Re: [Haskell-cafe] (no subject)

On Mon, Nov 28, 2011 at 04:20:54PM -0600, Antoine Latter wrote:

On Mon, Nov 28, 2011 at 4:12 PM, Willem Obbens wrote:

...

Hello, I get this error when I try to derive an instance of the Show typeclass: Abc.hs:21:60:     Couldn't match expected type `Vector' with actual type `[Point]'     In the first argument of `show'', namely `xs'     In the second argument of `(++)', namely `show' xs'     In the second argument of `(++)', namely `", " ++ show' xs' Failed, modules loaded: none. Here's the faulty code: newtype Point = Point Int instance Show Point where    show (Point a) = [chr $ a + 48]

data Vector = Vector [Point] instance Show Vector where    show (Vector ys) =       let show' (Vector [z])     = show z           show' (Vector (x:xs))  = show x ++ ", " ++ show' xs           show' (Vector [])      = []       in  "(" ++ show' ys ++ ")"

You've made show' :: Vector -> String, but I'm guessing you actually want to make it show' :: [Point] -> String; i.e. get rid of the Vector constructors in the show' patterns. -Brent