[Haskell-cafe] strict version of Haskell - does it exist?

Felipe Almeida Lessa felipe.lessa at gmail.com
Mon Jan 30 10:19:53 CET 2012

On Mon, Jan 30, 2012 at 6:21 AM, Herbert Valerio Riedel <hvr at gnu.org> wrote:
> On Sun, 2012-01-29 at 23:47 +0100, Marc Weber wrote:
>> So maybe also the JSON parsing library kept too
>> many unevaluated things in memory. So I could start either writing my
>> own JSON parsing library (being more strict)
> Jfyi, aeson has been added strict parser variants json' and value'
> some time ago, so you shouldn't have to write your own stricter JSON
> parsing library...

Also, besides using those variants, you may also use the
attoparsec-conduit library [1].  If you have

  processJson :: Value -> IO X

then you'd need just something like

  import Data.Aeson (Value, json')
  import Data.Attoparsec.Char8 (isSpace_w8)
  import qualified Data.ByteString as B
  import qualified Data.Conduit as C
  import qualified Data.Conduit.Attoparsec as CA
  import qualified Data.Conduit.Binary as CB
  import qualified Data.Conduit.List as CL

  main = do
    ret <- forM_ fileList $ \fp -> do
      C.runResourceT $
        CB.sourceFile fp C.$=
        jsonLines C.$=
        CL.mapM processJson C.$$
    print ret

  jsonLines :: C.Resource m => C.Conduit B.ByteString m Value
  jsonLines = C.sequenceSink () $ do
    val <- CA.sinkParser json'
    CB.dropWhile isSpace_w8
    return $ C.Emit () [val]

This code is extremely resource-friendly, since (a) you can't leak
file descriptors and (b) you just have to make sure that  processJson
function isn't too lazy.  It should be quite fast as well.

Cheers! =)

[1] http://hackage.haskell.org/package/attoparsec-conduit


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