David Eisinger

Elsewhere > Multi-line Memoization

Posted 2009-01-05 on viget.com

Here’s a quick tip that came out of a code review we did last week. One easy way to add caching to your Ruby app is to memoize the results of computationally expensive methods:

def foo
  @foo ||= expensive_method
end

The first time the method is called, @foo will be nil, so expensive_method will be called and its result stored in @foo. On subsequent calls, @foo will have a value, so the call to expensive_method will be bypassed. This works well for one-liners, but what if our method requires multiple lines to determine its result?

def foo
  arg1 = expensive_method_1
  arg2 = expensive_method_2
  expensive_method_3(arg1, arg2)
end

A first attempt at memoization yields this:

def foo
  unless @foo
    arg1 = expensive_method_1
    arg2 = expensive_method_2
    @foo = expensive_method_3(arg1, arg2)
  end

  @foo
end

To me, using @foo three times obscures the intent of the method. Let’s do this instead:

def foo
  @foo ||= begin
    arg1 = expensive_method_1
    arg2 = expensive_method_2
    expensive_method_3(arg1, arg2)
  end
end

This clarifies the role of @foo and reduces LOC. Of course, if you use the Rails built-in memoize method, you can avoid accessing these instance variables entirely, but this technique has utility in situations where requiring ActiveSupport would be overkill.