require "./parcom/*"

module Parcom
  VERSION = "0.2.0"

  # A ParserFail exception should be raised by `Parser#parse` when
  # a parse attempt is unsuccessful.
  class ParserFail < Exception
  end

  # Provides a more convenient syntax for combining parsers via `Parser#and_then`.
  # The first argument is a string literal used for the name of the parser.
  # The second and third arguments are types used for the parser's type.
  # These are followed by any number of 2-tuples containing a variable name and
  # an expression resolving to a `Parser(t.Class, _)`, whose success value will
  # be stored in the aformentioned variable. The `finally` named argument is an
  # expression that resolves to a `Parser(t.class, u.class)`.
  #
  # Example:
  # ```
  # any_word = Parser(Char, Char).satisfy(&.letter?).some.map(&.join)
  # ws = Parser(Char, Array(Char)).satisfy(&.whitespace?).many
  # two_of_same_word = parser_chain "two words", Char, String,
  #   {word, any_word},
  #   {_,    ws},
  #   finally: Parser.token_sequence(word.chars).map(&.join)
  #
  # tokens = Tokens.from_string("foo   foo")
  # result = two_of_same_word.parse(tokens)
  # result.value # => "foo"
  #
  # # The above definition of `two_of_same word`
  # # is an alternative way of doing this:
  # two_of_same_word = any_word.and_then do |word|
  #   ws.and_then do |_|
  #     Parser.token_sequence(word.chars).map(&.join)
  #   end
  # end.named("two words")
  # ```
  #
  # This macro is based on Haskell's do-notation.
  macro parser_chain(name, t, u, *steps, finally)
    Parser({{t}}, {{u}}).new({{name}}) do |tokens|
      {% for tup, index in steps %}
        {{tup.last}}.and_then do |{{tup.first}}|
      {% end %}
      {{finally}}
      {% for _, _ in steps %}
        end
      {% end %}
      .parse(tokens)
    end
  end
end