Op 2010-09-17 06:32, Dean Herington schreef:
At 10:54 AM +0200 9/16/10, Jeroen van Maanen wrote:
Op 2010-09-16 08:50, Jeroen van Maanen schreef: [...]
Gregs suggestion provided the way out: evaluate. My code now looks like this:
do let maybeUpdatedModel = f update startModel theCheckSum = maybe 0 checkSum maybeUpdatedModel logException exception = logger "Exception" [showString label, shows (exception :: SomeException)] >> return Nothing logger "Received update" [showString label, logs update] resultOrException <- try $ evaluate $ seq theCheckSum maybeUpdatedModel maybeNextModel <- either logException return resultOrException logger "Maybe next model" [showString label, logs maybeNextModel] -- ...
I still don't understand why evaluating maybeUpdatedModel to WHNF after seq evaluated theCheckSum to WHNF forces the lurking exception to the surface. Or is the phrase 'when the resultant IO action is executed' in the documentation of 'evaluate' significant here?
I still don't follow all your code, but presuming maybeUpdatedModel is of non-IO type, I'm guessing it's the evaluation of theCheckSum that causes evaluation of maybeUpdatedModel (which raises the exception).
What I don't understand is the difference between: try $ return $ seq theCheckSum maybeUpdatedModel or even try $! return $! seq theCheckSum maybeUpdatedModel and try $ evaluate $ seq theCheckSum maybeUpdatedModel How is it possible that the exception escapes the former two expressions, but gets caught by the third try? Cheers, Jeroen