Reflection on method parameters in Ruby

Question

Take the following class:

class Automator
  def fill_specific_form(fields)
    fields.each_pair do |key, value|
      puts "Setting '#{key}' to '#{value}'"
    end
  end
end

a = Automator.new
a.fill_specific_form :first_name => "Mads", :last_name => "Mobæk"

# => Setting 'first_name' to 'Mads'
# => Setting 'last_name' to 'Mobæk'

Is it possible to do the same without a hash? Since all parameters are required, I want a method with the following signature:

fill_specific_form(first_name, last_name)

In my mind this would be possible by having the method body reflect and iterate over its parameters, thus achieving the same result.

How would you implement this? Does a pattern/idiom for this exist already? Two obvious benefits would be parameter information in IDEs and not having to check if all hash keys are supplied.

What I want to avoid is:

puts "Setting first_name to #{first_name}"
puts "Setting last_name to #{last_name}"
# and so on

Answer 1

I don't quite understand. Do you want to receive all parameters within a single array?

def fill_specific_form *args
    #Process args
end

Answer 2

If you set no other local variables inside the method, local_variables will give you a list of the method's parameter names (if you do set other variables you can just call local_variables first thing and remember the result). So you can do what you want with local_variables+eval:

class Automator
  def fill_specific_form(first_name, last_name)
    local_variables.each do |var|
      puts "Setting #{var} to #{eval var}"
    end
  end
end

Automator.new().fill_specific_form("Mads", "Mobaek")

Be however advised that this is pure evil.

And at least for your example

puts "Setting first_name to #{first_name}"
puts "Setting last_name to #{last_name}"

seems much more sensible.

You could also do fields = {:first_name => first_name, :last_name => last_name} at the beginning of the method and then go with your fields.each_pair code.

Answer 3

To reflect on a method's (or Proc's) parameters, you can use Proc#parameters, Method#parameters or UnboundMethod#parameters:

->(m1, o1=nil, *s, m2, &b){}.parameters
# => [[:req, :m1], [:opt, :o1], [:rest, :s], [:req, :m2], [:block, :b]]

However, in your case, I don't see why you need reflection, since you already know the names of the parameters anyway.