[Haskell-beginners] Re: list of integers to list of nats

Christian Maeder Christian.Maeder at dfki.de
Sat Feb 20 10:21:55 EST 2010

kane96 at gmx.de schrieb:
> Hi,
> I have to write a function which maps a list of integers to a list of the corresponding nats. The following code is already there:
> data Nat = Z | S Nat deriving (Eq,Ord,Show)
> instance Enum Nat where
>     toEnum i | i < 0        = error "Enum_Nat.toEnum: Negative integer"
>              | i == 0       = Z
>              | otherwise    = S (toEnum (i-1))

this function has type "Int -> Nat" and should be wrapped into a
function of type "Int -> Maybe Nat" that does not produce a runtime
error if called with negative input.

> the function should be: mapIntsToNats :: [Int] -> Maybe [Nat]
> so for example: [2,0,1,3] should make: [S (S Z), Z, S Z, S (S (S Z))]
> how can I do that?

Have you heard about monads and mapM? (The type constructor Maybe is an
instance of the class Monad.)

Cheers Christian

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