On Sun, 7 Apr 2002, Eric wrote:
| Hi there, I am a beginner a haskell,and I have some
| difficulty in dealing with an assignment that is to
| translate a string representation of a list of
| appointments into a list of appointments. For example:
[requirements snipped]
| How can I finish the requirement make use of library
| functions such as words, unwords, lines and break.
Hey Leo,
Well, I'm just learning haskell too, but I decided to give
this a shot. I think I solved the main problem, but I did
hit a few snags of my own so I can't be sure. Here goes:
Basically, I defined appointment as a single type... But
I couldn't figure out how to print that type. So I can't
really test what I've done, other than I know it
compiles. :) Can someone show me how to fill in show down
here? )
module Main where
data Appt = Appt (Bool, Int, Int, String)
instance Show Appt where
show x = ???
I then used liness and words to break the multi-line string
into a list of lists of words:
strToApps :: String - [Appt]
strToApps x = map lineToApp (lines x)
lineToApp x = wordsToApp (words x)
Since the starting ! was optional, the structure of the
list could go in two directions here:
wordsToApp :: [String] - Appt
wordsToApp ws | head ws == ! = mkAppt True (tail ws)
| otherwise = mkAppt False (ws)
Now it's just a matter of parsing the rest of the line. For
simplicity's sake, I took the liberty of assuming you always
used the same number of digits for the hours, so I didn't
have to search for the -:
mkAppt :: Bool - [String] - Appt
mkAppt isImp (w:ws) = Appt (isImp, start, done, note)
where (hs, hd) = splitAt 3 w -- assumes zero-padded (eg 01-03)
start = atoi hs
done = atoi hd
note = foldr1 concat ws
concat a b = a ++ ++ b
That atoi function came from working through the exercises
in Rex Pages online book ( http://www.cs.ou.edu/cs1323h/textbook/haskell.shtml )
horner str = foldr1 op (reverse str)
where op d s = d + (10 * s)
atoi str = horner [digitToInt d | d - str]
Then I defined a main function:
-- I wonder what a refec is. :)
main = do print $ strToApps ! 10-11 lecture\n12-13 lunch at the refec :(
... And that's it! I think it would work if I could figure
out how to print Appt objects (or whatever you call them in
haskell). Meanwhile, I get this because of the way I defined
show:
Main main
[???,???]
Cheers,
- Michal http://www.sabren.net/ [EMAIL PROTECTED]
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