[Haskell-cafe] (no subject)

[Antoine Latter]

On Mon, Nov 28, 2011 at 4:12 PM, Willem Obbens <dubl-u at hotmail.com> 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