On Thu, Dec 17, 2009 at 04:03:59PM +0100, kane96 wrote:
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)'
I'm sure the error message goes on to say something like: in the first argument of (.) namely (find ((== myMonth) . fst) monthAndMaxDay) ... you have to interpret this error message a little bit. It is saying the composition operator (.) is expecting a function of type b -> c in it's first argument (or on the left-hand side), but you have provided something of type Maybe (Month, Day). This type is what that (find...) evaluates to. I'm missing the correct terminology here, but you cannot provide something that is fully evaluable on the left-hand side of (.). It needs to be something that evaluates to a function of one argument. That argument will be supplied by the function provided on the right hand side of (.). So, at a minimum, you need to swap your snd and your (find...), but that doesn't solve the problem of getting the tuple out of the Maybe monad that find returns. A