How do you set a boolean default value in Ruby?

Question

For other types of variables, I use ||=, but this doesn't work for booleans (x ||= true assigns x to true even if x was previously assigned to false).

I'd thought that this would work:

x = true unless defined?(x)

But it doesn't: it assigns x to nil for some reason. (An explanation here would be appreciated.)

I do know one method that works:

unless defined?(x)
  x = true
end

But it's rather verbose. Is there a more concise way to assign default values to boolean variables in Ruby?

Answer 1

You must have defined? first, else the parser reaches x = and then defines x (which makes it nil) before running the unless:

defined?(x) or x = true
x  #=> true

x = false
defined?(x) or x = true
x  #=> false

Doing a if/unless block (instead of post-if/unless one-liner) also works:

unless defined?(x)
  x = true
end
x  #=> true

x = false
unless defined?(x)
  x = true
end
x  #=> false

Answer 2

There are only two non-true values in Ruby: false and nil. All you need to do is differentiate between those. Until the new //= operator that does this automatically comes around, you're stuck with this:

if (x.nil?)
  x = true
end

Hopefully this can be abbreviated in future versions of ruby. 99% of the time you don't really care about the difference between the two non-true values, but that 1% of the time you do it becomes annoying to have to be so unusually verbose.

Remember that the defined? operator will always return "local-variable" for that condition because the variable x is "defined" as a local variable. Contrast with defined?(nope) and you'll get nil because that variable does not exist. Ruby is concerned with the variable or constant in question, not if that variable or constant has been defined with a value.