Question

I have the following Ruby code:

local_var = "Hello"

def hello
  puts local_var
end

hello

I get the following error:

local_variables.rb:4:in 'hello': undefined local variable or method 'local_var' 
for main:Object (NameError) from local_variables.rb:7:in '<main>'

I always thought that local variables are not accessible from outside of the block, function, closure, etc.

But now I defined local variable in the file and try to get an access from the function INSIDE the same file.

What’s wrong with my understanding?

Answer 1

In Ruby local variables only accessible in the scope that they are defined. Whenever you enter/leave a Class, a Module or a Method definiton your scope changes in Ruby.

For instance :

v1 = 1

class MyClass # SCOPE GATE: entering class
  v2 = 2
  local_variables # => ["v2"]

  def my_method # SCOPE GATE: entering def
    v3 = 3
    local_variables  # => ["v3"]
  end # SCOPE GATE: leaving def

  local_variables # => ["v2"]
end # SCOPE GATE: leaving class

These entering and leaving points are called Scope Gates. Since you enter through Scope Gate via method definition you cannot access your local_var inside hello method.

---

You can use Scope Flattening concept the pass your variable through these gates.

For instance instead of using def for defining your method you can use Module#define_method.

local_var = "Hello"

define_method :hello do
  puts local_var
end

In the same way you can define your classes via Class#New so that your scope does not change when you pass through class definition.

local_var = 'test'

MyClass = Class.new do
  puts local_var #valid
end

instead of

class MyClass
  puts local_var #invalid
end

In the same way you should use Module#New if you want to pass your local variables through Module gates.

Example is taken from Metaprogramming Ruby