Yes, thank you. Here's my simple fix: 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' [z] = show z show' (x:xs) = show x ++ ", " ++ show' xs show' [] = [] in "(" ++ show' ys ++ ")" And I added this function: createPoint :: Int -> PointcreatePoint x = Point x When I loaded the file containing all this into ghci and executed 'Vector $ map createPoint [1..5]' the result was '(1, 2, 3, 4, 5)' (without the quotes).This was actually more or less a test question as I'm new to haskell-cafe, but I hope people who will read this message will learn from my mistake. Thank you.

From: aslatter@gmail.com Date: Mon, 28 Nov 2011 16:20:54 -0600 Subject: Re: [Haskell-cafe] (no subject) To: dubl-u@hotmail.com CC: haskell-cafe@haskell.org

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 ++ ")"

Here you're treating the value 'ys' as if its type was 'Vector', but its type is '[Point]'.

Does that help?

Antoine