Named Parameters in Ruby Structs

Question

I'm pretty new to Ruby so apologies if this is an obvious question.

I'd like to use named parameters when instantiating a Struct, i.e. be able to specify which items in the Struct get what values, and default the rest to nil.

For example I want to do:

Movie = Struct.new :title, :length, :rating m = Movie.new :title => 'Some Movie', :rating => 'R' 

This doesn't work.

So I came up with the following:

class MyStruct < Struct   # Override the initialize to handle hashes of named parameters   def initialize *args     if (args.length == 1 and args.first.instance_of? Hash) then       args.first.each_pair do |k, v|         if members.include? k then           self[k] = v         end       end     else       super *args     end   end end  Movie = MyStruct.new :title, :length, :rating m = Movie.new :title => 'Some Movie', :rating => 'R' 

This seems to work just fine, but I'm not sure if there's a better way of doing this, or if I'm doing something pretty insane. If anyone can validate/rip apart this approach, I'd be most grateful.

UPDATE

I ran this initially in 1.9.2 and it works fine; however having tried it in other versions of Ruby (thank you rvm), it works/doesn't work as follows:

• 1.8.7: Not working
• 1.9.1: Working
• 1.9.2: Working
• JRuby (set to run as 1.9.2): not working

JRuby is a problem for me, as I'd like to keep it compatible with that for deployment purposes.

YET ANOTHER UPDATE

In this ever-increasing rambling question, I experimented with the various versions of Ruby and discovered that Structs in 1.9.x store their members as symbols, but in 1.8.7 and JRuby, they are stored as strings, so I updated the code to be the following (taking in the suggestions already kindly given):

class MyStruct < Struct   # Override the initialize to handle hashes of named parameters   def initialize *args     return super unless (args.length == 1 and args.first.instance_of? Hash)     args.first.each_pair do |k, v|       self[k] = v if members.map {|x| x.intern}.include? k     end   end end  Movie = MyStruct.new :title, :length, :rating m = Movie.new :title => 'Some Movie', :rating => 'R' 

This now appears to work for all the flavours of Ruby that I've tried.

Answer 1

Synthesizing the existing answers reveals a much simpler option for Ruby 2.0+:

class KeywordStruct < Struct   def initialize(**kwargs)     super(*members.map{|k| kwargs[k] })   end end 

Usage is identical to the existing Struct, where any argument not given will default to nil:

Pet = KeywordStruct.new(:animal, :name) Pet.new(animal: "Horse", name: "Bucephalus") # => #<struct Pet animal="Horse", name="Bucephalus">   Pet.new(name: "Bob") # => #<struct Pet animal=nil, name="Bob">  

If you want to require the arguments like Ruby 2.1+'s required kwargs, it's a very small change:

class RequiredKeywordStruct < Struct   def initialize(**kwargs)     super(*members.map{|k| kwargs.fetch(k) })   end end 

At that point, overriding initialize to give certain kwargs default values is also doable:

Pet = RequiredKeywordStruct.new(:animal, :name) do   def initialize(animal: "Cat", **args)     super(**args.merge(animal: animal))   end end  Pet.new(name: "Bob") # => #<struct Pet animal="Cat", name="Bob">