Ruby: Automatically Set Instance Variable as Method Argument

Ruby: Automatically set instance variable as method argument?

After some pondering, I wondered if it's possible to actually get the argument names from a ruby method. If so, I could use a special argument prefix like "iv_" to indicate which args should be set as instance variables.

And it is possible: How to get argument names using reflection.

Yes! So I can maybe write a module to handle this for me. Then I got stuck because if I call the module's helper method, it doesn't know the values of the arguments because they're local to the caller. Ah, but ruby has Binding objects.

Here's the module (ruby 1.9 only):

module InstanceVarsFromArgsSlurper
  # arg_prefix must be a valid local variable name, and I strongly suggest
  # ending it with an underscore for readability of the slurped args.
  def self.enable_for(mod, arg_prefix)
    raise ArgumentError, "invalid prefix name" if arg_prefix =~ /[^a-z0-9_]/i
    mod.send(:include, self)
    mod.instance_variable_set(:@instance_vars_from_args_slurper_prefix, arg_prefix.to_s)
  end

  def slurp_args(binding)
    defined_prefix = self.class.instance_variable_get(:@instance_vars_from_args_slurper_prefix)
    method_name = caller[0][/`.*?'/][1..-2]
    param_names = method(method_name).parameters.map{|p| p.last.to_s }
    param_names.each do |pname|
      # starts with and longer than prefix
      if pname.start_with?(defined_prefix) and (pname <=> defined_prefix) == 1
        ivar_name = pname[defined_prefix.size .. -1]
        eval "@#{ivar_name} = #{pname}", binding
      end
    end
    nil
  end
end

And here's the usage:

class User
  InstanceVarsFromArgsSlurper.enable_for(self, 'iv_')

  def initialize(iv_name, age)
    slurp_args(binding)  # this line does all the heavy lifting
    p [:iv_name, iv_name]
    p [:age, age]
    p [:@name, @name]
    p [:@age, @age]
  end
end

user = User.new("Methuselah", 969)
p user

Output:

[:iv_name, "Methuselah"]
[:age, 969]
[:@name, "Methuselah"]
[:@age, nil]
#<User:0x00000101089448 @name="Methuselah">

It doesn't let you have an empty method body, but it is DRY. I'm sure it can be enhanced further by merely specifying which methods should have this behavior (implemented via alias_method), rather than calling slurp_args in each method - the specification would have to be after all the methods are defined though.

Note that the module and helper method name could probably be improved. I just used the first thing that came to mind.

in Ruby can I automatically populate instance variables somehow in the initialize method?

You can use instance_variable_set like this:

params.each do |key, value|
  self.instance_variable_set("@#{key}".to_sym, value)
end

Shortcut to assigning instance variables

Person = Struct.new(:name, :artist, :duration) do
  # more code to the Person class
end

Your other option is to pass a Hash/keyword of variables instead and use something like ActiveModel::Model
https://github.com/rails/rails/blob/master/activemodel/lib/active_model/model.rb#L78-L81

def initialize(params={})
  params.each do |attr, value|
    self.instance_variable_set("@#{attr}", value)
  end if params
end

ruby initialize method - purpose of initialize arguments

You can set an instance variable in any method in your class.

initialize is a method that is executed immediately after calling Person.new.

All external data for new object is passed through the arguments of .new(args).

Your line @age = age - it's the same that @age = nil.

This is due to the fact that age is absent in the arguments of initialize.

Also you have attr_accessor :age.

It's equal, that you have methods:

def age
  @age
end

def age=(age)
  @age = age
end

So you can set instance variable like this:

john = Person.new('John')
p john.age #=> nil

john.age = 5
p john.age #=> 5

Is there a way to store an instance method inside a variable without running that function first? In Ruby

PLEASE DO NOT TELL ME I CAN DO method(:something) this does not help as it is an instance method being called by another instance.

I would really like to not tell you that, but unfortunately, that is the correct answer:

rerun = start_game_ui.method(:prompt)
# Then, later when you need to call it:
result = rerun.()

Not using Object#method, as you require in your question, leads to significant added complexity, e.g. by passing around the receiver and the name of the method separately:

rerun_receiver_and_message = [start_game_ui, :prompt]
# Then, later when you need to call it:
result = rerun_receiver_and_message.first.public_send(rerun_receiver_and_message.last)

Confused about send method to set instance variables

First off: you cannot set instance variables with Object#send. Object#send sends messages, it doesn't set instance variables. Of course, you can send a message which may or may not then in turn invoke a method which may or may not in turn then set an instance variable, but that's not the doing of Object#send, it's the doing of whatever method was invoked in response to the message you sent.

If you want to dynamically set instance variables, use Object#instance_variable_set:

class Dog
  def initialize(**attrs)
    attrs.each do |attr, value|
      instance_variable_set(:"@#{attr}", value)
    end
  end
end

dog = Dog.new(talk: 'bruuuf')

In this case, however, it looks like you don't actually want to set an instance variable but rather call a setter method. Setter methods have names that end with an = sign, e.g. Dog#talk=:

class Dog
  def initialize(**attrs)
    attrs.each do |attr, value|
      send(:"#{attr}=", value)
    end
  end
end

dog = Dog.new(talk: 'bruuuf')

Note, of course, that this assumes that the method Dog#talk= actually exists. If it doesn't, you will get a NoMethodError.

Ruby Method Arguments - Can they change truth value automatically?

To achieve what you are looking for, you should wrap the forest method in a class and define each argument as an instance variable.

class Forest

  attr_accessor :sword, :armour, :ring

  def initialize(sword = false, armour = false, ring = false)
    @sword = sword
    @armour = armour
    @ring = ring
  end

end

So now, you can declare an instance of Forest,

forest = Forest.new

All the variables default to false, unless you explicitly write true.

With the attr_accessor, you can access and set all the variables.

forest.sword #=> false
forest.sword = true
forest.sword #=> true

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