[Haskell-cafe] try, seq, and IO

[Neil Brown]

On 15/09/10 10:13, 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?
>    
I just looked at your code for a second time and I think I know.  Here's 
my guess:

It's all about the types, and weak head normal form.  The "theCheckSum" 
item has type Maybe a, and you are looking for an exception in the "a" 
part that's wrapped by Maybe (I presume: the Just/Nothing is determined 
by the function "f", not "checkSum").  When you use seq or $!, you only 
evaluated to weak head normal form,  For something of type "Maybe a", 
that's only the Just/Nothing part.  Forcing that aspect to be evaluated 
only requires some evaluation of the function "f", and the checkSum 
function doesn't need to be applied, even if it is a Just value.  
However, your shows function digs deeper and evaluates the whole thing, 
so hence it will trigger the error where seq and $! won't.

So, if I'm right, you need to change your function to make sure to 
evaluate the checksum itself.  The deepseq package on Hackage may help 
here (use the deepseq function in place of seq: 
http://hackage.haskell.org/packages/archive/deepseq/1.1.0.0/doc/html/Control-DeepSeq.html).

I'm a little suspicious of the code as a whole though: why does checkSum 
throw an exception rather than returning a Maybe value?  Or is it an 
exception somehow outside of your control?

Another thing:

x <- return $ blah

in a do block is bad style, and can always be replaced with:

let x = blah

Note that there is no "in" needed with let statements in do blocks (that 
confused me when first learning Haskell).

Hope that helps,

Neil.