SafeMemoize

Thread-safe memoization for Ruby that correctly handles nil and false values.

Ruby's common memoization pattern breaks with falsy values:

def user
  @user ||= find_user  # Re-runs find_user every time it returns nil!
end

SafeMemoize uses Hash#key? to distinguish "not yet cached" from "cached nil/false", so your methods are only computed once regardless of return value.

• Correctly memoizes nil and false return values
• Caches per unique arguments (positional and keyword)
• Thread-safe via double-check locking
• Zero runtime dependencies
• Simple prepend + memoize API
• Block arguments bypass cache (blocks aren't comparable)

Add to your Gemfile:

gem "safe_memoize"

Then run:

bundle install

Or install directly:

gem install safe_memoize

class UserService
  prepend SafeMemoize

  def current_user
    # This expensive lookup runs only once
    User.find_by(session_id: session_id)
  end
  memoize :current_user
end

Results are cached per unique argument combination:

class Calculator
  prepend SafeMemoize

  def compute(x, y)
    sleep(2)
    x + y
  end
  memoize :compute
end

calc = Calculator.new
calc.compute(1, 2)  # computes and caches
calc.compute(1, 2)  # returns cached result
calc.compute(3, 4)  # computes and caches (different args)

class Config
  prepend SafeMemoize

  def enabled?
    # Only called once, even though it returns false
    ENV["FEATURE_FLAG"] == "true"
  end
  memoize :enabled?
end

obj = MyService.new
obj.reset_memo(:current_user)  # Clears cache for one method
obj.reset_all_memos            # Clears all memoized values

SafeMemoize uses Ruby's prepend mechanism. When you call memoize :method_name, it creates an anonymous module with a wrapper method and prepends it onto your class. The wrapper calls super to invoke the original method and stores the result in a per-instance hash. Thread safety is achieved with a per-instance Mutex using double-check locking.

After checking out the repo, run bin/setup to install dependencies. Then, run bundle exec rspec to run the tests. You can also run bin/console for an interactive prompt.

Bug reports and pull requests are welcome on GitHub at https://github.com/eclectic-coding/safe_memoize.

The gem is available as open source under the terms of the MIT License.