On Fri, Dec 5, 2008 at 1:29 AM, Martijn van Steenbergen <martijn@van.steenbergen.nl> wrote:

Dmitri O.Kondratiev wrote:

-- How to define Show [MyType] ?

Define instance Show MyType and implement not only show (for 1 value of MyType) but also showList, which Show provides as well. You can do all the magic in there.

HTH,

Martijn.

Thanks everybody for your help!
I tried to implement showList, but get the same error:

{--
-- from Prelude:
type ShowS = String -> String
showList :: [a] -> ShowS
--}

myShows::ShowS
myShows s = s ++ "\n"

data ShipInfo = Ship {
      name :: String,
      kind :: String,
      canons :: Int
} deriving Show

-- I get this error again:
instance (Show [ShipInfo]) where
    showList [] = myShows []
    showList (x:xs) = myShows "x" ++ showList xs

    Illegal instance declaration for `Show [ShipInfo]'
        (The instance type must be of form (T a b c)
         where T is not a synonym, and a,b,c are distinct type variables)
    In the instance declaration for `Show [ShipInfo]'
Failed, modules loaded: none.

-- What am I doing wrong?
Thanks!