Cannot print own data type in Haskell

Question

Please read my problem below, after the bold text, before taking your time to scrutinize this code. I don't want to waste your time if you can't answer this.

Okay. I have created my own data type within Haskell. It is

data Dialogue= Choice String [(String, Dialogue)] 
            | Action String Event
  -- deriving (Show)

Note the commented out 'deriving (Show)' which is important for my problem below.

I have a function named dialogue defined as

dialogue:: Game -> Dialogue -> IO Game
dialogue (Game n p ps) (Action s e) = do 
   putStrLn s
   return (e (Game n p ps))
dialogue (Game n p ps) (Choice s xs) = do
  putStrLn s
  let ys = [ fst a | a <- xs ]
  let i = [1..length ys]
  putStrLn (enumerate 1 ys)
  str <- getLine
  if str `elem` exitWords
  then do
     return (Game n p ps)
  else do
     let c = read str::Int
     if c `elem` i 
     then do 
        let ds = [ snd b | b <- xs ]
        let d = ds !! c
        putStrLn $ show d
        return (Game n p ps)
     else do
        error "error"

My data type game is defined as

data Game = Game Node Party [Party] | Won 
  deriving (Eq,Show)

And an Event is a type, defined by myself as

type Event = Game -> Game

Now, this is where my problem occurs. When I go to load this file within cmd and I have not included deriving (Show) within my data type Dialogue, I get the following error:

* No instance for (Show Dialogue) arising from a use of `show'
* In the second argument of `($)', namely `(show d)'
  In a stmt of a 'do' block: putStrLn $ (show d)
  In the expression:
    do let ds = ...
       let d = ds !! c
       putStrLn $ (show d)
       return (Game n p ps)
    |
120 |          putStrLn $ (show d)

It would seem to me that I need to include the deriving (Show) in order to be able to print this data type to the console. However, when I do include deriving (Show), I get this error:

* No instance for (Show Event)
    arising from the second field of `Action' (type `Event')
    (maybe you haven't applied a function to enough arguments?)
  Possible fix:
    use a standalone 'deriving instance' declaration,
      so you can specify the instance context yourself
* When deriving the instance for (Show Dialogue)
   |
85 |   deriving Show

I have spent quite a long time trying to find out why this might be happening. But I cannot find anywhere online that seems to document this particular problem.

Any help would be perfect, or even just a link to an appropriate explanation.

**Edit: ** My Event is a type synonym and so I cannot add deriving Show to this

Thanks a lot

Answer 1

Event as you have defined it is a function which has no sensible method to show. How would you like to display this information? One solution is to import Text.Show.Functions, which has an instance.

For example:

Prelude Text.Show.Functions> show (+ 1)
"<function>"

Another solution is to define your own show instance:

instance Show (a -> b) where
    show _ = "_"