Parsing a simple expression grammar with maybe quoted strings

Parser.ex

defmodule ParseHelpers do
  import NimbleParsec

  def whitespace() do
    ascii_char([?\s])
    |> repeat
  end

  def quote_mark(combinator \\ empty()) do
    combinator
    |> ignore(optional(whitespace()))
    |> ascii_char([?"])
  end

  def and_(combinator \\ empty()) do
    combinator
    |> ignore(optional(whitespace()))
    |> ascii_char([?&])
  end

  def or_(combinator \\ empty()) do
    combinator
    |> ignore(optional(whitespace()))
    |> ascii_char([?|])
  end

  def left_paren(combinator \\ empty()) do
    combinator
    |> ignore(optional(whitespace()))
    |> ascii_char([?(])
  end

  def right_paren(combinator \\ empty()) do
    combinator
    |> ignore(optional(whitespace()))
    |> ascii_char([?)])
  end

  def input_string(combinator \\ empty()) do
    combinator
    |> choice([
      quoted_string(),
      unquoted_string()
    ])
  end

  @syntax Enum.map([?", ?(, ?), ?&, ?|, ?\s], &{:not, &1})
  def unquoted_string(combinator \\ empty()) do
    combinator
    |> ignore(optional(whitespace()))
    |> repeat(ascii_char(@syntax))
    |> reduce({List, :to_string, []})
  end

  def quoted_string(combinator \\ empty()) do
    combinator
    |> ignore(optional(whitespace()))
    |> ignore(quote_mark())
    |> repeat(maybe_escaped_char())
    |> ignore(quote_mark())
    |> reduce({List, :to_string, []})
  end

  def maybe_escaped_char(combinator \\ empty()) do
    combinator
    |> choice([
      ignore(string("\\")) |> ascii_char([]),
      ascii_char([{:not, ?"}])
    ])
  end

  def to_postfix([left, ?&, right]) do
    [:and, left, right]
  end

  def to_postfix([left, ?|, right]) do
    [:or, left, right]
  end
end

defmodule ParserGrammar do
  import NimbleParsec
  import ParseHelpers

  # quotation_mark ::= ?"
  # left_bracket   ::= ?(
  # right_bracket  ::= ?)
  # or_            ::= ?|
  # and_           ::= ?&

  # Based upon knowledge @ https://www.youtube.com/watch?v=dDtZLm7HIJs#t=15m54s
  # expression     ::= term or_ expression | term
  # term           ::= factor and_ term | factor
  # factor         ::= left_bracket expression right_bracket | input_string

  defparsec :expression,
    choice([
      parsec(:term) |> or_() |> parsec(:expression) |> reduce(:to_postfix),
      parsec(:term)
    ])

  defparsec :term,
    ignore(optional(whitespace()))
    |> choice([
      parsec(:factor) |> and_() |> parsec(:term) |> reduce(:to_postfix),
      parsec(:factor)
    ])

  defparsec :factor,
    ignore(optional(whitespace()))
    |> choice([
      ignore(left_paren()) |> parsec(:expression) |> ignore(right_paren()) |> wrap(),
      input_string()
    ])

  defparsec :parse,
    parsec(:expression)
    |> ignore(optional(whitespace()))
    |> eos()

end

defmodule ParseTest do
  @inputs [
    "\"1 quote\"",
    "\"2 quote\" | \"quote\"",
    "\"3 quote\" & foo | \"quote\"",
    "\"4 quote\" & (foo | bar) & \"quote\"",
    "5 & (foo | (bar | \"qu( & )ux\" & batz))",
    "6&(foo|(bar|\"qu( & )ux\"&batz))",
    "7bar&\"qu( & )ux\"",
    "8 | foo & (\"b a a r\" & quux) | batz",
    "99 | ab & \"c d\"",
    "10aa & bb | cc",
    "11aa | bb & cc"
  ]

  def test do
    for string <- @inputs do
      {:ok, result, "", _, _, _} = ParserGrammar.parse(string)
      result
    end
  end
end

defmodule Evaluator do
  def evaluate(term) when is_binary(term) do
    inspect(term)
  end

  def evaluate([:or, left, right]) do
    "#{evaluate(left)} OR #{evaluate(right)}"
  end

  def evaluate([:and, left, right]) do
    "#{evaluate(left)} AND #{evaluate(right)}"
  end

  def evaluate([expression]) do
    "( #{evaluate(expression)} )"
  end
end