Example type class for a type-level list with no instance for an empty list.

no-empty.hs

{-# LANGUAGE DataKinds           #-}
{-# LANGUAGE FlexibleInstances   #-}
{-# LANGUAGE KindSignatures      #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeApplications    #-}
{-# LANGUAGE TypeOperators       #-}

import           Data.Kind (Type)

data ExampleData (as :: [Type]) = ExampleData

class ExampleClass (as :: [Type]) where
  example :: ExampleData as -> String

instance {-# OVERLAPPING #-} ExampleClass '[a] where
  example _ = "one"

instance ExampleClass as => ExampleClass (a ': as) where
  example _ = "two or more, " ++ example (ExampleData @as)

main = putStrLn $ example (ExampleData @'[Char, Int, Bool])