Re: [Haskell-beginners] Enum for natural numbers

20 Dec 2009

      Maybe I didn't understand the exercise if have to do. It says:
"Write the instance Enum Nat where toEnum is defined as a total function that returns Z for negative integers. Some examples:
*Main> toEnum (-1) :: Nat
Z
*Main> toEnum 0 :: Nat
Z
*Main> toEnum 1 :: Nat
S Z
*Main> fromEnum (S (S Z))
2

so I did:
data Nat = Z | S Nat deriving (Eq,Ord,Show)
instance Enum Nat where
toEnum x|x > 0 = S Z
        |otherwise = Z

somehow it looks really wrong. Do you understand want I have to do and how it should look like?

-------- Original-Nachricht --------
...
Datum: Fri, 18 Dec 2009 18:59:40 +0100
Von: Deniz Dogan 
An: kane96@gmx.de
CC: beginners@haskell.org
Betreff: Re: [Haskell-beginners] Enum for natural numbers
...
2009/12/18  :
...
ahhh, ok, thanks so far. Here is how the output should looks like:
*Main> toEnum (-1) :: Nat
Z
*Main> toEnum 0 :: Nat
Z
*Main> toEnum 1 :: Nat
S Z
*Main> fromEnum (S (S Z))
2
the first three things work, but I don't know about the last one. Does
anybody how it should work and what's the sense of it?
Looks right to me. "Z" means 0 (Zero), "S x" means "the Successor of
x". So (S (S (S Z))) means "the successor of the successor of the
successor of zero", which is 3.
...
-------- Original-Nachricht --------
...
Datum: Fri, 18 Dec 2009 18:06:32 +0100
Von: Deniz Dogan 
An: kane96@gmx.de
CC: beginners@haskell.org
Betreff: Re: [Haskell-beginners] Enum for natural numbers
...
2009/12/17  :
...
Hi,
I have data Nat = Z | S Nat deriving (Eq,Ord,Show)
toEnum should return Z for negative numbers. I did something like
this
...
...
but the problem is I don't know how to set "less than". I tried > and
lt:
...
instance Enum Nat where
   toEnum (gt 0) = S Z
   toEnum (lt 1) = Z
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Fill in the gaps:
instance Enum Nat where
    toEnum x | x <= 0 = ...
             | otherwise = ...
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-- 
Deniz Dogan
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Re: [Haskell-beginners] Enum for natural numbers

kane96＠gmx.de