Parsec error

ex.hs:11:36:
    Couldn't match type ‘Text.Parsec.Prim.ParsecT s0 u0 m0 [Char]’
                  with ‘[Char]’
    Expected type: Text.Parsec.Prim.ParsecT
                     String () Data.Functor.Identity.Identity [Char]
      Actual type: Text.Parsec.Prim.ParsecT
                     String
                     ()
                     Data.Functor.Identity.Identity
                     (Text.Parsec.Prim.ParsecT s0 u0 m0 [Char])
    Relevant bindings include
      withPrefix :: forall u u1.
                    Text.Parsec.Prim.ParsecT
                      String
                      u
                      Data.Functor.Identity.Identity
                      (Text.Parsec.Prim.ParsecT s0 u1 m0 [Char])
        (bound at ex.hs:14:21)
    In the second argument of ‘(<|>)’, namely ‘withPrefix’
    In a stmt of a 'do' block: x <- noPrefix <|> withPrefix

ex.hs:17:54:
    No instance for (Text.Parsec.Prim.Stream s0 m0 Char)
      arising from a use of ‘digit’
    The type variables ‘s0’, ‘m0’ are ambiguous
    Relevant bindings include
      withPrefix :: Text.Parsec.Prim.ParsecT
                      String
                      u
                      Data.Functor.Identity.Identity
                      (Text.Parsec.Prim.ParsecT s0 u1 m0 [Char])
        (bound at ex.hs:14:21)
    Note: there are several potential instances:
      instance Monad m =>
               Text.Parsec.Prim.Stream Data.ByteString.Internal.ByteString m Char
        -- Defined in ‘Text.Parsec.Prim’
      instance Monad m =>
               Text.Parsec.Prim.Stream
                 Data.ByteString.Lazy.Internal.ByteString m Char
        -- Defined in ‘Text.Parsec.Prim’
      instance Monad m =>
               Text.Parsec.Prim.Stream Data.Text.Internal.Lazy.Text m Char
        -- Defined in ‘Text.Parsec.Prim’
      ...plus two others
    In the first argument of ‘many1’, namely ‘digit’
    In the expression: many1 digit
    In a case alternative: 'd' -> many1 digit

ex.hs:18:54:
    No instance for (Show b0) arising from a use of ‘show’
    The type variable ‘b0’ is ambiguous
    Note: there are several potential instances:
      instance Show a => Show (ZipList a)
        -- Defined in ‘Control.Applicative’
      instance Show Data.Text.Internal.Text -- Defined in ‘Data.Text’
      instance Show Data.Text.Internal.Lazy.Text
        -- Defined in ‘Data.Text.Lazy’
      ...plus 28 others
    In the first argument of ‘(.)’, namely ‘show’
    In the first argument of ‘fmap’, namely
      ‘(show . fst . head . readOct)’
    In the expression:
      fmap (show . fst . head . readOct) (many1 octDigit)

ex.hs:18:74:
    No instance for (Num b0) arising from a use of ‘readOct’
    The type variable ‘b0’ is ambiguous
    Note: there are several potential instances:
      instance Num Double -- Defined in ‘GHC.Float’
      instance Num Float -- Defined in ‘GHC.Float’
      instance Integral a => Num (GHC.Real.Ratio a)
        -- Defined in ‘GHC.Real’
      ...plus three others
    In the second argument of ‘(.)’, namely ‘readOct’
    In the second argument of ‘(.)’, namely ‘head . readOct’
    In the second argument of ‘(.)’, namely ‘fst . head . readOct’

ex.hs:19:54:
    No instance for (Show b1) arising from a use of ‘show’
    The type variable ‘b1’ is ambiguous
    Note: there are several potential instances:
      instance Show a => Show (ZipList a)
        -- Defined in ‘Control.Applicative’
      instance Show Data.Text.Internal.Text -- Defined in ‘Data.Text’
      instance Show Data.Text.Internal.Lazy.Text
        -- Defined in ‘Data.Text.Lazy’
      ...plus 28 others
    In the first argument of ‘(.)’, namely ‘show’
    In the first argument of ‘fmap’, namely
      ‘(show . fst . head . readHex)’
    In the expression:
      fmap (show . fst . head . readHex) (many1 hexDigit)

ex.hs:19:74:
    No instance for (Num b1) arising from a use of ‘readHex’
    The type variable ‘b1’ is ambiguous
    Note: there are several potential instances:
      instance Num Double -- Defined in ‘GHC.Float’
      instance Num Float -- Defined in ‘GHC.Float’
      instance Integral a => Num (GHC.Real.Ratio a)
        -- Defined in ‘GHC.Real’
      ...plus three others
    In the second argument of ‘(.)’, namely ‘readHex’
    In the second argument of ‘(.)’, namely ‘head . readHex’
    In the second argument of ‘(.)’, namely ‘fst . head . readHex’