On Tue, Oct 23, 2012 at 5:03 PM, "José A. Lopes"
Hey everyone,
I changed my code I now I get the following error message Main: thread blocked indefinitely in an MVar operation
Before the change, I was using the State monad with runState. Then, I changed the code to use the StateT monad transformer wrapped around IO monad and runStateT. And this change introduced the error message.
BTW I am using DoRec extension, maybe it is the source of the problem, but I don't know.
See if you can reproduce the problem using a small code sample. The problem is likely that your program is trying to use a state value that hasn't been produced yet. DoRec uses fixIO for the IO monad. fixIO passes a callback its own return value. It's not magic; it only works if the thunk is not forced within the callback. Take a look at how fixIO is implemented: http://hackage.haskell.org/packages/archive/base/latest/doc/html/src/System-...