[Haskell-cafe] Type Class Query

Tim Stitt timothy.stitt at uwimona.edu.jm
Mon Feb 2 12:32:25 EST 2004

I'm experimenting with type classes in Haskell and tried to implement a 
new class based on nullifying values of various types e.g.

nullify  6  = 0
nullify 7.6 = 0
nullify [1,2,3] = []
nullify Tree = Empty etc.

I set up my class as follows:

class Null a where

      nullify :: a -> a

instance Null Int where
         nullify x = 0

but get the following error when I invoke

nullify 6 on the Hugs command line:

ERROR - Unresolved overloading
*** Type       : (Num a, Null a) => a
*** Expression : nullify 6 

I then tried the following:

instance Num a => Null a where
         nullify x = 0

but got the following error in Hugs

ERROR "C:\zero.hs":5 - Syntax error in instance head (constructor expected)

Can anybody explain what I'm doing wrong ?

Thanks in advance,


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