require "./parser.cr"

module Parcom
  # `FirstOf` is a `Parser` that accepts multiple parsers, and tries to parse
  # with all of them, in order. As soon as one of the parsers succeeds,
  # that parser's result is returned. If none of the parsers are successful,
  # the parsing fails.
  class FirstOf(T, V) < Parser(T, V)
    @p : Parser(T, V)

    # Accepts the parsers to use. Raises an `ArgumentError` if
    # no parsers are provided.
    def initialize(ps : Iterable(Parser(T, V)))
      ps_iter = ps.each
      p = ps_iter.next

      if p.is_a?(Iterator::Stop)
        raise ArgumentError.new("FirstOf requires atleast one parser, got none")
      end

      # Combine all the parsers into one by wrapping them with `Alt`.
      @p = p

      loop do
        p = ps_iter.next
        break if p.is_a?(Iterator::Stop)
        @p = @p | p
      end
    end

    # Tries to parse with each of the given parsers. Either returns the first
    # successful result or fails if no parsers succeed.
    def parse(tokens : Tokens(T)) : Result(T, V)
      @p.parse(tokens)
    rescue ex : ParserFail
      raise ParserFail.new("FirstOf: #{ex.message}")
    end
  end
end