At 10:14 PM +0200 2006/8/3, [EMAIL PROTECTED] 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
[EMAIL PROTECTED] [EMAIL PROTECTED]
http://www.cs.utexas.edu/users/ham/richards
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