Question

So I’m using the awesome trollop gem to do option parsing, but I’m having a general problem with the scope of the variables it’s setting.

require 'trollop'

class MyClass
  opts = Trollop::options do
    opt :thing, "does something", default: "blah", type: String
  end

  def my_method
    puts opts[:thing]
  end
end

But I get:

undefined local variable or method `opts' for #<MyClass:0x0000010203c840> (NameError)

Any ideas what I’m doing wrong with my scope?

Answer 1

There are about six options here: instance variable, class instance variable, class variable, class constant, global variable, global constant. Which to use depends on your needs.

Instance Variable – each MyClass instance gets its own options:

class MyClass
  def initialize
    @opts = ...
  end

  def my_method
    puts @opts[:thing]
  end
end

Class Instance Variable – single value across the class that can be reassigned:

class MyClass
  @opts = ...
  class << self
    attr_accessor :opts
  end

  def my_method
    puts self.class.opts[:thing]
  end
end

Class Variable – each MyClass and all subclasses share the same value (convenient syntax, but rarely a good idea):

class MyClass
  @@opts = ...
  def my_method
    puts @@opts[:thing]
  end
end

Class Constant – single object that may be mutated, but not re-assigned. Easily accessed from this class, accessible from others via MyClass::OPTS:

class MyClass
  OPTS = ...
  def my_method
    puts OPTS[:thing]
  end
end

Global Variable – you can only have one of these in your entire app; often global variables are ill-advised, but perhaps appropriate for a standalone application’s options:

$opts = ...
class MyClass
  def my_method
    puts $opts[:thing]
  end
end

Global Constant – accessed from many classes, can’t be set to a new value, but may be mutated:

OPTS = ...
class MyClass
  def my_method
    puts OPTS[:thing]
  end
end