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

Duplicate of Ruby - print the variable name and then its value — Andrew Grimm
– Andrew Grimm, Commented Aug 11, 2010 at 23:26
This is a duplicate of Is there a way to return a method parameter names in ruby. — Jörg W Mittag
– Jörg W Mittag, Commented Aug 21, 2011 at 10:18

sepp2k · Accepted Answer · 2012-10-11 07:49:04Z

3

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.to_s}"
    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.

edited Oct 11, 2012 at 7:49

answered Aug 11, 2010 at 9:11

sepp2k

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4 Comments

Mads Mobæk Over a year ago

Pure evil is not intended and not what I want. Just curious if there was a way to do this. I will test your suggestions and get back to you

Mads Mobæk Over a year ago

fields = {:first_name => first_name, :last_name => last_name} is what I want. Thank you.

Javid Jamae Over a year ago

I tried with Ruby 1.9.3, and you have to do #{eval var.to_s} to get it to work, otherwise you get a TypeError: can't convert Symbol into String

sepp2k Over a year ago

@JavidJamae Yes, they've changed local_variables from an array of strings to an array of symbols in 1.9.

Daniel O'Hara · Accepted Answer · 2010-08-11 09:10:56Z

0

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

def fill_specific_form *args
    #Process args
end

answered Aug 11, 2010 at 9:10

Daniel O'Hara

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1 Comment

Mads Mobæk Over a year ago

Not quite. This only gets the values passed. By using explicit parameter names matching the name of a form element, I could then set name = value. See sepp2k's answer

Jörg W Mittag · Accepted Answer · 2010-08-11 15:32:28Z

0

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.

answered Aug 11, 2010 at 15:32

Jörg W Mittag

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2 Comments

sepp2k Over a year ago

Is this new in 1.9.2? This doesn't work for me in 1.8.7 or 1.9.1.

Jörg W Mittag Over a year ago

@sepp2k: Yes, Proc#parameters is new in 1.9.2.

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