Long Parameter List

Ruby supports positional method arguments which can lead to Long Parameter Lists.

Symptoms

• You can’t easily change the method’s arguments.
• The method has three or more arguments.
• The method is complex due to number of collaborating parameters.
• The method requires large amounts of setup during isolated testing.

Example

Look at this mailer for an example of long parameter list.

# app/mailers/mailer.rb
class Mailer < ActionMailer::Base
  default from: "from@example.com"

  def completion_notification(first_name, last_name, email)
    @first_name = first_name
    @last_name = last_name

    mail(
      to: email,
      subject: 'Thank you for completing the survey'
    )
  end
end

Solutions

• Introduce parameter object and pass it in as an object of naturally grouped attributes.

• Extract class if the method is complex due to the number of collaborators.

Anti-Solution

A common technique used to mask a long parameter list is grouping parameters using a hash of named parameters; this will replace connascence of position with connascence of name (a good first step). However, it will not reduce the number of collaborators in the method.