Nil and boolean in Ruby

Question

Can someone explain the reasoning being this? Just spent 30 mins trying to figure out why my boolean method returned nil and found out that in Ruby:

2.2.1 :001 > nil && true
 => nil
2.2.1 :002 > nil && false
 => nil

Since nil is a falsey value, I would have expected the output of nil && true to be false. Also it seems to go against the idea that conditional operators should return a boolean value.

What is the rationale behind this?

It makes sense that the boolean operator is not commutative:

nil && false != false && nil

For others seeing this, my issue was that in rails I had a statement like:

def some_method?
  object.attr && object.attr > something
end

But when object.attr is nil, the function will be nil. Which is fine in most cases but when chaining boolean methods together, not so much. I just changed it to this instead:

def some_method?
  object.attr.present? && object.attr > something
end

I could do the same thing in vanilla Ruby with:

def some_method?
  !!object.attr && object.attr > something
end

Answer 1

The statement goes through the conditions in order, will stop when a falsy result is obtained and return the value of the last evaluation performed.

In contrary to && which stops at a falsy value, || will stop at a truthy value instead.