parser.rb

require 'strscan'

# Methods can be added here to expand functionality available to the REPL
module Methods
  def self.add(*nums)
    nums.reduce(&:+)
  end

  def self.multiply(*nums)
    nums.reduce(&:*)
  end
end

# This parser supports any number of methods and arguments (including zero)
class Parser
  def initialize(str)
    @scanner = StringScanner.new(str)
  end

  def parse
    read_arg
  end

  private

  # Expects "add 1 2 3 4)" or "add)" or ")"
  def read_expr
    method = read_token.strip
    args = []
    until @scanner.peek(1) == ')'
      arg = read_arg
      raise BadSyntaxError, pretty_current if arg == false

      args << arg
    end
    @scanner.skip_until(/\s/) # To continue to the next expression if it exists
    evaluate(method, *args)
  end

  def read_arg
    first_char = @scanner.peek(1)
    case first_char
    when '('
      @scanner.getch
      read_expr
    when ->(x) { numeric?(x) }
      num = read_token
      return false if num.blank?

      num.strip.to_i
    else
      false
    end
  end

  def read_token
    after_bracket = @scanner.check_until(/\s/)&.include?(')')
    (after_bracket ? @scanner.scan_until(/[A-Za-z]*(?=\))/) : @scanner.scan_until(/\s|.(?=\))/))
  end

  def evaluate(method, *args)
    return nil if method.blank? # "()" becomes a nil argument
    return Methods.public_send(method) unless args # To support methods w/out args

    Methods.public_send(method, *args)
  end

  def numeric?(str)
    Float(str) != nil rescue false
  end

  # Prints: (add 3 ()))))
  #                 ^
  def pretty_current
    "\n#{@scanner.string}\n#{' ' * @scanner.pos}^\n"
  end

  class BadSyntaxError < StandardError; end
end

# ===== Quick monkeypatches for cleaner code =====

class String
  # Remove trailing bracket along with spaces
  def strip
    stripped = lstrip.rstrip
    return stripped[0...-1] if stripped[-1] == ')'

    stripped
  end
end

class Object
  def blank?
    respond_to?(:empty?) ? !!empty? : !self
  end
end

# ================================================

puts Parser.new(ARGV[0]).parse