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:
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.
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">