How to use the xml-conduit Cursor Interface for in

Question

The following question is based upon the accepted answer of this question. The author of the accepted answer said that the streaming helper API in xml-conduit was not updated for years (source: accepted answer of SO question), and he recommends the Cursor interface.

Based on the solution of the first question, I wrote the following haskell code which uses the Cursor interface of xml-conduit package.

import Text.XML as XML (readFile, def)
import Text.XML.Cursor (Cursor, ($/), (&/), ($//), (>=>), 
    fromDocument, element, content)
import Data.Monoid (mconcat)
import Filesystem.Path (FilePath)
import Filesystem.Path.CurrentOS (fromText)

data Page = Page
    { title :: Text
    } deriving (Show)

parse :: FilePath -> IO ()
parse path = do
    doc <- XML.readFile def path
    let cursor = fromDocument doc
    let pages = cursor $// element "page" >=> parseTitle
    writeFile "output.txt" ""
    mapM_ ((appendFile "output.txt") . (\x -> x ++ "\n") . show) pages

parseTitle :: Cursor -> [Page]
parseTitle c = do
    let titleText = c $/ element "title" &/ content
    [Page (mconcat titleText)]

main :: IO ()
main = parse (fromText "input.xml")

This code works on small XML files. However, when the code is run on a 30G XML file, the execution is killed by the OS.

How can I make this code work on a very large XML file?

Answer 1

The Cursor module requires that the entire contents be in memory, which seems to not be possible in this case. If you want to process files that large, you'll need to use the streaming interface.