Think twice before using double splat in Ruby

As of Ruby 2.5.1, double splat operator (**options, for example) is still considerably slower than accepting a normal argument and sets a default to empty hash.

Here’s a simple benchmark:

require "benchmark/ips"

def option_hash(options = {}); end
def double_splat(**options); end

Benchmark.ips do |x|
  x.report("option_hash") { option_hash(foo: 1, bar: 2) }
  x.report("double_splat") { double_splat(foo: 1, bar: 2) }
end

… and result (bigger number is better):

Warming up --------------------------------------
         option_hash   123.232k i/100ms
        double_splat    61.146k i/100ms
Calculating -------------------------------------
         option_hash      1.798M (± 3.6%) i/s -      8.996M in   5.011461s
        double_splat    708.545k (± 5.4%) i/s -      3.546M in   5.020927s

What’s the optimal usage of double splat?

You should use double splat in the method definition only if that method defines optional arguments.

# This is a valid use case since otherwise Ruby will set a hash
# in argument name `optional`.
def valid_use_of_double_splat(optional = nil, **options)
# Both of these use case can just use `options = {}` 
def perform_delivery(recipient:, **options)
def associate_user(user, **options)