Your problem seems to be related to the Num class, and the fact that the compiler can't figure out what '5' is -- see pp 223/224 of Simon Thompson's book. The code below offers two possible resolutions, one commented: [[ class Show a => Visible a where toString :: a -> String size :: a -> Int toString x = show x size x = 1 {- instance Visible Int where toString x = show x size _ = 1 -} instance Num a => Visible a where toString x = show x size _ = 1 {- t1 - toString (5::Int) -} t1 = toString 5 ]] #g -- At 23:58 27/06/04 -0400, kasager wrote:
Hi, in my last posting i asked for help with exercise 12.5 however, i forgot to give my code... here is my code and the error messege that hugs gives when i try to run it (it compiles fine, however when i test it, it gives an error)...
Code:
class Show a => Visible a where toString :: a -> String size :: a -> Int
toString x = show x size x = 1
instance Visible Int where toString x = show x size _ = 1
-- i have tried writing this without the show function, and the same error messege still is given... i also have other instances in the class, which work fine and should not effect this issue...
Error:
Prelude> :l "C:\\WorkArea\\Haskell\\homework\\ch12_5.hs" Main> toString 5 ERROR - Unresolved overloading *** Type : (Num a, Visible a) => [Char] *** Expression : toString 5
If you can help in any way or give any suggestions its greatly appreciated... once again thanks much.
------------ Graham Klyne For email: http://www.ninebynine.org/#Contact