Ruby's "foo = true if !defined? foo || foo.nil?" won't work

I thought in the following, foo should be true

$ irb

ruby-1.9.2-p0 > foo = true if !defined? foo || foo.nil?
 => nil 

ruby-1.9.2-p0 > foo
 =&g开发者_如何学Ct; nil 

because foo was at first not defined, but the foo = true part make it temporarily has a nil value, so the !defined didn't catch it, but the foo.nil? should catch it, and make it true... but why is it still nil?

this is related to Ruby's "foo = true if !defined? foo" won't work as expected

Be careful when skipping parenthesis. You meant:

foo = true if !defined?(foo) || foo.nil?

As per your other question, the defined?(foo) will always be true, so really you want to write:

foo = true if foo.nil?