Having recently worked with a Ruby codebase where anything could be nil at anytime, I consider even references to nil to be a code smell. Developers are inherently prone to ignoring error handling, so they'll happily ignore the fact that Order#customer really means Order#customer_or_nil. And when they see errors in production, they'll slap an andand on it and call it fixed. The code becomes extremely difficult to reason about.

I prefer that functions throw an exception when unable to do what they promised, rather than return nil or a null object. The try-catch block serves as documentation that the function might fail. If someone forgets to catch the exception, it will shut everything down rather than leave the program in an unanticipated state that could lead to an error elsewhere.

I'm not sure I like it, but this works

    def nil.method_missing(*args, &block); end

So now you can call nil.whatever.you.like and get nil. It's also still falsy, but now every nil value in your app silently works and doesn't throw an error.

It's both impressive and scary that Ruby lets you (or that new contractor you're not sure about) change the fundamentals of the language in one line of code.

I've said this in a comment at rosania.org, but I think it bears repeating here: this is why we need a #blank? or #null? protocol that user classes can participate in as part of core. #nil? and an inextensible FalseClass just aren't enough to do the sorts of thing that Avdi was trying to do in his original post. My tentative proposal is languishing here: http://redmine.ruby-lang.org/issues/5372

I guess that overriding the behavior of any built-in classes in Ruby is not a great idea, as most gems rely on that unmodified behavior.

Conceptually, nil != false. You shouldn't use 'nil' to mean 'false'. You should always test explicitly for nil-ness using #nil?. If you write

  if finished?
    do_something
  end

then finished? is not expected, and may not, return nil.

If you expect some_value to be able to be nil (which, for boolean values, is hardly ever), you should write

  if !some_value.nil? && some_value
    do_something
  end

In the end, these 'convenient' coercions that allow you to use any value as a boolean only come back to bite you, because unexpected stuff happens when stuff is unexpectedly nil. If it makes my coworkers cry that an object is nil and true at the same time, then they are at fault, not me.

Edit: and more importantly (I forgot to point this out explicitly) a non-nil value != true. That any string or integer is 'true' can lead to very hard-to-find bugs.

Isn't this a flaw in Ruby, though? It also means that you can't create a delegate/decorator/proxy object for a false object and have it be false as well, which goes against the general "everything is determined by sending messages" vibe Ruby has going on.

I know the reasoning (it's much faster to do math on object IDs than it is to call a method), but there are workarounds for this (e.g. only allowing frozen objects to be boolean-false, and having the object's truth-ness/false-ness represented by the return value of its #false? method at the time of freezing—thus allowing the interpreter to locate it in a semantically-meaningful part of object-ID space that can later be masked for in a TEST instruction.)

    if account = Account.create(params[:account])
      ...
    else
      complainAbout account.errors
    end

Such code is everywhere in the Ruby community and explicitly endorsed by many. I have never seen code like your second example in the wild, and it's far more complicated than the idiomatic way of writing it:

   if some_value
     do_something
   end

It's a core expectation of the language that !!nil_thing == false, so why violate it?

  if !some_value
     do_something
  end

  if some_value
    do_something
  end

  if !some_value.nil?
    do_something
  end

If the language ships with falsy nils, I don't think it's unreasonable to assume that nil values are also falsy.

By the same argument, nil != true.

This could lead to a tri-state ifs (with true/false/nil branches).

Or you can just consider that 2-branch 'if' runs the first branch if the test expr is true and the second 'otherwise'. (So that doesn't make nil false, it just makes it non-true).

And then your interpretation and everyone else co-exists and we can all code together.

You probably shouldn't use 'nil' to mean 'false', but since pretty much every compiler / interpreter out there will take the same branch when given a condition that's either nil or false, I'd say it's much safer to stick w/ what's existed for decades rather than accommodate some sort of tri-state boolean.

IME using any value as boolean never caused me any problem, neither in ruby nor in other languages where the same is possible.

What stuff should happen when something is unexpectedly nil if you tested it for truth-ishnes? Once you have written

    if not some_value
      something with some_value
    end

the only things you can do with `x` are things you can do with `false`, which are not many.

  if some_value
    something with some_value
  end