[Haskell-beginners] Parsing a file with data divided into sections

3 Feb 2009

      Hi all,

I wanted to parse a file that looks like this:

MONDAY
        JOHN
        JIM
        LINDA
TUESDAY
        BILL
        BOB
WEDNESDAY
THURSDAY
        SAM
        TODD
        LARRY
        LUKE
FRIDAY
        TED

in order to count the number of people for each day. After a (very)
long time and a lot of trial and
error, I managed to do it with this program:

import Char (isSpace)

main = interact (unlines . countSections . lines)
   where countSections = map (show) . snd . foldr compileSections (0, [])
         compileSections line (n, acc) =
             if isSection line
             then (0, (line, n) : acc)
             else (n + 1, acc)
         isSection line = not . isSpace . head $ line

which outputs:

("MONDAY",3)
("TUESDAY",2)
("WEDNESDAY",0)
("THURSDAY",4)
("FRIDAY",1)

I had quite a hard time figuring out how to keep count of the number
of records in each sections.
Is there a more obvious way to handle these types of problems? Are
there some builtins that could
of made it easier?

In Perl I would probably have used a hash and a variable to keep count
of the current day, incrementing
the hash value for each person until I got to the next day, but it's
not obvious to me how to transpose this
technique to functional programming.

Thanks a lot,

Patrick
-- 
=====================
Patrick LeBoutillier
Rosemère, Québec, Canada