[Haskell-cafe] Parsec to parse tree structures?

david fries djf at gmx.ch
Wed Mar 17 16:36:36 EDT 2010

Thanks for the links!

I didn't realize that Iteratee can also do random access. I looked at
Oleg's slides a while back, but I haven't wrapped my head around it.

On Mon, 2010-03-15 at 08:57 -0500, John Lato wrote:
> Hi Dave,
> > From: david fries <djf at gmx.ch>
> >
> > Hello Café
> >
> > Some time ago I wrote a parser for a project of one our customers. The
> > format was proprietary and binary. The data was structured as a tree
> > with tables pointing to sub tables farther in the file. (Well actually
> > there was one or two cases where branches joined together, so I guess it
> > was a directed graph.) Also it had no defined endianess, some tables
> > were bigendian others were little endian and some were in their very own
> > in-house-endianess.
> > All in all, the typical binary data structure that has been in use and
> > continuously extended for the last 15 years. You know the kind.
> >
> > My parser was written in C# and wasn't particularly elegant, but it
> > worked reliably. I was wondering how you would parse tree-like
> > structures with Parsec (or other functional parsers)? Up to know, all
> > examples I've seen were of sequential data. To parse trees, you'd
> > essentially need to be able to follow pointers, parse whatever is there
> > and then jump back. I guess I'd have to mess around with the internal
> > state of the parser to do that, which is something I'd rather avoid.
> Have you seen Edward Kmett's Parsing Trifecta talk/slides, available
> at http://comonad.com/reader/2009/iteratees-parsec-and-monoid/ ?  I
> think a similar approach would allow you to parse this data structure.
>  The trick is to create a data source that can read from arbitrary
> locations.
> Also see http://okmij.org/ftp/Haskell/Iteratee/Tiff.hs for an
> iteratee-based TIFF reader.  TIFF files have a similar structure, so
> this code could apply pretty directly.  The parsing is quite
> low-level, but you could use iteratee-parsec [1] or
> attoparsec-iteratee [2] to use parser combinators with this technique.
> [1] http://hackage.haskell.org/package/iteratee-parsec
> [2] http://hackage.haskell.org/package/attoparsec-iteratee

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