[Haskell-beginners] A first try

[Manfred Lotz]

On Sun, 26 Jun 2011 09:27:47 -0400
David Place <d at vidplace.com> wrote:

> 
> On Jun 26, 2011, at 8:25 AM, Heinrich Apfelmus wrote:
> 
> > What about the combinator
> > 
> >  withFile :: FilePath -> (String -> a) -> IO a
> >  withFile name f = bracket (openFile name ReadMode) hClose $ \h ->
> >      evaluate . f =<< hGetContents h
> > 
> > ? It gives you the same thing as Iteratees - a way to apply a
> > function to the contents of a file - without the need to rewrite
> > all the existing list functions like  map , lines , words , and so
> > on.
> 

When I stumbled upon lazy IO newbie-wise I was pointed to withFile
resp. bracket by Daniel Fischer and now that I know how to do it it
seems fine to me. It also alerted me to pay more attention to lazyness
as this is a Haskell immanent thingie. 


> How would you, for instance, implement the program for counting all
> the words in a list of files that Oleg describes in his message?
> 
> > http://okmij.org/ftp/Haskell/Iteratee/Lazy-vs-correct.txt
> 

I'm not quite sure if it boils down to the same problem
structure-wise but I had to scan thru some 4000 small xml files, and I
did it like this (after Daniel lifted me up...):


  import qualified System.IO.UTF8 as U

  ...
  dt <- scanDir ccbd 
  let xmlfiles = filter (\x -> "xml" `isSuffixOf`  x) dt
  -- insert xml contents into two maps s, and ht.
  (s,ht) <- foldM insertXml (M.empty,M.empty) xmlfiles
  ...
and 

insertXml (stat, m) xf =
  U.withBinaryFile xf ReadMode
     (\handle -> do ct <- getXmlContent xf handle
          ... some code...   
         return $! (stat',m'))

and

getXmlContent xf inh = do
    xml <- U.hGetContents inh
    let content = parseXMLDoc xml
    ...

 

-- 
Manfred