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a[x] ||= some_value

In many books and articles on Ruby the form:

a <op>= b

Where <op> is an operator like + Is described as syntactic sugar:

a = a <op> b

While this is true in most cases, it isn’t’ true if the operator is ||.

This comes to light when the left hand side is a method call, to an accessor, or accessor-like method. For example

h = Hash.new("hello")
h[:fred] ||= ""
h #=> {}

Some find it surprising that the assignment doesn’t cause the hash to have :fred as a key. What this code snippet shows is that the assignment doesn’t actually assign anything if the left hand expression returns a logically true value. A Hash with a default value will return the default value when accessed by any key which is not present in the hash. Since h[:fred] returns the default value, the assignment doesn’t happen.

This affects any object which has ‘accessor’ methods. Here’s a class cooked up just to explore this aspect of Ruby.

class ChattyCathy

  def initialize(x=nil)
    @x = x
    puts "created x is now #{x.inspect}"
  end

  def x
    puts "x read x is #{@x.inspect}"
    @x
  end

  def x=(val)
    puts "x written, now #{val.inspect}"
    @x = val
  end
end

The purpose of this class is simply to let us see exactly when the x attribute is read and written. Now if we run this code

c = ChattyCathy.new(42)
puts "about to evaluate c.x ||= 43"
c.x ||= 43
c = ChattyCathy.new
puts "about to evaluate c.x ||= 43"
c.x ||= 43

We get the following output:

Which clearly illustrates just when the assignment actually happens.

The real expansion of x ||= y

Matz explains that the real expansion of x ||= y is:

x || x = y

The expectation that x ||= y is the same as x = x || y, does seem reasonable to someone ‘coming from’ C or one of it’s derivative languages. As far as I can determine, C introduced the notion of assignment operators like += and -=. And K&R defined these assignment operators as a shorthand for x = x + y, etc.

On the other hand, although C has logical operators || and && which, like Ruby have ‘short-circuit’ evaluation, it doesn’t allow ||=, or &&= as assignment operators.

Since || is a ‘short-circuit’ boolean operator, the left hand operand expression is only evaluated if the right hand operand expression evaluates to a logically false value, i.e. either nil or false.

The way that Matz included ||= as an assignment operator makes perfect sense to me. The ||= assignment operator reserves the short-circuit nature of ||.

So what about x &&= y

Although I haven’t seen this discussed anywhere, &&= in Ruby has similar behavior:

c = ChattyCathy.new
puts "about to evaluate c.x &&= true"
c.x &&= true
puts "about to evaluate c.x = \"hi\""
c.x = "Hi"
puts "about to evaluate c.x &&= true"
c.x &&= true

So the expansion of x &&= y is x && x = y