Ruby Code explained

Question

Could somebody please explain this piece of Ruby code:

def add_spec_path_to(args) # :nodoc:
  args << {} unless Hash === args.last
  args.last[:spec_path] ||= caller(0)[2]
end

I have seen the << operator used for concatenation strings or as a bitwise operator in other languages but could somebody explain it in this context. Is it somehow appending a blank lamda onto args or am I totally wrong?

I can also see it used like this:

before_parts(*args) << block

Is the Hash a keyword?

I am also unsure of what the ||= operator is saying.

I am equally in the dark as to what caller(0)[2] is.

Answer 1

I'm presuming that args is an Array.

Hash is the name of a class - the first line pushes an empty hash {} onto args unless the last element of args is already a Hash (the === operator for classes tests whether an object is of a certain class).

The ||= operator is similar to the += operator: it's more or less equivalent to:

args.last[:spec_path] = args.last[:spec_path] || caller(0)[2]

So, it will set args.last[:spec_path] if and only if it is currently unset.

The caller method returns info about the calling method.