Patrick LeBoutillier wrote:
Heinrich Apfelmus wrote:
import Data.ByteString.Lazy.Char8 as B
parse = map (zipWith ($) formats . B.split '\t') . B.lines where formats = [str, str, int, int, int, int, int, int, int, float, float] int = fst . fromJust . readInt float = \s -> read (unpack s) :: Double str = id
I've been looking at this example and I can't figure it out how it works. Seems to me that "formats" is a list of functions that return different types. How does this work?
Oops. It doesn't. :D Mea culpa, the functions having different types is indeed a problem. Fortunately, the trick from Oliver Danvy's "Functional Unparsing" http://www.brics.dk/RS/98/12/BRICS-RS-98-12.pdf applies. Here's the code: import qualified Data.ByteString.Lazy.Char8 as B import Data.Maybe parse = map (convert format . B.split '\t') . B.lines where format = str . str . int . int . int . int . int . int . int . float . float int = lift $ fst . fromJust . B.readInt float = lift $ \s -> read (B.unpack s) :: Double str = lift $ id lift :: (a -> b) -> ([a] -> c) -> ([a] -> (b,c)) lift f k (x:xs) = (f x, k xs) convert k = k nil where nil [] = () This time, I've also tested it in GHCi. Try :type parse to see the magic type. Regards, Heinrich Apfelmus -- http://apfelmus.nfshost.com