I tried: map toEnum [1,2,3,4]) :: [Nat] which works normally, but not for Maybe in: mapIntsToNats :: [Int] -> Maybe [Nat] -------- Original-Nachricht --------

Datum: Sat, 20 Feb 2010 19:18:57 +0100 Von: kane96@gmx.de An: jonas.duregard@gmail.com CC: beginners@haskell.org Betreff: Re: [Haskell-beginners] list of integers to list of nats

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.

-------- Original-Nachricht --------

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Datum: Sat, 20 Feb 2010 13:19:32 +0100 Von: "Jonas Almström Duregård" An: kane96@gmx.de Betreff: Re: [Haskell-beginners] list of integers to list of nats

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Homework?

Should mapIntsToNats [-1] be Nothing?

One way to to do this is to check if any integer in the list is negative (for instance using the any function) and if so return Nothing, otherwise Just map an Int-to-Nat function across the values in the list (there is already such a function in your code).

A slightly more elegant solution uses the fact that Maybe is a Monad, so the function

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

Hope this helps /Jonas Duregård

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Hi, I have to write a function which maps a list of integers to a list of

On 20 February 2010 12:39, wrote: the corresponding nats. The following code is already there:

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data Nat = Z | S Nat deriving (Eq,Ord,Show)

instance Enum Nat where    toEnum i | i < 0        = error "Enum_Nat.toEnum: Negative

integer"

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            | i == 0       = Z             | otherwise    = S (toEnum (i-1))

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?

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