[Haskell-cafe] newbie questions (read, etc., with Data.ByteString.Lazy.Char8)

George Pollard porges at porg.es
Tue Oct 7 17:19:48 EDT 2008

On Mon, 2008-10-06 at 21:06 -0500, Mike Coleman wrote:
> There's a readInt method, which I guess I could use, but it returns a
> Maybe, and I don't see how I can easily strip that off.

You can use Data.Maybe's 'mapMaybe' function
"The mapMaybe function is a version of map which can throw out elements.
In particular, the functional argument returns something of type Maybe
b. If this is Nothing, no element is added on to the result list. If it
just Just b, then b is included in the result list."

So we change this:
> -- lazy version (doesn't compile)
> -- file: ch08/SumFile.hs
> import qualified Data.ByteString.Lazy.Char8 as L
> main = do
>          contents <- L.getContents
>          print (sumFile contents)
>              where sumFile = sum . map read . L.words

To this:
import qualified Data.ByteString.Lazy.Char8 as B
import Data.Maybe (mapMaybe)

main = do
        contents <- B.getContents
        print (sumFile contents)
        where sumFile = sum . map fst . mapMaybe B.readInt . B.words

Now, there is  a problem with this; the original version will not
read integers that are followed by garbage, whereas this one will.
The difference is between readInt and read:

(read "2a") will fail,
(readInt "2a") will return Just (2,"a")

readInt is really designed with parsers in mind. So, we need to filter
out anything readInt returns with a non-empty string in the right part
of the tuple. Ending up with this:

sumFile = sum . map fst . filter (B.null . snd) . mapMaybe B.readInt . B.words

Actually, after saying this, the original version probably
just dies when given garbage :P
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