Question
What does the following code mean in Ruby?
||=
Does it have any meaning or reason for the syntax?
Answer
This question has been discussed so often on the Ruby mailing-lists and Ruby blogs that there are now even threads on the Ruby mailing-list whose only purpose is to collect links to all the other threads on the Ruby mailing- list that discuss this issue.
Here's one: [The definitive list of ||= (OR Equal) threads and pages](https://www.ruby-forum.com/t/the-definitive-list-of-or-equal-threads- and-pages/136446)
If you really want to know what is going on, take a look at Section 11.4.2.3 "Abbreviated assignments" of the Ruby Language Draft Specification.
As a first approximation,
a ||= b
is equivalent to
a || a = b
and not equivalent to
a = a || b
However, that is only a first approximation, especially if a is undefined.
The semantics also differ depending on whether it is a simple variable
assignment, a method assignment or an indexing assignment:
a ||= b
a.c ||= b
a[c] ||= b
are all treated differently.
