kane96@gmx.de schrieb:
the function should produce Nothing if there is a negative Int in the input list and otherwise Just with the list of the corresponding Nats
mapM looks like the right function for that, so I tried some examples that work like I need it. But in case of my exercise:
mapM toEnum [1,2,3,4] :: Nat
doesn't work.
Surely, it does not work, because the type of your first argument "toEnum" does not match the expected type "Int -> Maybe Nat", as pointed out below. Furthermore the overall result type is not "Nat" nor "[Nat]", but "Maybe [Nat]". Did you have higher order functions (like mapM or map) in your course? If not, you are supposed to program "mapIntsToNats" using explizit recursion over lists (and using case-distinctions aka pattern matching for Lists and the Maybe data type). Cheers Christian P.S. If you're looking for a ready solution, you're (most likely) wrong here. We only try to help you, to find a solution yourself.
mapM :: Monad m => (a -> m b) -> [a] -> m [b]
is also mapM :: (Int -> Maybe Nat) -> [Int] -> Maybe [Nat].
so mapIntsToNats = mapM f, for some function f :: Int -> Maybe Nat
You should be able to program such a function "f" (using "toEnum" or programming it from scratch similar to "toEnum").