At 10:14 PM +0200 2006/8/3, aldirithms@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@cs.utexas.edu hrichrds@swbell.net http://www.cs.utexas.edu/users/ham/richards ------------------------------------------------------------------