Ruby Underscore

As a Ruby programmer, you probably have seen methods or blocks look like this:

def test(a, b, _); end
proc { |_, a| ... }

But what exactly is this _ thingy, and how is it special?

Underscore is a valid object identifier

Yes, you can access, and assign values to _ and you are also able to use it as a method name.

def a(_); puts _; end # a(3) prints 3

Yet it does not work exactly the way you might expect in IRB, because it’s explicitly configured to return the last used value in the Ruby console

_ = 1
2
_ # returns 2, not 1

Not double underscore __ tho:

__ = 1
2
__ # returns 1

Underscore is treated a bit differently than other object identifiers

You could have (>= Ruby 1.9)

def test(_, _); end
def test(__, __); end
def test(__x, __x); end

however, you cannot have

def test(x, x); end # this will cause an error

Ruby explicitly allows duplicate parameter names when they’re made of underscore _.

It is most common for people to use _ to represent unused parameters (placeholders), yet as the examples above showed, you can use other symbols to avoid “unused var” warnings too 🙂