Why does ruby method with = at the end not allow more then one argument

Question

This is in a shared library, I have to make this backward compatible.

Original method

   def rrp_exc_sales_tax=(num)
      price_set(1, num, currency_code)
   end

Need to enhance and add currency_code

   def rrp_exc_sales_tax=(num, currency_code=nil)
      print "num=#{num}"
      print "currency_code=#{currency_code}"
      price_set(1, num, currency_code)
   end


some_class.rrp_exc_sales_tax=2, "USD"

num=[2, "USD"]
currency_code=

No value gets assigned to currency_code

Answer 1

If you want it to be backwards compatible, leverage on the powers of the array:

def rrp_exc_sales_tax=(arr)
  num, currency_code = arr
  price_set(1, num, currency_code)
end

some_class.rrp_exc_sales_tax=2, "USD"
# => num=2
# => currency_code="USD"

some_class.rrp_exc_sales_tax=2
# => num=2
# => currency_code=nil

Answer 2

Because it's meant to look like a simple assignment operation. If that operation needs to be parameterized, it makes more sense to make it look like a method call instead. Also, having multi-parameter assignment statements complicates the language grammar.