Double splat and default value in method profile

Question

While a method with a default value profile can accept nil (besides a hash):

def f(options = {})
  options
end

f(hoge: "AAA", foo: "BBB") #=> {:hoge=>"AAA", :foo=>"BBB"}
f(nil) #=> nil

A method with double splat raises an error with nil:

def f(**options)
  options
end

f(hoge: "AAA", foo: "BBB") #=> {:hoge=>"AAA", :foo=>"BBB"}
f(nil) # => wrong number of arguments (1 for 0) (ArgumentError)

When should I use double splat and when should I use = {}?

I think this might be a duplicate of [What does a double * (splat) operator do](http://stackoverflow.com/questions/18289152/what-does-a-double-splat-operator-do) — Linus Oleander, Nov 19 '15 at 23:54
[ruby - What does a double * (splat) operator do - Stack Overflow](http://stackoverflow.com/questions/18289152/what-does-a-double-splat-operator-do) not say error. — shingo.nakanishi, Nov 20 '15 at 00:13
It explains what the `**` operator does. Passing `nil` raises an error as there are no named arguments thus giving the error *wrong number of arguments*. — Linus Oleander, Nov 20 '15 at 00:16
[This article - Ruby and double splat operator](http://blog.simplificator.com/2015/03/20/ruby-and-the-double-splat-operator/_) explains the scenario you have mentioned - error is reported for non-hash inputs to double splat operator. It also explains the typical use of double splat. — Wand Maker, Nov 20 '15 at 12:12

Wand Maker · Accepted Answer · 2015-11-20T12:26:07.583

If the input to the method MUST be options hash, then, use double splat operator **.

Using options = {} only declares the default value to be empty hash, however, it does not necessarily guarantee that caller will pass hash - she may pass non-hash values and nil.

If the function was implemented using double splat (**) - as evident in examples you have provided - then non-hash and nil values will not be accepted and will be reported as error.

Double splat and default value in method profile

1 Answers1