2. It seems perfectly reasonable to want to have the Haskell called-back code throw an exception that is caught by the Haskell code

that called out to foreign code. "Reusing" the Haskell thread is necessary (though not sufficient) to achieve such behavior.

This is a particularly tricky problem, and I prefer to leave it up to the programmer to handle such things. Propagating the exception to the original (Haskell) caller automatically is unlikely to be the right thing to do, because the foreign code will almost certainly want to do some clean up. If the foreign language supports exceptions then it is conceivable that we could turn a Haskell exception into a foreign exception (and vice-versa), but for plain C I think we should leave well alone. Cheers, Simon