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

kane96 at gmx.de kane96 at gmx.de
Sun Feb 21 09:57:09 EST 2010

```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 at gmx.de
> An: jonas.duregard at gmail.com
> CC: beginners at 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 --------
> > Datum: Sat, 20 Feb 2010 13:19:32 +0100
> > Von: "Jonas Almström Duregård" <jonas.duregard at gmail.com>
> > An: kane96 at gmx.de
> > Betreff: Re: [Haskell-beginners] list of integers to list of nats
>
> > 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
> >
> >
> > On 20 February 2010 12:39,  <kane96 at gmx.de> wrote:
> > > 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))
> > >
> > > 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?
> > >
> > >
> > >
> > > --
> > > NEU: Mit GMX DSL über 1000,- ¿ sparen!
> > > http://portal.gmx.net/de/go/dsl02
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> > > Beginners at haskell.org
> > > http://www.haskell.org/mailman/listinfo/beginners
> > >
>
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