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]) end
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]) end
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
private
def clean_summary
self.summary = summary.squish
end
end
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 squish1 on nil. Oftentimes, the fix looks like this:
def clean_summary self.summary = summary && summary.squish end
which looks eerily similar to our current_user memoization expansion. So instead, try:
def clean_summary self.summary &&= summary.squish end
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.
11 Comments
Florian Gilcher June 21, 2011
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!
Steve Richert June 21, 2011 http://collectiveidea.com
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 &&=.
Dan June 21, 2011
First, hilarious title / comparison
Second, I wasn’t aware of this, thanks for sharing!
Dan
Luigi Montanez June 21, 2011 http://luigimontanez.com
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.
p8 June 21, 2011 http://deheus.net/petrik
You can also use the xor one for :
a ^= true # true
a ^= true # false
a ^= true # true
a ^= true # false
Dave Fayram June 21, 2011 http://kirindave.tumblr.com
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.
Brian Armstrong June 21, 2011 http://www.startbreakingfree.com
I think this syntax is more clear:
self.summary = summary.squish if summary.present?
But I hadn’t seen the squish method, very cool!
Mark Wilden June 21, 2011 http://mwilden.blogspot.com/
You can also use && in a similar way to mimic #try:
name = user && user.name
dude June 21, 2011
In rails you can just do:
self.summary = summary.try(:squish)
But yes, quite good :)
xn June 22, 2011
Fun! How about this?
array_of_methods.inject(obj, :try)
jarmo March 21, 2013
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.
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