Why keyword arguments must be passed as hash with symbol keys, not string keys in Ruby?

Question

We cannot pass keyword arguments as hash with string keys, keyword arguments works only with hash as symbol keys.

A simple example:

def my_method(first_name:, last_name: )
  puts "first_name: #{first_name} | last_name: #{last_name}"
end

my_method( {last_name: 'Sehrawat', first_name: 'Manoj'}) 
#=> first_name: Manoj | last_name: Sehrawat

my_method( {first_name: 'Bob', last_name: 'Marley'})
#=> first_name: Bob | last_name: Marley

my_method( {'first_name' => 'Kumar', 'last_name' => 'Manoj'})
#=> Error: missing keywords: first_name, last_name (ArgumentError)

What is the reasoning behind it?

Answer 1

The short version would be because Matz says so - on this rubymine issue he comments

I am negative for the proposal. My opinion is that you should not (or no longer) use strings as keywords.

That actual issue is around something that happens as a consequence of this, but if Matz says no it's unlikely to happen. I don't know if he has further expounded on why he is against this.

Answer 2

The implementation of * and ** could be relevant:

def gather_arguments(*arguments, **keywords)
  puts "arguments: #{arguments.inspect}"
  puts " keywords: #{keywords.inspect}"
end

gather_arguments('foo' => 1, bar: 2, 'baz' => 3, qux: 4)

Output:

arguments: [{"foo"=>1, "baz"=>3}]
 keywords: {:bar=>2, :qux=>4}