On Thu, Dec 17, 2009 at 3:03 PM, kane96
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) 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' 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