Normally, if you have a private method, you can’t call it with an explicit receiver, even if that receiver is self. So you can’t say

def foo
 self.bar  # explicit receiver
end

private
def bar
  123
end

Instead, foo needs to call simply bar, leaving the self implicit:

def foo
 bar  # implicit receiver
end

However, when you call setters, you always need an explicit receiver, or you’ll just assign a local variable:

def assign_things
  self.a = 123
  b = 456
end

def a=(v)
  puts "This one gets called."
end

def b=(v)
  raise "This one never does; the other method makes a local called `b` instead."
end

So, what do you do if you have a private setter? You call it with an explicit receiver:

def assign_things
  self.a = 123
end

private
def a=(v)
  puts "This is called successfully."
end

There’s a crazy special exception in Ruby that lets you use an explicit receiver of self with a setter just so that you can call private setters.

This strikes me as weird. Why can’t you call any private method explicitly on self? I thought it was just easier to implement Ruby if you couldn’t, but if they made it work for setters, I’m not sure what the big deal is.