You can see the final product of this article here

as of Ruby 2.1.0, method definitions grew a return value. Method Definitions now return the method name as a symbol. This allow for some interesting things like inline private method declaration.

private def foo


def foo
private :foo

Note: Inline private method declarations only works for instance methods. Because def is returning a symbol representing the method name, it does not know the owning object.

so this got me thinking, what else could you use in front of a method definition?

Cache method

Note: Throughout this post I used cache and memoize interchangeably. In reality, memoization is a special case of caching. In this article, all caching is memoization caching.

I see the @expensive_value ||= expensive_computation pattern far too much. I wonder if we can do that automatically by using the return value of def.

cache def foo

would replace

def foo
@foo ||= expensive_computation

I do like how cache def looks and after exploring this problem, turns out the implementation is not that ridiculous either.

module MemoizeMethod
def cache(method)
recompute_method = "recompute_" + method.to_s
cache_var = "@" + method.to_s + "_cache"
alias_method recompute_method, method
define_method(method) { |*args|
instance_eval(cache_var) ||
instance_eval("#{cache_var} ||= #{send(recompute_method, *args)}")

NOTE: I dislike the double instance_eval, but I couldn’t find a better way of sending arguments along to the recompute method.

To get this new behavior just extend MemoizeMethod in your class and enjoy method caching.

Lets walk through the code

  • We alias the method we have just defined to the recompute_ prefixed version of itself.
  • Name the variable we will be storing the expensive computation in.
  • Then, redefine the method we original method to first check the instance variable, then do the computation if it isn’t already saved.

an added advantage of this approach is that we have access to the recompute_method method elsewhere in our code. This lets us bust the cache if we want. There are tons of uses for having a return value from function definition. you can store source annotations, or use it to register the method with a callback.

One thing I wonder after this exploration is have we actually made the code better? I am honestly unsure. We pulled a common idiom into a single word that actually provides additional functionality that was not there before. I suppose it is up to the user if it is worth having this extra layer of magic to provide a different way of caching the return value of a method.