This is several questions wrapped in one. I saw the method "<<=" used recently. I was able to piece together what is does, but not anything else about it.
$ dec = 2
$ dec <<= 1
$ dec <<= 2
$ a = 2 * 2 ** 1
$ a = a * 2 ** 2
$ 4 <<= 1
=> SyntaxError: unexpected tOP_ASGN, expecting end-of-input
1 <<= 1
This is called an abbreviated assignment and is covered in section 188.8.131.52 Abbreviated Assignments of the ISO Ruby Language Specification; specifically it is an abbreviated variable assignment (§184.108.40.206.2).
An abbreviated variable assignment is evaluated (roughly) like this:
a ω= b
is the same as
a = a ω b
ω is an arbitrary non-equality Ruby operator (
Note: this simple translation applies only to abbreviated variable assignments. It does not apply to abbreviated indexing assignments (
a[b] ω= b) or abbreviated method assignments (
a.b ω= c).
- Where is there documentation for this?
While the ISO Ruby Language Specification covers only a small subset of the intersection of Ruby 1.8 and Ruby 1.9, abbreviated assignments are covered there.
- What class or module does this live in?
None. Assignments are a language feature, not a library feature.
- Why does is only work if you assign it to a variable first?
I am not sure what you mean. It works perfectly fine without assigning to a variable first:
class NilClass; def <<(*) :haha end end a <<= a #=> :haha