Singletons/Error.hs:0:0: Splicing declarations
    singletons
      [d| head :: [a] -> a
          head (a : _) = a
          head [] = error "Data.Singletons.List.head: empty list" |]
  ======>
    Singletons/Error.hs:(0,0)-(0,0)
    head :: forall a. [a] -> a
    head (a GHC.Types.: _) = a
    head GHC.Types.[] = error "Data.Singletons.List.head: empty list"
    type HeadSym1 (t :: GHC.Types.[] a) = Head t
    instance SuppressUnusedWarnings HeadSym0 where
      suppressUnusedWarnings _
        = snd (GHC.Tuple.(,) HeadSym0KindInference GHC.Tuple.())
    data HeadSym0 (l :: TyFun (GHC.Types.[] a) a)
      = forall arg. KindOf (Apply HeadSym0 arg) ~ KindOf (HeadSym1 arg) =>
        HeadSym0KindInference
    type instance Apply HeadSym0 l = HeadSym1 l
    type family Head (a :: GHC.Types.[] a) :: a where
      Head ((GHC.Types.:) a z) = a
      Head GHC.Types.[] = Apply ErrorSym0 "Data.Singletons.List.head: empty list"
    sHead ::
      forall (t :: GHC.Types.[] a). Sing t -> Sing (Apply HeadSym0 t)
    sHead (SCons sA _)
      = let
          lambda ::
            forall a wild. t ~ Apply (Apply (:$) a) wild =>
            Sing a -> Sing (Apply HeadSym0 (Apply (Apply (:$) a) wild))
          lambda a = a
        in lambda sA
    sHead SNil
      = let
          lambda :: t ~ GHC.Types.[] => Sing (Apply HeadSym0 GHC.Types.[])
          lambda
            = applySing
                (singFun1 (Proxy :: Proxy ErrorSym0) sError)
                (sing :: Sing "Data.Singletons.List.head: empty list")
        in lambda