Ruby

First, foo(b: 1) has been exactly the same as foo({b: 1}) since Ruby 1.8 at least. It is parsed exactly the same way. It's syntax sugar.

require 'ripper' Ripper.sexp('foo(a : 1)') == Ripper.sexp('foo({a : 1})') # => true 

As you note, the ** operator is not needed in many cases, but there are cases where it matters. You can see a difference in these three cases:

a) It merges keyword arguments:

h = {a: 1, b: 2} p(h, c: 3) # => {:a=>1, :b=>2}, then {:c=>3} p(**h, c: 3) # {:a=>1, :b=>2, :c=>3} 

b) It insures that a hash is viewed as keyword arguments:

h = {'a' => 1} p(h) # {"a"=>1} p(**h) # => TypeError (hash key "a" is not a Symbol) 

c) It differentiates between an actual empty hash {} and nothing at all

def foo(x) p x end e = {} foo('hello', **e) # => 'hello' foo('hello', e) # => ArgumentError (wrong number of arguments (given 2, expected 1)) 

Note that some corner cases may not perfectly handled yet (#15078)

In summary: using ** improves legibility by making the intention crystal clear, makes your code stricter and allows you to easily merge options. There is also discussion to make the use of ** required in some cases in Ruby 3.0 (see #14183).

I'm closing this, but will reopen if need be.