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

24 Dec 2012

      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

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

Emmanuel Touzery