a naive implementation of dataflow variables in ruby

dataflow.rb

# Usage: run tests with `ruby dataflow.rb`

require 'bundler/setup'

module DataFlow
  class BasicObject < ::BasicObject
    undef_method :==
    undef_method :equal?

    def raise(*args)
      ::Object.send(:raise, *args)
    end
  end

  class DataFlowVar < BasicObject
    def initialize
      @mutex = ::Mutex.new
      @locked = false
    end

    def value=(value)
      if @locked
        raise ::StandardError, "value already assigned"
      else
        @mutex.synchronize do
          @locked = true
          @value = value
        end
      end
    end

    def value
      @value
    end
  end
end

if __FILE__ == $0
  require 'minitest/autorun'

  class DataFlowVarTest < MiniTest::Unit::TestCase
    def setup
      @var = DataFlow::DataFlowVar.new
    end

    def test_assignment
      @var.value = "foo"
      @var.value.must_equal "foo"
    end

    def test_double_assignment_raises
      @var.value = "foo"
      -> { @var.value = "bar" }.must_raise StandardError
    end
  end
end

Gemfile

source :rubygems
gem 'minitest'

You should also move require 'bundler/setup' into the test condition.