When do you need to pass arguments to `Thread.new`?

Question

Local variables defined outside of a thread seem to be visible from inside so that the following two uses of Thread.new seem to be the same:

a = :fo         


        

Answer 1

When you pass a variable into a thread like that, then the thread makes a local copy of the variable and uses it, so modifications to it do not affect the variable outside of the thread you passed in

a = "foo"
Thread.new{ a = "new"}
p a # => "new"
Thread.new(a){|d| d = "old"} 
p a # => "new"
p d # => undefined