[Haskell-cafe] Re: Rethinking OO idioms

Graham Klyne GK at ninebynine.org
Thu Sep 30 05:03:49 EDT 2004

At 01:44 30/09/04 +0000, John Goerzen wrote:
>On 2004-09-29, Ben.Yu at combined.com <Ben.Yu at combined.com> wrote:
> > 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?
> > 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:
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.)


Graham Klyne
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