How to constrain type family to Show - haskell

Question

I'm currently working on an interpreter, which should be able to handle multiple variations of a language.
Therefore I'm designing my AST with type families (a simple example is given below).
How do I tell GHC that my type families need to have Show (and Eq) instances?
I have tried to use StandaloneDeriving, but can't find out how to define the dependency/ constraint.
-- Types.hs
{-# LANGUAGE TypeFamilies #-}
module Types where
data Statement v = CommonStatement (CommonStatement v)
| VariantStatement (VariantStatement v)
deriving (Show)
data CommonStatement v = Skip deriving (Show)
data family VariantStatement v
-- Coroutine.hs
{-# LANGUAGE TypeFamilies #-}
module Coroutine (module Coroutine, module Types) where
import Types
newtype Coroutine = Coroutine [Statement Coroutine]
data instance VariantStatement Coroutine = SomeStatement deriving (Show)
When trying to build this (with stack and resolver lts-16.16), it fails since it can't deduce an instance of Show for (VariantStatement v):
• No instance for (Show (VariantStatement v))
arising from the first field of ‘VariantStatement’
(type ‘VariantStatement v’)
Possible fix:
use a standalone 'deriving instance' declaration,
so you can specify the instance context yourself
• When deriving the instance for (Show (Statement v))
|
7 | deriving (Show)
| ^^^^

Answer 1

This is basically just a matter of chasing down and squashing the errors:
{-# LANGUAGE TypeFamilies, StandaloneDeriving, UndecidableInstances #-}
main= print SomeStatement
data Statement v
= CommonStatement (CommonStatement v)
| VariantStatement (VariantStatement v)
-- StandaloneDeriving and UndecidableInstances
-- are required for this one.
deriving instance Show (VariantStatement v) => Show (Statement v)
data CommonStatement v = Skip deriving (Show)
data family VariantStatement v
newtype Coroutine = Coroutine [Statement Coroutine]
data instance VariantStatement Coroutine = SomeStatement deriving Show