Thanks all. I think for my function, I only need to throw an error
message for the out of range index. But through this, I know more some
ways to deal with the polymorphic type.
On 6/22/06, Brian Hulley
Sara Kenedy wrote:
Hello all,
Now I am trying with the function of polymorphic type: This function returns the Nth element of list with type a. I try it as below.
getNthElem :: Int -> [a] -> Maybe a getNthElemt _ [] = Nothing getNthElem 0 _ = Nothing getNthElem n s
n > length s = Nothing otherwise = Just ((drop (n-1) (take n s))!!0)
getNthElem 2 ["a","b","c"] Just "b"
However, I do not satisfy with this function because I want to return the Nth element of type a, not (Maybe a). For example, I want this function: getNthElem :: Int -> [a] -> a
But, I do not know how to define the empty element of type a.
getNthElemt _ [] = ???? getNthElem 0 _ = ????
If you have some ideas about this, please give me some clues. Thanks a lot.
You might find it's always a lot easier to start counting from zero rather than 1, so that "a" is the 0th element, "b" is the 1st element etc. Just like a building with 2 floors has a ground floor and a first floor, and if you want to find what day of the week it is in 46 days from today you just use (today + 46) `mod` 7 instead of (((today - 1) + 46) `mod` 7) + 1
That aside, why not just throw an error when the function is called with an index that's out of range?
getNthElemt _ [] = error "getNthElemt"
Regards, Brian. -- Logic empowers us and Love gives us purpose. Yet still phantoms restless for eras long past, congealed in the present in unthought forms, strive mightily unseen to destroy us.