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 : _) = a
    head [] = error "Data.Singletons.List.head: empty list"
    type HeadSym1 (a0123456789876543210 :: [a0123456789876543210]) =
        Head a0123456789876543210
    instance SuppressUnusedWarnings HeadSym0 where
      suppressUnusedWarnings = snd (((,) HeadSym0KindInference) ())
    data HeadSym0 :: forall a0123456789876543210.
                     (~>) [a0123456789876543210] a0123456789876543210
      where
        HeadSym0KindInference :: forall a0123456789876543210
                                        arg. SameKind (Apply HeadSym0 arg) (HeadSym1 arg) =>
                                 HeadSym0 a0123456789876543210
    type instance Apply HeadSym0 a0123456789876543210 = Head a0123456789876543210
    type family Head (a :: [a]) :: a where
      Head ( '(:) a _) = a
      Head '[] = Apply ErrorSym0 "Data.Singletons.List.head: empty list"
    sHead ::
      forall a (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")
    instance SingI (HeadSym0 :: (~>) [a] a) where
      sing = (singFun1 @HeadSym0) sHead