[Haskell-beginners] try, seq, and IO

[Jeroen van Maanen]

The past year I have been working on a port of my machine learning project named LExAu from Java to Haskell. I'm still very glad I took the jump, because the complexity curve appears to be log shaped rather than exp shaped. In one year I almost got to the functionality that had taken me five years to produce in Java (of course it helped a lot that I had a working prototype this time).

There is one thing that still bothers me though: when I write seq or $! it doesn't seem to have any effect!

Currently I am trying to add some exception handling to help me debug the system, but the code that I managed to produce depends on the logging statement to produce the desired result. :-( It looks like this, and only works when I uncomment the line '-- logger "Check sum": [...]', otherwise the exception is caught by the try around the body of the thread that this code runs in:

         do logger "Received update" [showString label, logs update]
            result <-
              try $!
                do maybeUpdatedModel <- return $ f update startModel
                   theCheckSum <- return $ liftM checkSum maybeUpdatedModel
--                   logger "Check sum" [showString label, shows theCheckSum]
                   return $! seq theCheckSum maybeUpdatedModel
            maybeNextModel <-
              case result of
                Right theMaybeNextModel -> return theMaybeNextModel
                Left exception ->
                  do let exc :: SomeException
                         exc = exception
                     logger "Exception" [showString label, shows exception]
                     return Nothing
            logger "Maybe next model" [showString label, logs maybeNextModel]

For more context see:

  http://lexau.svn.sourceforge.net/viewvc/lexau/branches/totem/src/LExAu/Pipeline/Concurrent.hs?revision=326&view=markup

after line 241.

Can someone explain why a few showStrings a shows and a putStrLn are more effective in forcing the check sum to be computed (which necessarily evaluates the complete updated model and reveals the lurking exception) than the seq on the line just below the logging statement?

Cheers,  Jeroen