Undefined method error for main:Object when using gem

Question

Testing out making a gem, figured logic gates would be more or less effortless(if useless) to actually implement so I used them. I have this code in my lib/logic.rb file:

require "logic/version"

module Logic
  def or_gate(a, b)
    a || b
  end

  def and_gate(a, b)
    a && b
  end

  def nand_gate(a, b)
    !(a && b)
  end

  def nor_gate(a, b)
    !(a || b)
  end

  def not_gate(a)
    !a
  end

  def xor_gate(a, b)
    !(a == b)
  end

  def xnor_gate(a, b)
    a == b
  end
end

I can build and install the gem without issue, but when testing with irb calling the or_gate method for example just returns a "NoMethodError: undefined method 'or_gate' for main:Object'. Doing something like

Logic.or_gate

or

Logic::Gate.or_gate

(putting methods in a Gate class) both have the same problem. What am I doing wrong?

anothermh · Accepted Answer · 2019-06-01T20:36:11.080

You have defined instance methods, not module methods. Change:

def or_gate(a, b)

to:

def self.or_gate(a, b)

and it'll work the way you expect:

Logic.or_gate(1,2)
 => 1

Repeat this change for all your method definitions.

Alternatively, you can use extend self to accomplish the same goal without having to add self. to each method definition:

module Logic
  extend self

  def or_gate(a, b)
    a || b
  end
end

This adds/copies all the instance methods as module methods.

There is more discussion of this here, and this answer goes into some more detail on how methods are defined in modules.

Undefined method error for main:Object when using gem

1 Answers1