Allow for multiple linker instances

[jcpetruzza]

Right now there is only one RTS linker with a single symbol table.This means, for example, that one cannot have multiple instances of the GHC interpreter in the same process running simultaneously.

[jcpetruzza]

Example that fails due to this limitation

[hgolden]

I looked at the attachment. Is it a significant problem to use separate processes when two interpreters are needed? Could you give a use case for this feature that shows its usefulness? Thanks.

[jcpetruzza]

Replying to hgolden:

[...] Could you give a use case for this feature that shows its usefulness? Thanks.

You may want to use ghc in interactive mode to implement some sort of evaluator as part of a Haskell IDE (e.g., like  this). You will then want to have one evaluator per opened project and each evaluator naturally corresponds to a ghc session. One can implement this with multiple processes but in an arguably less convenient way.

[simonmar]

This is indeed a significant ugliness. I think fixing it would be fairly mechanical, just package up the linker's state as an object that gets passed in to every call, and then in GHC the PersistentLinkerState becomes the holder for the RTS linker state and is carried around in the HscEnv rather than being a global variable.