Excerpts from Patrick LeBoutillier's message of Sun Aug 08 13:14:41 -0400 2010:
The getVersion and getDocument functions use the Data.Binary.Get monad to decode the byte string into various objects. I tried sprinkling "trace" calls a bit everywhere and I realize they don't always get called at the expected time. Most of them are only called when I reach the "putStrLn $ show stuff" statement. [snip]
Is this happening because I'm using lazy IO to read the file? Is there a way to force the evaluation of these "trace" calls? Are there any other ways to debug this kind of stuff in Haskell?
To be more precise, this is happening because Haskell is processing the file lazily. My feeling is forcing the evaluation of the trace calls using seq or similar won't be too helpful, because your error is earlier than that, in your definition of the Get monad. If you trace in the monad before you exhaust the input stream, you should get useful info; the function getRemainingLazyByteString may also come in handy. Cheers, Edward