Re: [Haskell-cafe] How to define Show [MyType] ?

7 Dec 2008

      Daniel, thanks!
I used your advice, it works, yet some more questions, if you don't mind :)
So:
{--
Why showList returns a function of type:
type ShowS = String -> String
In fact showList returns a function which already contains all values
from list converted to a single string.
Why return function instead of a ready to use string?
--}
data ShipInfo = Ship {
      name :: String,
      kind :: String,
      cannons :: Int
}

s1 = Ship {name ="HMS Fly", kind = "sloop", cannons=16}
s2 = Ship {name ="HMS Surprise", kind = "frigate", cannons=42}

fleet = [s1,s2]

instance Show ShipInfo where
    show x = "Ship "++ show (name x) ++" is a " ++ show(kind x) ++ "
with " ++ show (cannons x) ++ " cannons "
    showList xs = showString (foldr (\x y -> x ++ " <*> "++y) [] (map show xs))

{--
Now when I type 'fleet' I get:

*ShowFleet> fleet
Ship "HMS Fly" is a "sloop" with 16 cannons  <*> Ship "HMS Surprise"
is a "frigate" with 42 cannons  <*>
*ShowFleet>

On the other hand evaluating  t2 :
--}

t2 xs = foldr (\x y -> x ++ " <*> "++y) [] (map show xs)

{--
provides:

*ShowFleet> t2 fleet
"Ship \"HMS Fly\" is a \"sloop\" with 16 cannons  <*> Ship \"HMS Surprise\" is a
 \"frigate\" with 42 cannons  <*> "
*ShowFleet>

More questions:
1)Where characters \" come from in this case?
2)How showList function 'removes' characters \" from the output?
What magic goes behind the scenes here?
--}

Thanks!
Dmitri

On 12/6/08, Daniel Fischer <daniel.is.fischer@web.de> wrote:
...
Am Samstag, 6. Dezember 2008 00:13 schrieb Dmitri O.Kondratiev:
...
Thanks everybody for your help!
I tried to implement showList, but get the same error:
{--
-- from Prelude:
type *ShowS* =
String<file:///C:/usr/ghc-6.6.1/doc/html/libraries/base/Prelude.html#t%3ASt
ring>->
String<file:///C:/usr/ghc-6.6.1/doc/html/libraries/base/Prelude.html#t%3ASt
ring> *showList* :: [a] ->
ShowS<file:///C:/usr/ghc-6.6.1/doc/html/libraries/base/Prelude.html#t%3ASho
wS> --}
myShows::ShowS
myShows s = s ++ "\n"
data ShipInfo = Ship {
      name :: String,
      kind :: String,
      canons :: Int
} deriving Show
No, if you derive the Show instance for ShipInfo, it automatically
implements
the standard showList, too. You have to do it yourself:
data ShipInfo = Ship {
  name :: String,
  kind :: String,
  canons :: Int
      }
instance Show ShipInfo where
  showsPrec p (Ship name kind canons) = ...
  showList xs = showString (unlines $ map show xs)
or whatever you want for showList.
However, somebody said it before, the Show instance should not be used for
pretty-printing.
...
-- 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!
-- 
Haste makes waste - Lose not a moment!
Dmitri O. Kondratiev
dokondr@gmail.com
http://www.geocities.com/dkondr