Jules Bean wrote:
Unfortunately, it's not going to work. It's not going to work because some of the procedures take callbacks, and the callbacks are values of type IO (). I can see two solutions to this:
a) revert to using an IORef, and use lexical scoping to define my callbacks in a location such that the reference to the environment is in scope. This will work, but it's just not convenient to define all your functions inside another function so you can do a lexical scoping trick...
b) write the callbacks as values of type
StateT Env IO ()
Is this the right approach? Is there a better one?
Yes: implicit parameters and implicit configurations. The end user code is often identical in both of these approaches (but the latter may require a bit more preparation. OTH, implicit parameters are supported by the compiler whereas the latter is just a Haskell library). Both approaches are compared in Appendix A of the paper http://www.eecs.harvard.edu/~ccshan/prepose/prepose.pdf The example is writing JNI functions in Haskell. Incidentally, the example illustrates passing of the implicit state (JNIEnv pointer) _around_ the exception handler. The problem is that the function 'handle' has the type handle :: (Exception -> IO a) -> IO a -> IO a rather than handle :: (Exception -> MonadIO a) -> MonadIO a -> MonadIO a
revert to using an IORef, and use lexical scoping to define my callbacks in a location such that the reference to the environment is in scope. This will work, but it's just not convenient to define all your functions inside another function so you can do a lexical scoping trick...
The stated motivation of the implicit configurations is precisely that: make it appear that the IORef is `lexically scoped' over all of its clients without putting all the clients inside one giant function. The master file for the paper http://www.eecs.harvard.edu/~ccshan/prepose/prepose.lhs is a literate Haskell code, which you can load into GHCi as it is. You can then run all of the examples in the paper. If you have trouble using implicit configurations, have a particular application in mind and can turn it into a self-contained example (by replacing various functions with stubs wherever possible: e.g., see the very end of prepose.lhs), you may want to let Ken and me know. I'll see what can be done.