[Haskell-beginners] A first try

[Manfred Lotz]

On Mon, 27 Jun 2011 12:39:42 +0900
Antoine Latter <aslatter at gmail.com> wrote:

> On Mon, Jun 27, 2011 at 11:57 AM, Manfred Lotz
> <manfred.lotz at arcor.de> wrote:
> > On Sun, 26 Jun 2011 17:32:02 -0400
> > David Place <d at vidplace.com> wrote:
> >
> >>
> >>
> >> On Jun 26, 2011, at 4:59 PM, Manfred Lotz wrote:
> >>
> >> > Is there any problem in the code snippet I pasted? If so I would
> >> > like to get a hint of course.
> >>
> >> There really isn't enough code in your snippet to be able to say.
> >> The rule is that if you return a lazy data structure from withFile
> >> you may find the file has been closed before you have read the
> >> data. If your parseXMLDoc is strict then you will get away with
> >> it.   I feel that isn't very nice.
> >>
> >
> > What I had learned (and it seems to work fine) is to force the
> > result which gets done here in return $!
> >
> > insertXml (stat, m) xf =
> >  U.withBinaryFile xf ReadMode
> >     (\handle -> do ct <- getXmlContent xf handle
> >          ... some code...
> >         return $! (stat',m'))
> >
> > So I hope that is the one and important recipe to add when using
> > withBinaryFile.
> >
> 
> The operator ($!) isn't recursive, so all that it does is force the
> tuple you're returning, not the contents of the tuple.
> 

Hmm, you are right. 


> Internally it calls 'seq', which only forces its argument enough to
> determine which constructor it was called with. Sometimes this can be
> enough to do what you want, but not usually. And certainly not in this
> case.
> 


I tried now:

getXmlContent :: String -> Handle -> IO Ctan
getXmlContent xf inh = do
    xml <- U.hGetContents inh
    let content = xml `deepseq` parseXMLDoc xml
    case content of
    ...



deepseq really ensures parseXmlDoc gets the full file as a string.

It is unclear to me how this could be done without deepseq.




-- 
Manfred