Wrong type for the `pParens` parser combinator?

ParensType.hs

{-# LANGUAGE UnicodeSyntax #-}

-- From        : Swiestra (2009). Combinator Parsing: A Short Tutorial
-- Tested with : GHC 7.8.4

------------------------------------------------------------------------------

infix  7 <$>
infixl 7 <$
infixl 5 <*>, <*

newtype Parser s t =  P ([s] → [(t, [s])])

-- unP ∷ Parser s t → [s] → [(t, [s])]
-- unP (P p) = p

pSym ∷ Eq s ⇒ s → Parser s s
pSym a = P (\inp → case inp of
                     (s : ss) | s == a    → [(s, ss)]
                              | otherwise → []

                     _                    → []
           )

pReturn ∷ a → Parser s a
pReturn a = P (\inp → [(a, inp)])

(<*>) ∷ Parser s (b → a) → Parser s b → Parser s a
P p1 <*> P p2 = P (\inp → [ (v1 v2, ss2) | (v1, ss1) ← p1 inp
                                         , (v2, ss2) ← p2 ss1
                          ]
                  )

(<*) ∷ Parser s a → Parser s b → Parser s a
p <* q = const <$> p <*> q

(<$>) ∷ (b → a) → Parser s b → Parser s a
f <$> p = pReturn f <*> p

(<$) ∷ a → Parser s b → Parser s a
f <$ p = const <$> pReturn f <*> p

pParens ∷ Parser s a → Parser s a
pParens p = id <$ pSym '(' <*> p <* pSym ')'

-- Error:
-- /tmp/ParensType.hs:45:24:
--     Couldn't match type `s' with `Char'
--       `s' is a rigid type variable bound by
--           the type signature for pParens :: Parser s a -> Parser s a
--           at /tmp/ParensType.hs:45:1
--     In the first argument of `pSym', namely '('
--     In the second argument of `(<$)', namely pSym '('
--     In the first argument of `(<*>)', namely id <$ pSym '('