Haskell: Creating an instance of Show

Question

In the code below, I define an algebraic data type and I (attempt to) make it an instance of Show. However, I'm getting a compile-time error (included below). What am I doing wrong?

I believe I'm using the correct syntax (atleast as per this post). For context, I'm working Problem #13 in '99 Haskell Problems'

data RepeatType a = Multiple (Int, a) | Single a

instance Show RepeatType where
  show (Multiple (n,x)) = "(" ++ n ++ " " ++ show x ++ ")"
  show (Single x)       = show x

I'm getting the following compile-time error:

test.hs:3:15:
    Expecting one more argument to `RepeatType'
    In the instance declaration for `Show RepeatType'
Failed, modules loaded: none.

For example, the goal is for it to work as follows in GHCi:

ghci> Multiple (5,'C')
(5 C)
ghci> Single 'D'
D

EDIT: Sorry for the totally irrelevant post title - changed now.

Answer 1

Your problem is that RepeatType by itself is not really a type, it's a type constructor. Show can only be instantiated for "proper" types, e.g. RepeatType a. However, for this to work you need to promise that a itself will be an instance of Show, so you will end up with something like this:

instance (Show a) => Show (RepeatType a) where
  show (Multiple (n,x)) = "(" ++ show n ++ " " ++ show x ++ ")"
  show (Single x)       = show x

(Note: you will also need to call show on n in the first part of the definition, since you cannot concatenate Ints and Strings.)