[Confusion]

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.

[necubi]

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?

[Confusion]

Because when you write

  if !some_value
     do_something
  end

you often don't actually want it to be executed when some_value is nil instead of false. And when it is, you go scratching your head and searching where some_value came from, to discover some silly typo.

In some ways, the reverse is even worse: when you write

  if some_value
    do_something
  end

and you change some_value from nil to some sensible default like '', you will forget to update this clause and you will not understand why the code is being executed. If you use

  if !some_value.nil?
    do_something
  end

you don't have that problem, because the nil-test sticks out like a sore thumb.

[aphyr]

I disagree. I find it most natural to assume nil and false are negatively charged because that's how the language is defined. If I want to differentiate between nil and false, I use .nil? or == false.

Somehow this has never been a problem for me in eight years and hundreds of thousands of Ruby lines--but every programmer is different, and I'd like to hear other views.

change the base case of some_value from nil to 0, nil to NullObject

change some_value from nil to some sensible default like ''

What, what, what are you doing? Who uses an empty string to mean "uninitialized"? Nil and {0, "", etc) have very different semantic meanings. How would you discriminate between the default "" and a user-submitted ""? If you mean to check whether it has changed from the default, compare it to a default constant. If you mean to check emptiness, use #empty? or #blank?. I read these kinds of explicit .nil? checks as fighting the language, and in some ways, a violation of the cultural contract in Ruby and Lisp around truth charge.

[csallen]

I agree with aphyr's response to you. This has never been a problem for me. I think your issue is that you consider an empty string to be a sensible default representing false or nil.

[Confusion]

I fully agree that those sane defaults indeed aren't right for representing false or nil. What I'm arguing is rather the reverse: nil is often initially used as an (insane?) default and that is later changed without updating all related checks (because they don't stick out and demand attention, which makes them easy to overlook or 'overthink'), which causes bugs.

[perfunctory]

I actually agree with Confusion. All these implicit coercions _will_ bite you. Don't understand why his post is downvoted.

[anonymoushn]

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

[jbert]

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.

[schukin]

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.

[Confusion]

Well, the tri-state boolean was a worst-case, but consistent, example. I did mention you should hardly ever need that ;)

[riffraff]

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.

[Confusion]

See my response to necubi. The main problem is the reverse:

  if some_value
    something with some_value
  end

where you first use nil to signal false, but later change the base case of some_value from nil to 0, nil to NullObject or nil to some other base value. Since you think of these base cases as 'nothing', you will forget to change this related conditional the first time around, because it seems it would still do the right thing.