Responds to method AND is not null check in Ruby?

Question

I'm working with a large codebase where I see the following code a lot:

if foo.respond_to?(:bar)
  unless foo.bar.nil?
    # do stuff
  end
end

Is there a way to do that in one line?

[Final Answer]

if foo.respond_to?(:bar) and not foo.bar.nil?
  # do stuff
end

Other than the trivial case of using `&&` to combine the conditionals? — Max, Dec 30 '13 at 21:45
For some reason it seems like a code smell to me if you see that *a lot*. Wouldn't it be better if all `foo`s had some implementation of `bar`, even if it possibly does nothing? Or maybe use the visitor pattern if "do stuff" changes. — millimoose, Dec 30 '13 at 21:55
@millimoose lol. Yeah. Lots of things would be better if was used in the legacy code base. That's why its called "legacy". :) — bitcycle, Dec 30 '13 at 21:57
@bitcycle Fair enough. My reasoning was: if you're already going to change all that code, it might be feasible to actually do it right. (Or it might not, no idea what other constraints you're operating under.) — millimoose, Dec 30 '13 at 22:00

score 1 · Accepted Answer · answered Dec 30 '13 at 21:45

1

One line or one method call? unless is simply a logically inverted if so you can put it on one line like this...

if foo.respond_to?(:bar) && !foo.bar.nil?
  # do stuff
end

Otherwise you can create a method on foo or some sort of helper that makes the checks in a single call.

answered Dec 30 '13 at 21:45

dee-see

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`foo.respond_to?(:bar)` come first.. Otherwise there is a chance to get error as **no method** – Arup Rakshit Dec 30 '13 at 21:46
Awesome. I was wondering if it would _have_ to evaluate both before determining the result of the if-condition. – bitcycle Dec 30 '13 at 21:51
@bitcycle With `&&`, the second expression won't get evaluated if the first one is false. – dee-see Dec 30 '13 at 21:52
Does it not also work that way if I use the word "and" instead of "&&" ? – bitcycle Dec 30 '13 at 21:52
It's *almost* the same. See this other SO question http://stackoverflow.com/questions/1426826/difference-between-and-and-in-ruby – dee-see Dec 30 '13 at 21:53

score 1 · Answer 2 · answered Dec 30 '13 at 21:59

If you are using ActiveSupport >= 4.0, Object#try will return nil if the object does not respond to the method. As mentioned in the Rails 4 release notes.

Object#try will now return nil instead of raise a NoMethodError if the receiving object does not implement the method, but you can still get the old behavior by using the new Object#try!.

So, you could do what you want with unless foo.try(:bar). Maybe not useful in this case if you're not using ActiveSupport, but others who find this page might be interested to know this.

Responds to method AND is not null check in Ruby?

2 Answers2