C# ?? operator in Ruby?

Is it possible to implement the ?? operator in Ruby?

a = nil
b = 1

x = a ?? b # x should == 1
x = b ?? 2 # x should == 1

回答1:

You're looking for conditional assignment:

a ||= b  # Assign if a isn't already set

and the || operator

a = b || 2 # Assign if b is assigned, or assign 2

回答2:

In Ruby, the short-circuiting Boolean operators (||, &&, and and or) do not return true or false, but rather the first operand that determines the outcome of the entire expression. This works, because Ruby has a rather simple idea of truth. Or rather, it has a rather simple idea of falsehood: nil is false, and obviously false is false. Everything else is true.

So, since || is true when at least one of its operands is true, and operands are evaluated from left to right, this means that a || b returns a, when a is true. But when a is false, then the outcome of the expression is solely dependent on b, and thus b is returned.

That means that, because nil is false, you can just use || instead of ?? for the examples that you gave. (There is also the nifty a ||= b operator, which kind of works like a || a = b, but not quite.)

However, that only works, because you don't use Booleans in your examples. If you expect to deal with Boolean values, that won't work:

b = false

x = b || 2 # x should be == false, but will be 2

In that case, you will have to use #nil?, and a conditional expression:

b = false

x = unless b.nil? then b else 2 end # x should be == 2

or using the ternary conditional operator:

b = false

x = b.nil? ? 2 : b # x should be == false

If you want to, you can wrap that up in a nice method:

class Object
  def _? b = nil
    return self
  end
end

class NilClass
  def _? b = nil
    return yield if block_given?
    return b
  end
end

b = false

x = b._? { 2 } # x should be == false
x = b._? 2 # x should be == false

This cute snippet brought to you by polymorphism, open classes and the fact that nil is actually an object representing nothingness (unlike, say, Java, where null is actually nothing).

回答3:

x = b || 2

It (?? in C#) is called the coalesce operator.

回答4:

There is the coalesce gem, which is as close as you'll get.

nil || 5 # => 5
false || 5 # => 5 :(
false._? 5 # => false :)