[Hugs-users] natural numbers

[Hamilton Richards]

At 10:14 PM +0200 2006/8/3, aldirithms at gmx.net wrote:
>Hello,
>
>thanks a lot for the expeditious replies to my trifling question. 
>Here is another one:
>
>Can you tell me why the following does not function? I mean the last 
>part for converting number into the new data type. If this is not a 
>quick fix, please ignore it. As an utter beginner, I cannot estimate 
>the significance of my questions.
>
>
>
>data Nat = Zero | Succ Nat
>
>natToInt Zero = 0
>natToInt (Succ n) = (natToInt n) + 1
>
>number n = case n of
>	0 -> Zero
>	_ -> Succ(number(n-1))
>
>

Most likely the problem you encountered looked something like

    Main> number 0
    ERROR - Cannot find "show" function for:
    *** Expression : number 0
    *** Of type    : Nat


The easy solution is to add

    deriving (Show)

to the definition of Nat.

Or, if you want to do it yourself,

    instance Show Nat where
       show Zero = "Zero"
       show (Succ n) = "(Succ " ++ show n ++ ")"

Either way, you're providing a conversion from Nat to String, which 
is necessary if Nat values are ever to be displayed.

Cheers,

--Ham



-- 
------------------------------------------------------------------
Hamilton Richards, PhD           Department of Computer Sciences
Senior Lecturer (retired)        The University of Texas at Austin
ham at cs.utexas.edu                hrichrds at swbell.net
http://www.cs.utexas.edu/users/ham/richards
------------------------------------------------------------------