[Haskell-cafe] Hexpat: Lazy I/O problem with huge input files

[thinkingeric]

Hi Aleks, 
Did you (or anyone) ever resolve this? I'm having precisely the same
problem. 

Eric



Aleksandar Dimitrov wrote
> 
> Hello Daniel,
> 
>> I don't know Hexpat at all, so I can only guess.
>>
>> Perhaps due to the laziness of let-bindings, mError keeps a reference to
>> the entire tuple, thus preventing tree from being garbage collected as it
>> is consumed by print.
> 
> Thanks for your input. I think you are right, the parse tree isn't
> freed as the parse proceeds if mError is forced later on in the
> program (anywhere.) I don't think it has something to do with the
> tuple constructor or 'let' itself, but I'm also not very proficient at
> figuring these kinds of things out, so I may be very wrong. I did do
> the following test to support my hypothesis:
> 
>> import Text.XML.Expat.Tree
>> import System.Environment (getArgs)
>> import Control.Monad (liftM)
>> import qualified Data.ByteString.Lazy as C
>>·
>> -- This is the recommended way to handle errors in lazy parses
>> main = do
>>     f <- liftM head getArgs >>= C.readFile
>>     let (_, mError) = parse defaultParseOptions f :: (UNode String, Maybe
>> XMLParseError)
> 
>>     case mError of
>>          Just err -> putStrLn $ "It failed : "++show err
>>          Nothing -> putStrLn "Success!"
> 
> I.e., keeping the parse tree is forced by the evaluation of mError.
> There is not a single reference to the parse tree within the program
> itself (unless I'm not noticing some sort of do-notation magic in the
> whole thing here...) It is interesting (and rather unfortunate) that
> just evaluating a potential error seems to block garbage collection.
> 
> If I'm correct (and I hope I'm not!) this seems to prevent using lazy
> I/O in Hexpat if you want to know if there's a parse error (and if so,
> what that would be.) I'll contact the author, maybe it's a genuine
> bug?
> 
> Thanks again :-)
> Aleks
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