Re: [Haskell-beginners] find element of tupels

thanks Magnus, great, works so far. I added to check that myDay have to be greater 0: legalDate :: Date -> Bool legalDate (myDay, myMonth, myYear) = maybe False id $ do days <- lookup myMonth monthAndMaxDay return (not (myDay <= 0) && (myDay <= days)) Another thing is, that I have to check for leapYears. I have a function that checks if a year is a leap year isLeapYear :: Int -> Bool isLeapYear year = mod year 4 == 0 monthAndMaxDay has 30 days for February which have to be fix. So i have to check in legalDate if the year is a leap year. If so I have to check myDay in February for 29 days, otherwise for 28 days. How can I implement this to isLegalDate? -------- Original-Nachricht --------

Datum: Thu, 17 Dec 2009 15:21:13 +0000 Von: Magnus Therning An: kane96 CC: beginners@haskell.org Betreff: Re: [Haskell-beginners] find element of tupels

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Hi, I have a list of tuples: [(Jan, 31),(Feb, 28),(Mar, 31),...] called monthAndMaxDay

Date is: type Day = Int data Month = Jan | Feb | Mar | Apr | May | Jun | Jul | Ago | Sep | Oct | Nov | Dec deriving (Eq,Enum,Show) type Year = Int type Date = (Day,Month,Year)

Now I want to check if a date is legal: legalDate :: Date -> Bool legalDate (myDay, myMonth, myYear) =    not (myDay <= 0) && myDay >=  (find ((== myMonth) . fst) monthAndMaxDay) . snd

the find works to search for the month in the first element of every tupel but at the end I have to check the second value of the tupel (the day)

On Thu, Dec 17, 2009 at 3:03 PM, kane96 wrote: that it issn't higher than the maximal number of days in that month. Where do I have to set the "snd" correctly. Here I get the error: Couldn't match expected type `b -> c'

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          against inferred type `Maybe (Month, Day)'

First of all I'd take a look at the function Prelude.lookup, it'll be useful in this case.

Using that function I'd do something like this:

legalDate (myDay, myMonth, myYear) = maybe False id $ do days <- lookup myMonth monthAndMaxDay return (myDay <= days)

/M

-- Magnus Therning (OpenPGP: 0xAB4DFBA4) magnus＠therning．org Jabber: magnus＠therning．org http://therning.org/magnus identi.ca|twitter: magthe

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