On Mon, Feb 26, 2007 at 01:22:57PM -0600, Thomas Nelson wrote:
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 The other way round: instance class type where
Your errors have nothing to do with classes. Just try module Main where data MetaSine = MetaSine {period :: Integer } data Sine = Sine { period :: Integer } main = print "test" which won't compile and show the same error Solution? 1) single module solution: a) using records: Use prefixes data MetaSine = MetaSine {msPeriod :: Integer } data Sine = Sine { sPeriod :: Integer } b) using the same accessor function "period" requires classes I think this is what you had in mind when introducing ISine Of course you can add this to a), too. data MetaSine = MetaSine Integer data Sine = Sine sPeriod class Period where period :: Integer instance Period Sine where period (Sine p) = p instance Period MetaSine where period (MetaSine p) = p 2) use for each datatype another module. Then you can define record name period more than once. But you have to access them using ModuleSine.period or ModuleMetaSine.period which is not what you want I think. HTH Marc