Re: [Haskell-beginners] Laziness and XML parsing

The following should work. The basic idea is: * Try to parse a <person> * If it's not a <person>, recursively try again. {-# LANGUAGE OverloadedStrings #-} import Text.XML.Stream.Parse import Data.Text (Text, unpack) import Control.Monad (join) import Data.Enumerator (Iteratee) import Data.XML.Types (Event) data Person = Person { age :: Int, name :: Text } deriving Show parsePerson :: Monad m => Iteratee Event m (Maybe [Person]) parsePerson = tagName "person" (requireAttr "age") $ \age -> do name <- content return [Person (read $ unpack age) name] parseWrapper :: Monad m => Iteratee Event m (Maybe [Person]) parseWrapper = parsePerson `orE` (fmap . fmap) concat (tagPredicate (const True) ignoreAttrs (const $ many parseWrapper)) main = parseFile_ def "people.xml" $ force "people required" parseWrapper Michael On Tue, Nov 8, 2011 at 7:28 AM, Sean Hess wrote:

Thanks all for your help so far. Using xml-enumerator, is there any way to parse the following xml, and ignore the people tag? In other words, can I parse it by only providing an Iteratee for Person, no matter where a <person> tag appears nested within a document?

<?xml version="1.0" encoding="utf-8"?> <people> <person age="25">Michael</person> <person age="2">Eliezer</person> </people>

On Nov 8, 2011, at 7:33 AM, Michael Snoyman wrote:

Thanks for the heads-up, it's just a few minor tweaks in the 0.3->0.4 transition. I'll update later, and add a link to the blog post, and release a new version to Hackage.

On Tue, Nov 8, 2011 at 6:03 AM, Sean Hess wrote:

Thanks so much to both of you that sent that link.

Sorry, my email totally wasn't clear. I meant that the example in the

package description doesn't

run: http://hackage.haskell.org/packages/archive/xml-enumerator/0.4.3.1/doc/html/...

I'll read through that article.

On Nov 8, 2011, at 7:01 AM, Michael Snoyman wrote:

Here's a blog post on the package:

http://www.yesodweb.com/blog/2011/10/xml-enumerator . It doesn't cover

the streaming interface, but it might give you a good overview of the

package in general. I'm not sure what you mean by "it doesn't run,"

but you'll need at least a basic understanding of enumerators to get

off the ground.

On Tue, Nov 8, 2011 at 5:38 AM, Sean Hess wrote:

I cannot seem to find a working example of xml-enumerator. It doesn't run:

the names seem to have changed for some things, and I'm too much of a

beginner to figure it out easily.

http://hackage.haskell.org/packages/archive/xml-enumerator/0.4.3.1/doc/html/...

On Nov 7, 2011, at 7:59 PM, Felipe Almeida Lessa wrote:

On Tue, Nov 8, 2011 at 12:45 AM, Sean Hess wrote:

I want to parse a large xml file (2GB), without putting the whole thing into

memory. It's pretty simple with a sax parser in most languages, you just

stream bytes to the sax parser, and wait for sax events.

I recommend you taking a look at xml-enumerator [1] and

libxml-enumerator [2].  They are the SAX parsers you know from the

imperative world but much easier to write =).  In particular, you

don't need to rely on lazyness.

Cheers,

[1] http://hackage.haskell.org/package/xml-enumerator

[2] http://hackage.haskell.org/package/libxml-enumerator

--

Felipe.

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