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

module Parcom
  # `Recover` is a `Parser` that tries to parse with another parser,
  # but does not fail the parser chain if the wrapped parser fails.
  # If the wrapped parser fails, a `Recover` will not modify the input
  # stream, and return an arbitrary value of type `V` instead.
  class Recover(T, V) < Parser(T, V)
    @p : Map(T, V?, V)

    # Accepts the parser to use, and the default value
    # to return if the parser fails.
    def initialize(p : Parser(T, V), default : V)
      @p = Optional.new(p).map { |x| x.nil? ? default : x }
    end

    # Tries to parse with the given parser, succeeds with the
    # default value instead of failing. This parser does not
    # consume input if the wrapped parser fails.
    def parse(tokens : Tokens(T)) : Result(T, V)
      @p.parse(tokens)
    end
  end
end