require "./parser.cr"
require "./map.cr"

module Parcom
  # `Right` is a `Parser` that tries to parse with two different
  # parsers in succession and fails if either of the two parsers fails.
  # This parser behaves the same as `Plus`, but only returns the result
  # of the second parser.
  class Right(T, V, U) < Parser(T, U)
    @p : Map(T, {V, U}, U)

    # Accepts the two parsers to use, in order.
    def initialize(p1 : Parser(T, V), p2 : Parser(T, U)) 
      @p = (p1 + p2).map(&.last)
    end

    # Tries to parse with the two given parsers, and returns the
    # result of the second parser if they both succeed.
    def parse(tokens : Tokens(T)) : Result(T, U)
      @p.parse(tokens)
    rescue ex : ParserFail
      raise ParserFail.new("Left: #{ex.message}")
    end 
  end 
end