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
||= 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
@current_user is both defined and truthy (not
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
squish 1 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.
Otherwise: thanks for the reminder!
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 &&=.
First, hilarious title / comparison
Second, I wasn’t aware of this, thanks for sharing!
You can also use the xor one for :
a ^= true # true
a ^= true # false
a ^= true # true
a ^= true # false
&&= 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.
I think this syntax is more clear:
self.summary = summary.squish if summary.present?
But I hadn’t seen the squish method, very cool!
You can also use && in a similar way to mimic #try:
name = user && user.name
In rails you can just do:
self.summary = summary.try(:squish)
But yes, quite good :)
Fun! How about this?
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.
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.
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