Safe handling of Ruby constant redefinition (though you should not have to do it in the first place)

constant.rb

module Constant
  NAMESPACE_SEPARATOR = "::".freeze

  module_function

  def defined?(name)
    Object.const_defined?(name)
  end

  def ensure(name, value)
    return if self.defined?(name) && get(name) == value

    redefine(name, value)
  end

  def get(name)
    Object.const_get(name)
  end

  def redefine(name, value)
    namespace, basename = unpack(name)

    namespace.send(:remove_const, basename) if self.defined?(name)
    namespace.const_set(basename, value)
  end

  def set(name, value)
    fail NameError, "Constant '#{name}' already defined" if self.defined?(name)

    redefine(name, value)
  end

  def split(name)
    namespace, _, basename = name.to_s.rpartition(NAMESPACE_SEPARATOR)

    [namespace.empty? ? nil : namespace, basename]
  end

  def unpack(name)
    namespace, basename = split(name)

    namespace = namespace.nil? ? Object : get(namespace)

    [namespace, basename]
  end
end

constant_spec.rb

require "constant"
require "securerandom"

describe Constant do
  before do
    Object.send(:remove_const, :FOOBAR) if Object.const_defined?(:FOOBAR)
  end

  describe ".defined?" do
    context "when the constant is defined" do
      it "returns true" do
        expect(described_class.defined?(:RUBY_REVISION)).to be true
      end
    end

    context "when the constant is not defined" do
      it "returns false" do
        expect(described_class.defined?(:FOOBAR)).to be false
      end
    end
  end

  describe ".ensure" do
    let(:old_value) { SecureRandom.uuid }
    let(:new_value) { SecureRandom.uuid }

    before do
      allow(Object).to receive(:remove_const).with("FOOBAR").and_call_original
    end

    context "when the constant is not defined" do
      it "assigns the value" do
        described_class.ensure(:FOOBAR, new_value)

        expect(FOOBAR).to eq new_value
      end
    end

    context "when the constant was already defined" do
      context "with the desired value" do
        before do
          Object.const_set(:FOOBAR, new_value)
        end

        it "assigns the value" do
          expect(Object).to_not receive(:remove_const).with("FOOBAR")
          expect { described_class.ensure(:FOOBAR, new_value) }.to_not change { FOOBAR }
        end
      end

      context "with a different value" do
        before do
          Object.const_set(:FOOBAR, old_value)
        end

        it "assigns the value" do
          expect { described_class.ensure(:FOOBAR, new_value) }.to change { FOOBAR }.from(old_value).to(new_value)
        end
      end
    end
  end

  describe ".get" do
    context "when the constant is defined" do
      it "returns true" do
        expect(described_class.get(:RUBY_REVISION)).to eq RUBY_REVISION
      end
    end

    context "when the constant is not defined" do
      it "returns false" do
        expect { described_class.get(:FOOBAR) }.to raise_error(NameError)
      end
    end
  end

  describe ".redefine" do
    let(:old_value) { SecureRandom.uuid }
    let(:new_value) { SecureRandom.uuid }

    context "when the constant is not defined" do
      it "assigns the value" do
        described_class.redefine(:FOOBAR, new_value)

        expect(FOOBAR).to eq new_value
      end
    end

    context "when the constant was already defined" do
      before do
        Object.const_set(:FOOBAR, old_value)
      end

      it "assigns the value" do
        expect { described_class.redefine(:FOOBAR, new_value) }.to change { FOOBAR }.from(old_value).to(new_value)
      end
    end
  end

  describe ".set" do
    let(:old_value) { SecureRandom.uuid }
    let(:new_value) { SecureRandom.uuid }

    context "when the constant is not defined" do
      it "assigns the value" do
        described_class.set(:FOOBAR, new_value)

        expect(FOOBAR).to eq new_value
      end
    end

    context "when the constant was already defined" do
      before do
        Object.const_set(:FOOBAR, old_value)
      end

      it "assigns the value" do
        expect { described_class.set(:FOOBAR, new_value) }.to raise_error(NameError)
      end
    end
  end

  describe ".split" do
    it "returns the namespace (if present) and basename of the constant" do
      expect(described_class.split("::SecureRandom::FOOBAR")).to eq ["::SecureRandom", "FOOBAR"]
      expect(described_class.split("SecureRandom::FOOBAR")).to eq ["SecureRandom", "FOOBAR"]
      expect(described_class.split("::FOOBAR")).to eq [nil, "FOOBAR"]
      expect(described_class.split("FOOBAR")).to eq [nil, "FOOBAR"]
    end
  end

  describe ".unpack" do
    it "returns the namespace (if present) and basename of the constant" do
      expect(described_class.unpack("::SecureRandom::FOOBAR")).to eq [SecureRandom, "FOOBAR"]
      expect(described_class.unpack("SecureRandom::FOOBAR")).to eq [SecureRandom, "FOOBAR"]
      expect(described_class.unpack("::FOOBAR")).to eq [Object, "FOOBAR"]
      expect(described_class.unpack("FOOBAR")).to eq [Object, "FOOBAR"]
    end
  end
end