Checking function in Haskell

Question

I'm working on some CS homework and this is my first experience with Haskell, so I have a question.

I've declared a data type

data Date = Date Int Int Int deriving (Show)

and I have this function testdate:

testdate :: Date -> Maybe Date
testdate (Date m d y) = if 1 <= m && m <= 12 && 1 <= d && d <= 31 && y >= 0
    then
        True
    else
        False

I want to make a new function that checks to see if testdate is true or false, then returns to you the date. something like:

betterdate :: Date -> Maybe Date
betterdate (Date m d y) if (testdate = True) //I know this part doesn't work
     then Just (Date m d y) 
     else Nothing

How would I do this?

Answer 1

You want to pass your date to the testdate function. This should work:

betterdate d = if testdate d
    then Just d
    else Nothing

If you really want to unpack the Date in the parameter list then you can do this to avoid duplicating (Date m d y) everwhere:

betterdate date@(Date m d y) = if testdate date
    then Just date
    else Nothing