[Ruby] retry a block until a condition is met

token.rb

module Token
  module_function

  MAX_RETRIES = 3

  TokenTakenError = Class.new(StandardError)

  def token_generator
    # SecureRandom.hex(32)
    ("a".."z").to_a.sample
  end

  def token_exists?(token_candidate)
    # ApiToken.where(token: token_candidate).exists?
    ("a".."m").to_a.include?(token_candidate)
  end

  # More DB-like behaviour (unique index constraint)
  def raising_token_exists?(token_candidate)
    fail TokenTakenError, "Token already taken!" if token_exists?(token_candidate)
  end

  # retry times loop
  def generate_token_with_times
    MAX_RETRIES.times do
      token_candidate = token_generator
      return !token_exists?(token_candidate) && token_candidate
    end
  end

  # begin while loop
  def generate_token_with_while
    begin
      token_candidate = token_generator
    end while token_exists?(token_candidate)
    token_candidate
  end

  # loop loop
  def generate_token_with_loop
    token = loop do
      token_candidate = token_generator
      break token_candidate unless token_exists?(token_candidate)
    end
  end

  # retry loop
  def generate_token_with_rescue_retry
    token_generator.tap do |token_candidate|
      raising_token_exists?(token_candidate)
    end
  rescue TokenTakenError
    retry if _retries = (_retries || 0) + 1 and _retries < MAX_RETRIES
    fail "Too many retries!"
  end

  # endless retry loop
  def generate_token_with_rescue_endless_retry
    token_generator.tap do |token_candidate|
      raising_token_exists?(token_candidate)
    end
  rescue TokenTakenError
    retry
  end
end

# calls
Token.generate_token_with_times
Token.generate_token_with_while
Token.generate_token_with_loop
Token.generate_token_with_rescue_retry
Token.generate_token_with_rescue_endless_retry