Re: [Haskell-cafe] Re: Rethinking OO idioms

At 01:44 30/09/04 +0000, John Goerzen wrote:

On 2004-09-29, Ben.Yu@combined.com wrote:

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You can use state monad to mimic mutation.

Is that really what I want? In other words, is there a completely different, more Haskellish, way to look at this?

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Also, take a look at the recursive decent monadic parsec library. It may have done what you are trying to do.

Thanks for the pointer. I'll take a look.

Sometimes what you want really *is* a mutable value of some kind, but far less than you might expect. I recently implemented an RDF/XML parser using Parsec to parse from an "event stream" of XMLish things. Parsec is Monadic, and provides for user state in the parser, which I duly used. But the amount of user state I used was tiny: [[ data RDFParserState = RDFParserState { nodegen :: Int , listgen :: Int } ]] I.e., just two counters that were used for generating identifiers within the parsing process. Everything else was quite comfortably treated as immutable values. The code can be seen at: http://www.ninebynine.org/Software/HaskellRDF/RDF/Harp/RDFXMLParser.hs The function that invokes Parsec being 'parseEventsToRDF'. (This may not be the best introductory example, because it touches a lot of other logic. There is a test suite in the same directory if you want to dig deeper.) #g ------------ Graham Klyne For email: http://www.ninebynine.org/#Contact