.? (existence) operator in ruby?

Question

Is there a .? operator in Ruby that checks if an object is nil before calling a method on it?

For instance if I have to code:

if person and person.neighbor and person.neighbor.house and person.neighbor.house.rooms
    person.neighbor.house.rooms.each do |room|
        blah
    end
end

Is there a better way than having to do an if check on everything?

And please don't say "code in such a way that these objects cannot ever be nil", I am getting these things from an API call and can't control it.

The [Law of Demeter](http://en.wikipedia.org/wiki/Law_of_Demeter) may be interesting to you, as your code violates it heavily. — Andrew Marshall, Mar 12 '12 at 00:05
possible duplicate of [Is there an equivalent null prevention on chained attributes of groovy in ruby?](http://stackoverflow.com/questions/8805582/is-there-an-equivalent-null-prevention-on-chained-attributes-of-groovy-in-ruby) — Andrew Marshall, Mar 12 '12 at 00:06
@AndrewMarshal, I definitely agree. However I can't think of a good way to follow LoD in my case. I am basically making an API call to a 3rd party authentication platform (ie facebook twitter etc) and getting back a huge nested object of information. Within this, I only want to grab 1 piece of info (specifically, the user's education history), which is deeply nested. Unless I write 5 different methods each of which only makes a call to its descendent, I am forced to violate LoD. Maybe there's a better way? — Razor Storm, Mar 12 '12 at 00:12

score 6 · Answer 1 · answered Jan 13 '16 at 06:42

As of Ruby 2.3.0, the &. "safe navigation" operator does exactly this. So instead of

person and person.neighbor and person.neighbor.house and person.neighbor.house.rooms

you can now just do

person&.neighbor&.house&.rooms.

http://mitrev.net/ruby/2015/11/13/the-operator-in-ruby/

score 3 · Accepted Answer · edited May 23 '17 at 11:54

3

Easiest thing to do is use the andand gem, although there are some other, pretty trivially-implemented options, like this, or this, or etc.

edited May 23 '17 at 11:54

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answered Mar 11 '12 at 23:59

Dave Newton

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score 1 · Answer 3 · edited May 23 '17 at 12:17

1

The method you are looking for is nil?

Example:

foo.nil?

For a more elegant approaches to programming defensive against nil values, see my question on the same topic. For example you could write:

person.neighbor.house.rooms.each do |room|
    blah
end
rescue false

edited May 23 '17 at 12:17

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answered Mar 11 '12 at 23:55

wintersolutions

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You don't want to rescue arbitrary exceptions, though. – Dave Newton Mar 12 '12 at 00:01
@DaveNewton: If theres something like a nil exception please feel free to edit my answer – wintersolutions Mar 12 '12 at 00:04

score 1 · Answer 4 · answered Mar 12 '12 at 00:06

For second case, checking for does the object have a list of methods or not, you may use try method

require "active_support/core_ext/object/try"
person.try(:neighbor).try(:house).try(:rooms).each do |room|
   blah
end

Or my version of try

class Object
  def try(*args, &block)
    if args.empty? and block_given?
      begin
        instance_eval &block
      rescue NameError => e
        puts e.message + ' ' + e.backtrace.first
      end
    elsif respond_to?(args.first)
      send(*args, &block)
    end
  end
end

Then you can do it in short way:

person.try{neighbor.house.rooms}.each do |room|
   blah
end

score 1 · Answer 5 · answered Mar 12 '12 at 00:32

why not

begin
    person.neighbor.house.rooms.each do |room|
        ...
    end
rescue NameError
    ...
end

Exception handlers are made for handle exception :) So if you depend on someone else code, you should not prevent but handle your code for crash, not to check if everything is ok.

.? (existence) operator in ruby?

5 Answers5