[Haskell-cafe] Trouble with record syntax and classes

26 Feb 2007

      I'm brand new to haskell and I'm having trouble using classes.  The basic idea 
is I want two classes, Sine and MetaSine, that are both instances of ISine. 
This way I can use the act method and recurse through the metasines and sines. 
Here's my code:

module Main
     where

class ISine a where
     period :: a -> Integer
     offset :: a -> Integer
     threshold :: a -> Integer
     act :: (ISine b) => Integer -> a -> b
     on :: Integer -> a -> Bool
     --on needs offset, period, threshold
     on time self = (mod (time-(offset self)) (period self)) < (threshold self)

data Sine =
     Sine {
         period :: Integer,
         offset :: Integer,
         threshold :: Integer,
         letter :: String
         }

instance Sine ISine where
     act time (Sine self)
         |on time self = [letter self]
         |otherwise = []

data MetaSine =
     MetaSine {
         period :: Integer,
         offset :: Integer,
         threshold :: Integer,
         sines :: (ISine a) => [a]
         }

instance MetaSine ISine where
     act time (MetaSine self)
         |on time self  = foldr (++) (map (act time) (sines self))
         |otherwise = []

The errors I get involve multiple declarations of period, offset, and 
threshold.

Any help would be greatly appreciated.
-Thomas

[Haskell-cafe] Trouble with record syntax and classes

Thomas Nelson