**args as function parameter in Ruby

Question

I understand that **args is interpreted as a hash containing all key value pairs passed to a function but I don't understand why that would be preferred over a typical parameter. For example, I have the following two functions.

def test(some_string, hash)
    puts hash
    puts hash.class # => Hash
end


def test_two(some_string, **hash)
    puts hash
    puts hash.class # => Hash
end 

calling test("test string", a: 1, b: 2) or test_two("test string", a: 1, b: 2) produces the exact same result. What is the benefit of using ** as a parameter value?

Answer 1

Ruby 2.7 started more clearly differentiating between keyword arguments and regular hashes. **args is for keyword arguments. Some implications:

def test3(some_string, foo:, **args)
  puts args
end

test3('a', foo: 'b', bar: 'c') # => {:bar=>"c"}

works as expected, however

def test3(some_string, foo:, hash)
  puts args
end # => syntax error

def test3(some_string, hash, foo:)
  puts args
end # works so far

test3('a', foo: 'b', bar: 'c')
# warning: Passing the keyword argument as the last hash parameter is deprecated
# ArgumentError (missing keyword: :foo)

Once you upgrade to ruby 3, the warnings turn to errors.

Answer 2

The benefit of having the double splat operator ** as argument is mainly that you can avoid passing any argument at all. Much like the same of what happens when you use the single splat operator *.

Using your examples and calling:

test "s"
# raises ArgumentError (wrong number of arguments (given 1, expected 2))

test_two "s"
# works, prints `{} Hash`

This is useful in methods where you want to have a "main" argument, and usually some options, without cluttering the arguments space. For example, imagine a CSV row parser:

def parse_row_1(row, **options)
  separator = options.fetch :separator, ","
  quote_char = options.fetch :quote_char, null
  # ...
end

def parse_row_2(row, separator = ",", quote_char = null)
  # ...
end

# To parse a string like this:
s = "'ABC','123','DEF'"

# With ** method you can do just this:
parse_row_1 s, quote_char: "'"

# Without ** you must specify every time the arguments, because they are positional:
parse_row_2 s, ",", "'"