[Haskell-beginners] Using Debug.Trace

[Patrick LeBoutillier]

Hi,

I tried the bang patterns, it seems to force evaluation enough in most
cases to triger the "trace" call.

Thanks a lot,

Patrick


On Sun, Aug 8, 2010 at 2:18 PM, Daniel Fischer <daniel.is.fischer at web.de> wrote:
> On Sunday 08 August 2010 19:14:41, Patrick LeBoutillier wrote:
>> Hi all,
>>
>> I'm writing a parser for a binary format, and I'm trying to debug it
>> since I have a bug and the code is getting lost in the bits and bytes.
>> Basically my main is like this:
>>
>>   import Data.Binary.Get
>>   import Debug.Trace
>>   import qualified Data.ByteString.Lazy as L
>>
>>   main = do
>>     bytes <- L.readFile "song.gp4"
>>     let (version, bytes') = getVersion bytes
>>     putStrLn version
>>
>>     let stuff = runGet (getDocument version) bytes'
>>     putStrLn $ show stuff
>
> putStrLn $ show stuff === print stuff
>
>>
>>     return ()
>
> Unneeded
>
>>
>> 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.
>
> trace prints its first argument when the second is demanded, so to print
> earlier, you can in general add more strictness to your programme, but I'm
> not sure if that makes a difference for Get, since that has no freedom to
> reorder the sequence in which the values are read. So with traces in the
> right places, you should get tracing output while the deserialisation is
> underway automatically.
> Whether in
>
> getSomeObject = do
>    foo <- get
>    !bar <- trace ("foo is " ++ show foo) get
>    let !baz = trace ("bar is " ++ show bar) $ fiddle foo bar
>    return $! trace ("baz is " ++ show baz) (wibble baz foo bar)
>
> the bangs make a difference regarding trace output, I don't know.
>
>> Most of them are only
>> called when I reach the "putStrLn $ show stuff" statement.
>> Unfortunately that doesn't help me because I get a
>>
>>   *** Exception: too few bytes. Failed reading at byte position 35939
>>
>> before that.
>
> That usually means the file hasn't the correct format, e.g. some size
> (length of list) has been written in little-endian order and is read in
> big-endian, so get tries to read more items than there are.
>
>>
>> Is this happening because I'm using lazy IO to read the file?
>
> Unlikely. What is the file size on disk? If it's larger than 35939 bytes,
> you have an IO problem, but still Data.ByteString.Lazy.readFile wouldn't be
> the first on my list of suspects.
>
>> Is there a way to force the evaluation of these "trace" calls?
>
> Seeing more of the code could help coming up with ideas.
>
>> Are there any other ways to debug this kind of stuff in Haskell?
>>
>
> Break things down into smaller pieces and test those.
> And there's the ghci-debugger, I hear if one has learned to use it, it's
> quite helpful.
>
>>
>> Thanks a lot,
>>
>> Patrick
>
>



-- 
=====================
Patrick LeBoutillier
Rosemère, Québec, Canada