Re: forcing a haskell object to WHNF from C

Should/can I use rts_eval()?

Yes.

Any pointers on what this does (start new threads, cause garbage collection ...) would be appreciated. I can (and have) gone over the code but a more high level description would be helpful.

rts_eval() may cause garbage collection to happen. This means that every HaskellObj value might be invalid after a call to rts_eval (except for the return value). rts_eval starts a new concurrent Haskell thread to do the evaluation.

In essence I want a C version of seq (perhaps I can call to Haskell to perform a seq on it, but I think this might be overkill).

Calls to Haskell are implemented using rts_eval/rts_evalIO too, so writing it in Haskell is the same thing with a little bit more overhead. Cheers, Wolfgang