On Tue, Dec 4, 2012 at 9:37 PM, Johan Tibell <johan.tib...@gmail.com> wrote:
> Hi Oleg, > > On Tue, Dec 4, 2012 at 9:13 PM, <o...@okmij.org> wrote: > > I am doing, for several months, constant-space processing of large XML > > files using iteratees. The file contains many XML elements (which are > > a bit complex than a number). An element can be processed > > independently. After the parser finished with one element, and dumped > > the related data, the processing of the next element starts anew, so > > to speak. No significant state is accumulated for the overall parsing > > sans the counters of processed and bad elements, for statistics. XML > > is somewhat like JSON, only more complex: an XML parser has to deal > > with namespaces, parsed entities, CDATA sections and the other > > interesting stuff. Therefore, I'm quite sure there should not be > > fundamental problems in constant-space parsing of JSON. > > > > BTW, the parser itself is described there > > http://okmij.org/ftp/Streams.html#xml > > It certainly is possible (using a SAX style parser). What you can't > have (I think) is a function: > > decode :: FromJSON a => ByteString -> Maybe a > > and constant-memory parsing at the same time. The return type here > says that we will return Nothing if parsing fails. We can only do so > after looking at the whole input (otherwise how would we know if it's > malformed). > I thought it was possible to get around this with lazy patterns such "Wadler's force function" [1]? (untested code) force y = let Just x = y in Just x lazyDecode :: FromJSON a => ByteString -> Maybe a lazyDecode = force . decode [1] http://www.haskell.org/haskellwiki/Maintaining_laziness
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