[Haskell-beginners] Bytestring question

[Peter Braun]

Hi everyone,

as an exercise for learning Haskell i'm writing a program that converts 
Ascii Stl files (a simple format for 3D model data) into binary Stl 
format. In my first attempt i used normal strings and the result was 
therefore very slow. Now i rewrote the program to use lazy bytestrings 
instead.

But well... it got even slower, so i'm probably doing something terribly 
wrong ;)

Here's what i do (the relevant parts):

...
asciiFile <- L.readFile (args!!0)
binHandle <- openBinaryFile (args!!1) WriteMode
let asciiLines = L.split (c2w '\n') asciiFile
...
parseFile binHandle (Normal, tail asciiLines) -- First line contains a 
comment
...

where L is Data.ByteString.Lazy. readFile ought to be lazy so it should 
not read the whole file into ram at this point. But when i split the 
lines and pass them to a function, is this still carried out lazily?

parseFile processes a line, depending on the StlLineType and then calls 
itself recursive like this:

parseFile :: Handle -> (StlLineType, [L.ByteString]) -> IO ()
...
parseFile h (Vertex1, s) = do
     let vals = extractVertex (head s)
     L.hPutStr h $ runPut (writeFloatArray vals)
     parseFile h (Vertex2, tail s)
...

extractVertex looks like this:

extractVertex :: L.ByteString -> [Float]
extractVertex s = let fracs = filter (\n -> L.length n > 0) $ L.split 
(c2w ' ') s
                                     in    [read (C.unpack(fracs!!1)) :: 
Float,
                                             read (C.unpack(fracs!!2)) 
:: Float,
                                             read (C.unpack(fracs!!3)) 
:: Float]

where C is Data.ByteString.Lazy.Char8. It splits a byte string, filters 
out the whitespaces and converts certain entries to floats. Maybe unpack 
is an expensive operation. Is there a better way to convert a Bytestring 
to float?

I know, this is bad Haskell code ;) But where is my grand, obvious 
misuse of Bytestring?

I'm grateful for any suggestion to improve that code. I'm using ghc, 
version 6.12.1.

Thank you,
Peter