Ruby class with static method calling a private method?

First off, static is not really part of the Ruby jargon.

Let’s take a simple example:

class Bar
  def self.foo
  end
end

It defines the method foo on an explicit object, self, which in that scope returns the containing class Bar.
Yes, it can be defined a class method, but static does not really make sense in Ruby.

Then private would not work, because defining a method on an explicit object (e.g. def self.foo) bypasses the access qualifiers and makes the method public.

What you can do, is to use the class << self syntax to open the metaclass of the containing class, and define the methods there as instance methods:

class Foo
  class << self

    def bar
      do_calc
    end

    def baz
      do_calc
    end

    private

    def do_calc
      puts "calculating..."
    end
  end
end

This will give you what you need:

Foo.bar
calculating...

Foo.baz
calculating...

Foo.do_calc
NoMethodError: private method `do_calc' called for Foo:Class