issue accessing instance variable via included module's method

Question

The example code and output can be found here:

module A
  def self.included(base)
    base.include InMethods
  end

  module InMethods
    def mem
      @mem ||= []
    end

    def add(n)
      mem += n
    end
  end
end

class Help
  include A
end

h = Help.new
h.add(1)
# in `add': undefined method `+' for nil:NilClass (NoMethodError)

Basically, I am included one module which includes a submodule, but the real problem is with the methods and the instance variable. this is a common pattern for me, but since i am trying to do it from a module i've included, i'm having trouble.

Max · Accepted Answer · 2017-09-18 21:01:26Z

2

This actually has nothing to do with modules/inclusion/inheritance/whatever. You get the same error in a regular class.

class Help
  def mem
    @mem ||= 0
  end

  def add(n)
    mem += n
  end
end

Help.new.add(1)
# test.rb:16:in `add': undefined method `+' for nil:NilClass (NoMethodError)

When you use the syntactic sugar mem +=, Ruby actually expands this into mem = mem +. The first problem is that this is interpreted left-to-right, so the mem = defines a local variable mem that masks your method. So then the second mem references a defined-but-unset local variable and you get nil.

The second problem is that you didn't define a setter mem= method, so it can't change @mem at all!

To fix both problems, you could do

attr_writer :mem

def add(n)
  self.mem += n
end

edited Sep 18, 2017 at 21:01

answered Sep 18, 2017 at 20:37

Max

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1 Comment

Max Over a year ago

True, my wording is wrong. I meant "unset" for the second one. A weird artifact of how Ruby defines local variables.

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