Define function that accepts only a specific data constructor on a type

gistfile1.hs

{-# LANGUAGE GADTs #-}

data Nil
data Cons

data StrongList x tag where
  Nil :: StrongList a Nil
  Cons :: a -> StrongList a b -> StrongList a Cons

safeHead :: StrongList x Cons -> x
safeHead (Cons a b) = a

{-
   ghci> safeHead (Cons 1 (Cons 2 Nil))
   1
   ghci> safeHead Nil
   <interactive>:51:10:
       Couldn't match expected type `Cons' with actual type `Nil'
       Expected type: StrongList x0 Cons
         Actual type: StrongList x0 Nil
       In the first argument of `safeHead', namely `Nil'
       In the expression: safeHead Nil
-}