci/Linker.hs:90:GLOBAL_VAR_M(v_PersistentLinkerState, ... compiler/ghci/Linker.hs:91:GLOBAL_VAR(v_InitLinkerDone, ....
This isn't the only global state had by the linker; have a look at rts/Linker.c. Unfortunately this would take a fair amount of work to resolve. The easiest solution here would probably be to put a lock around the linker and have all sessions in a process use the same linker.
compiler/main/StaticFlags.hs:94:GLOBAL_VAR(v_opt_C, .... compiler/main/StaticFlags.hs:95:GLOBAL_VAR(v_opt_C_ready, ....
We've been meaning to get rid of the remaining static flags for quite some time. I was a bit surprised to find that opt_NoStateHack is static. This is a bit of an ugly one: the only user is a test in Id.isStateHackType which is buried inside several layers [1] of pure predicates that have no DynFlags. It would be a shame to pass a DynFlags through all of these calls just to support the state hack (which we threaten drop about once every six months). One option, of course, would be to merge all of the static flags into DynFlags and rework their users to use unsafeGlobalDynFlags. [1] One particular callpath is, Id.isStateHackType Id.typeOneShot CoreArity.typeArity CoreArity.exprArity CorePrep.rhsToBody CorePrep.cpeBody
compiler/main/DynFlags.hs:4453:GLOBAL_VAR(v_unsafeGlobalDynFlags,....
The principle user of this is Outputable, which uses it to provide DynFlags for pprTrace and friends. As you say, these are rather special cases and it's probably fine if they behave a bit funky in the case of more than one session in a process.
ghc/GHCi/UI.hs:149:GLOBAL_VAR(macros_ref,....
It is completely unclear why this is global at all. It seems like it would fit just fine in the GHCi monad. I've opened D1789 doing exactly this. There may be other global state that I'm not thinking of, but if this is everything then it seems quite possible to fix this up for 8.2. Cheers, - Ben