Dynamically defining a Object#initialize for a Ruby class - ruby-on-rails

Question

In my code base, I have a bunch of objects that all adhere to the same interface, which consists of something like this:
class MyTestClass
def self.perform(foo, bar)
new(foo, bar).perform
end
def initialize(foo, bar)
#foo = foo
#bar = bar
end
def perform
# DO SOMETHING AND CHANGE THE WORLD
end
end
The differentiating factor between the classes is the arity of the self.perform and initialize, plus the body of the #perform class.
So, I'd like to be able to create an ActiveSupport::Concern (or just a regular Module if that would work better) which allowed me to do something like this:
class MyTestClass
inputs :foo, :bar
end
which would then use some meta-programming to define self.perform and initialize of the above methods whose airty would depend on the airty specified by the self.inputs method.
Here is what I have so far:
module Commandable
extend ActiveSupport::Concern
class_methods do
def inputs(*args)
#inputs = args
class_eval %(
class << self
def perform(#{args.join(',')})
new(#{args.join(',')}).perform
end
end
def initialize(#{args.join(',')})
args.each do |arg|
instance_variable_set(##{arg.to_s}) = arg.to_s
end
end
)
#inputs
end
end
end
This seems to get the arity of the methods correct, but I'm having a tough time figuring out how to handle the body of the #initialize methods.
Can anybody help me figure out a way that I can successfully meta-program the body of #initialize so it behaves like the example I provided?

Answer 1

You could use this as body for #initialize:
#{args}.each { |arg| instance_variable_set("#\#{arg}", arg) }
However, I wouldn't string eval it. It usually leads to evil things. That said, here is an implementation which gives an incorrect Foo.method(:perform).arity, but still behaves as you would expect:
module Commandable
def inputs(*arguments)
define_method(:initialize) do |*parameters|
unless arguments.size == parameters.size
raise ArgumentError, "wrong number of arguments (given #{parameters.size}, expected #{arguments.size})"
end
arguments.zip(parameters).each do |argument, parameter|
instance_variable_set("##{argument}", parameter)
end
end
define_singleton_method(:perform) do |*parameters|
unless arguments.size == parameters.size
raise ArgumentError, "wrong number of arguments (given #{parameters.size}, expected #{arguments.size})"
end
new(*parameters).perform
end
end
end
class Foo
extend Commandable
inputs :foo, :bar
def perform
[#foo, #bar]
end
end
Foo.perform 1, 2 # => [1, 2]

Answer 2

You were so close! instance_variable_set takes two arguments, first is the instance variable and second is the value you want to set it to. You also need to get the value of the variable, which you can do using send.
instance_variable_set(##{arg.to_s}, send(arg.to_s))