typeclasses.hs

{-# LANGUAGE FlexibleInstances, OverlappingInstances #-}
import Data.List

toJson1 :: [String] -> String
toJson1 elems = toArray $ map quote elems
    where quote elem = '"' : elem ++ "\""
          toArray elems = '[' : concat (intersperse ", " elems) ++ "]"

sample1 = toJson1 ["a", "b", "c"]

toJson2 elems = toArray $ map show elems
    where toArray elems = '[' : concat (intersperse ", " elems) ++ "]"

data Conversation = Conversation { from :: Int, to :: Int, text :: String }
    deriving (Show)

-- instance Show Conversation where
--     show (Conversation from to text) =
--       concat [ "{\"from\":", show from, ",",
--                 "\"to\":", show to, ",",
--                 "\"text\":", show text, "}" ]

class EncodeJson a where
  toJson :: a -> String

instance EncodeJson String where
    toJson = show

instance EncodeJson Int where
    toJson = show

instance EncodeJson Conversation where
    toJson (Conversation from to text) =
      concat [ "{\"from\":", show from, ",",
                "\"to\":", show to, ",",
                "\"text\":", show text, "}" ]

toJson3 :: EncodeJson a => [a] -> String
toJson3 elems = toArray $ map toJson elems
    where toArray elems = '[' : concat (intersperse ", " elems) ++ "]"

instance EncodeJson a => EncodeJson [a] where
    toJson elems = toArray $ map toJson elems
        where toArray elems = '[' : concat (intersperse ", " elems) ++ "]"