-- auxiliar para inserir uma string entre parêntesis
paren :: String -> String
paren s = "(" ++ s ++ ")"

-- show de expressoes regulares optimizado por precedências
showREPrec :: Int -> RegExp-> String
showREPrec n = cataRegExp ("@",
                           \a   -> [a],
                           \a b -> let s = (showREPrec 3 a) ++ "+" ++ (showREPrec 3 b)
                                    in if (n<=3) then s else paren s,
                           \a b -> let s = (showREPrec 4 a) ++ (showREPrec 4 b)
                                    in if (n<=4) then s else paren s,
                           \a   -> let s = (showREPrec 5 a) ++ "*"
                                    in if (n<=5) then s else paren s,
                           \a   -> let s = (showREPrec 6 a) ++ "?"
                                    in if (n<=6) then s else paren s,
                           \a   -> let s = (showREPrec 7 a) ++ "^"
                                    in if (n<=7) then s else paren s
                          )
-- instanciamento do tipo RegExp na classe Show através da showREPrec
instance Show RegExp where
 show = showREPrec 0