The Billy Baldwin of Conditional Assignment

For those of you who don’t know, Billy Baldwin is the lesser-known and generally-less-useful little brother of famed actor Alec Baldwin.

In the world of Ruby’s conditional assignment operators, ||= is Alec Baldwin; charming and versatile. But not many people know about ||=’s little brother… the &&= operator.

Meet Billy

The ||= conditional assignment operator is popular in Ruby and for good reason. One common usage is for simple memoization. Ryan Bates outlined this technique in his very first RailsCast.

def current_user
  @current_user ||= User.find(session[:user_id])

The purpose here is to avoid re-finding the user every time the current\_user method is called. The ||= operator above has the same effect as:

def current_user
  @current_user = @current_user || User.find(session[:user_id])

If @current\_user is both defined and truthy (not false or nil), that value is simply returned. Otherwise the instance variable is set and returned.

Enough about Alec. What about Billy?

The magic of the &&= operator is that it only sets variables that are… already set! The usefulness of this may not be immediately apparent, but maybe you’ve written code like this:

class Article < ActiveRecord::Base
  validates :title, :body, :presence => true

  before_save :clean_summary

    def clean_summary
      self.summary = summary.squish

The problem with this code is that the article summary isn’t required. What if an article’s summary is nil? We’ll get a NoMethodError trying to call squish[1] on nil. Oftentimes, the fix looks like this:

def clean_summary
  self.summary = summary && summary.squish

which looks eerily similar to our current\_user memoization expansion. So instead, try:

def clean_summary
  self.summary &&= summary.squish

Sure, it’s a bit of a one-trick pony, but sometimes that one trick is exactly what you need. Have you ever seen Backdraft Every dog has its day.

[1] String#squish changes consecutive whitespace to single spaces.


Post a Comment

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  1. Tiny nitpick:   = has the same effect as:
        @value   @value = ….
    This avoids assigning the variable when its already set and a double lookup if you use   = when accessing a Hash member.

    Otherwise: thanks for the reminder!
    Florian Gilcher
    June 21, 2011 at 19:11 PM
  2. Florian: Absolutely right, thank you! I’d originally written the current_user expansion just like that but ultimately wanted to keep the effect clearer for the sake of comparison to &&=.

    June 21, 2011 at 19:28 PM
  3. First, hilarious title / comparison

    Second, I wasn’t aware of this, thanks for sharing!

    June 21, 2011 at 19:48 PM
  4. Just make sure you don’t use   = and &&= for boolean values like flags. For that reason, I prefer to explicitly check for .nil?, even though it’s more verbose.
    June 21, 2011 at 19:54 PM
  5. You can also use the xor one for : a ^= true # true a ^= true # false a ^= true # true a ^= true # false
    June 21, 2011 at 20:42 PM
  6. It’s sad to me that more rubyists don’t realize what terrific tools &&= and   = are. 

    &&= chaining is actually almost like a limited “Maybe Monad” for Ruby. It’s really great when you’re dealing with a long string of computations that may fail, you can just write them in a really naive fashion and avoid writing if(x != nil) guards. 

    June 21, 2011 at 22:08 PM
  7. I think this syntax is more clear:

    self.summary = summary.squish if summary.present?

    But I hadn’t seen the squish method, very cool!

    June 21, 2011 at 22:15 PM
  8. You can also use && in a similar way to mimic #try:

    name = user &&

    June 21, 2011 at 23:18 PM
  9. In rails you can just do:

        self.summary = summary.try(:squish)

    But yes, quite good :)
    June 22, 2011 at 0:23 AM
  10. Fun! How about this? array_of_methods.inject(obj, :try)
    June 22, 2011 at 7:40 AM
  11. In Objective-C just go ahead and call summary.squish (actually [summary squish]).  If summary is nil, it will do nothing.  No need to pre-test if (summary != nil) or have null pointer exception handling.  Surprising how useful this behavior actually is and how quickly you accept it.
    March 22, 2013 at 1:36 AM
  12. Heya pretty much started programming in the ruby programming language so I am quite a bit of a newcommer! However, have found your internet site very interesting and instructive. Appreciate it!

    April 07, 2014 at 18:35 PM
  13. Nice!… I never knew that the operator   could also be used this way as well. Thanks for the info…
    March 28, 2015 at 13:54 PM
  14. Thanks for share such a useful information with us. Keep sharing such kinds of posts. I really like reading this one.

    May 07, 2015 at 7:26 AM