Haskell: Algebraic data types whose type variables must be an instance of a class

Question

I am trying to define an algebraic type:

data MyType t = MyType t

And make it an instance of Show:

instance Show (MyType t) where
  show (MyType x) = "MyType: " ++ (show x)

The GHC complains because it cannot deduce that the type 't' in 'Show (MyType t)' is actually an instance of Show, which is needed for (show x).

I have no idea where and how to declare 't' as an instance of Show?

Answer 1

Add a type constraint to the type t:

instance Show t => Show (MyType t) where
  show (MyType x) = "MyType: " ++ (show x)

Answer 2

You can also simply:

data MyType t = MyType t
    deriving Show

if you want to have a regular display format.

Answer 3

- GADT:

data MyType t  where 
   MyType :: Show a => a -> MyType a