[Haskell-cafe] try, seq, and IO

Gregory Crosswhite gcross at phys.washington.edu
Wed Sep 15 12:22:20 EDT 2010

  Check out the "evaluate" function in Control.Exception.

Also note that if you apply seq to an IO action, you do *not* force the 
result, only the action that will eventually produce the result.


On 9/15/10 2:13 AM, Jeroen van Maanen wrote:
> 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
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