Re: [Haskell-beginners] Enum for natural numbers

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Datum: Mon, 21 Dec 2009 11:25:46 +0100 Von: Deniz Dogan An: kane96@gmx.de CC: beginners@haskell.org Betreff: Re: [Haskell-beginners] Enum for natural numbers

2009/12/21 :

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Now everything works but to print Z also for negative Integers. Don't know how to implement the <=0 or le0 in this case again:

instance Enum Nat where        toEnum 0 = Z        toEnum (n+1) = S(toEnum n)        fromEnum Z = 0        fromEnum (S n) = 1 + fromEnum n

Again, look into guards. You are definitely on the right track.

Again, you should look into guards.

instance Enum Nat where toEnum n | n <= 0 = Z | otherwise = ... -- do what with n? fromEnum Z = 0 fromEnum (S n) = 1 + fromEnum n

I did it that way: instance Enum Nat where toEnum n | n <= 0 = Z | otherwise = S(toEnum (n-1)) fromEnum Z = 0 fromEnum (S n) = 1 + fromEnum n In another task I should create an infinite list: allNats :: [Nat] I would have done it in a recursion but there issn't a parameter to call allNats with -- GRATIS für alle GMX-Mitglieder: Die maxdome Movie-FLAT! Jetzt freischalten unter http://portal.gmx.net/de/go/maxdome01