Singletons/Error.hs:(0,0)-(0,0): Splicing declarations
    singletons
      [d| head :: [a] -> a
          head (a : _) = a
          head [] = error "Data.Singletons.List.head: empty list" |]
  ======>
    head :: [a] -> a
    head (a GHC.Types.: _) = a
    head GHC.Types.[] = error "Data.Singletons.List.head: empty list"
    type HeadSym1 (t :: [a0123456789876543210]) = Head t
    instance SuppressUnusedWarnings HeadSym0 where
      suppressUnusedWarnings
        = snd ((GHC.Tuple.(,) HeadSym0KindInference) GHC.Tuple.())
    data HeadSym0 (l :: TyFun [a0123456789876543210] a0123456789876543210)
      = forall arg. SameKind (Apply HeadSym0 arg) (HeadSym1 arg) =>
        HeadSym0KindInference
    type instance Apply HeadSym0 l = Head l
    type family Head (a :: [a]) :: a where
      Head ((:) a _) = a
      Head '[] = Apply ErrorSym0 "Data.Singletons.List.head: empty list"
    sHead :: forall (t :: [a]). Sing t -> Sing (Apply HeadSym0 t :: a)
    sHead (SCons (sA :: Sing a) _) = sA
    sHead SNil
      = sError (sing :: Sing "Data.Singletons.List.head: empty list")