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

[Patrick LeBoutillier]

> Nice program, especially your use of function composition is good style.

Thanks, I must admit I put alot of time into refactoring it. There's so many
different ways of doing stuff (and then simplifying them) in Haskell!

>> I think that you've pretty much used accumulators in the way they most
>> often are used.  In many cases you don't _have_ to keep count though.
>> Here's one way to get the same result, but without keeping count:
>>
>> countDays [] = []
>> countDays ls = let
>>         day = head ls
>>         count = length $ takeWhile (isSpace . head) $ tail ls
>>     in (day, count) : countDays (drop (1 + count) ls)
>>
>> main = interact (unlines . map show . countDays . lines)
>
> Here's a version using  span  from the Prelude:
>
>  main = interact $ unlines . map show . countDays . lines
>
>  countDays []       = []
>  countDays (day:xs) = (day, length people) : countDays xs'
>     where (people, xs') = span (isSpace . head) xs
>

Both of these examples are great and exactly what I was looking for: a
different approach to the problem. I guess the step that was missing in my
"thought process" is that recursion doesn't have to imply processing the list
elements 1 by 1 (1 recursive call for each element). Of course it makes
perfect sense once you see it....

Thanks a lot to everyone, as usual the people on this list are always very
kind and helpful.


Patrick

>
> Note that this file format is very simple and it's ok to use  lines  and
>  isSpace  to parse it. But the tool of choice are parser combinators
> like  Text.Parsec  or  Text.ParserCombinators.ReadP .
>
>
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> http://apfelmus.nfshost.com
>
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-- 
=====================
Patrick LeBoutillier
Rosemère, Québec, Canada