[Haskell-beginners] parsec and source material with random order lines

[Emmanuel Touzery]

I think you are right, this is probably the right track. A little more
googling with permutation parsers gave me this, which is also about parsing
iCal using parsec:
http://stackoverflow.com/questions/3706172/haskell-parsec-and-unordered-properties

I'll review all this and see if that solves the problem... Thank you!

Emmanuel


On Tue, Dec 25, 2012 at 3:28 AM, Brent Yorgey <byorgey at seas.upenn.edu>wrote:

> Hi Emmanuel,
>
> Sounds like you want a permutation parser, perhaps?  Check out
>
>
> http://hackage.haskell.org/packages/archive/parsec/latest/doc/html/Text-Parsec-Perm.html
>
> -Brent
>
> On Tue, Dec 25, 2012 at 12:18:37AM +0100, Emmanuel Touzery wrote:
> > Hi,
> >
> >  I'm trying to parse ical files but the source material doesn't matter
> > much. First, I know there is an icalendar library on hackage, but I'm
> > trying to learn as well through this.
> >
> >  Now the format is really quite simple and actually I'm parsing it, it
> > works, but I don't like the code I'm writing, it feels wrong and I'm sure
> > there is a better way. Actually for now I'm parsing it to an array of
> > arrays, but I want to fill a proper "data" structure.
> >
> >  For my purpose the file contains a bunch of records like this:
> >
> > BEGIN:VEVENT
> > DTSTART:20121218T103000Z
> > DTEND:20121218T120000Z
> > [..]
> > DESCRIPTION:
> > [..]
> > END:VEVENT
> >
> > There are a bunch of records I don't care about and also I want to parse
> no
> > matter what is the order of directives (so, i want to parse also if DTEND
> > appears before DTSTART for instance, and so on).
> >
> > That last part is my one problem. I can't do:
> >
> > parseBegin
> > start <- parseStart
> > end <- parseEnd
> > skipRows
> > desc <- parseDesc
> > skipRows
> > end <- parseEnd
> > return Event { eventStart = start, eventEnd = end ...}
> >
> > my current working code is:
> >
> > parseEvent = do
> >     parseBegin
> >     contents <- many1 $ (try startDate)
> >             <|> (try endDate)
> >             <|> (try description)
> >             <|> unknownCalendarInfo
> >     parseEnd
> >     return contents
> >
> > But then contents of course returns an array, while I want to return only
> > one element here.
> >
> > SOMEHOW what I would like is:
> >
> > parseEvent = do
> >     parseBegin
> >     contents <- many1 $ (start <- T.try startDate)
> >             <|> (end <- T.try endDate)
> >             <|> (desc <- T.try description)
> >             <|> unknownCalendarInfo
> >     parseEnd
> >     return Event { eventStart = start, eventEnd = end ...}
> >
> >  But obviously as far as Parsec is concerned startDate could occur
> several
> > times and also it's just not valid Haskell syntax.
> >
> >  So, any hint about this problem? Parsing multi-line records with Parsec,
> > when I don't know the order in which the lines will appear? I mean sure I
> > can convert my array to the proper data structure... I find which element
> > in the array contains the start date and then which contains the end
> > date... and build my data structure.. But I'm sure something much nicer
> can
> > be done... I just can't find how.
> >
> >  I see the author of iCalendar fixed the problem but I can't completely
> > understand his source, it's too many things at the same time for me, I
> need
> > to take this one step at a time.
> >
> >  Thank you!
> >
> > Emmanuel
>
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> > Beginners at haskell.org
> > http://www.haskell.org/mailman/listinfo/beginners
>
>
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