On Sat, Oct 23, 2010 at 11:24:05PM -0400, Edward Turpin wrote:
I can't seem to get my head around types and typeclasses (as well as many other things in Haskell). Here's the following code I need help with.
In module 1: //////////////////////// Start of Code ////////////////////////////// data NewtonRow = NewtonRow { iteration :: Integer, xn :: Double, fxn :: Double, errEst :: Double }
instance Show NewtonRow where show row = show (iteration row) ++ "\t" ++ showEFloat (Just 8) (xn row) "\t" ++ showEFloat (Just 2) (fxn row) "\t" ++ showEFloat (Just 2) (errEst row) "\n"
newtype NewtonTable = NewtonTable { getNewtonTable :: [NewtonRow] }
instance Show NewtonTable where show table = "n\t" ++ "xn\t\t\t" ++ "fxn\t\t" ++ "xn - xn-1\n" ++ concatMap show (getNewtonTable table)
instance Monoid NewtonTable where mempty = NewtonTable [] table1 `mappend` table2 = NewtonTable $ (getNewtonTable table1) ++ (getNewtonTable table2)
cons :: NewtonRow -> NewtonTable -> NewtonTable row `cons` table = NewtonTable (row : getNewtonTable table)
How about something like this? The idea is to make the Table type polymorphic, and to abstract the row-type-specific table header into a new type class. {-# LANGUAGE GeneralizedNewtypeDeriving #-} newtype Table a = Table { getTable :: [a] } deriving Monoid -- derive the Monoid instance from the -- underlying list instance cons :: a -> Table a -> Table a row `cons` (Table t) = Table (row : t) class HasTableHeader a where tableHeader :: a -> String instance TableHeader NewtonRow where tableHeader _ = "n\t" ++ "wn\t\t\t" ++ "fxn\t\t" ++ "xn - xn-1\n" instance HasTableHeader a => Show (Table a) where show (Table []) = "---" show (Table (r:rs)) = showHeader r ++ concatMap show (r:rs) I think you can see how to extend this to work with the other row type as well. -Brent