Singletons/Error.hs:(0,0)-(0,0): Splicing declarations
    singletons
      [d| head :: [a] -> a
          head (a : _) = a
          head [] = error "head: empty list" |]
  ======>
    head :: [a] -> a
    head (a : _) = a
    head [] = error "head: empty list"
    type HeadSym0 :: (~>) [a] a
    data HeadSym0 :: (~>) [a] a
      where
        HeadSym0KindInference :: SameKind (Apply HeadSym0 arg) (HeadSym1 arg) =>
                                 HeadSym0 a0123456789876543210
    type instance Apply HeadSym0 a0123456789876543210 = Head a0123456789876543210
    instance SuppressUnusedWarnings HeadSym0 where
      suppressUnusedWarnings = snd ((,) HeadSym0KindInference ())
    type HeadSym1 :: [a] -> a
    type family HeadSym1 (a0123456789876543210 :: [a]) :: a where
      HeadSym1 a0123456789876543210 = Head a0123456789876543210
    type Head :: [a] -> a
    type family Head (a :: [a]) :: a where
      Head ('(:) a _) = a
      Head '[] = Apply ErrorSym0 "head: empty list"
    sHead ::
      (forall (t :: [a]). Sing t -> Sing (Apply HeadSym0 t :: a) :: Type)
    sHead (SCons (sA :: Sing a) _) = sA
    sHead SNil = sError (sing :: Sing "head: empty list")
    instance SingI (HeadSym0 :: (~>) [a] a) where
      sing = singFun1 @HeadSym0 sHead