A blog about web development

Paul Sturgess

Ruby Metaprogramming: Define_method and Instance_exec

This blog post runs through an example of iterating on some Ruby code. The bizarre fictional scenario develops to make the requirements ever more complicated. There’s certainly many different ways to solve the underlying problem, but I wanted to show an example where define_method and instance_exec can be combined with class methods to make for some very powerful and concise code.

For each change in requirements, I’ll post the code in full for how I’ve handled the new scenario.

I will say upfront that these two metaprogramming methods should be used with caution. Powerful and concise code does not necessarily make for easy to understand/debug/maintain code.

The Curry Restaurant

So, there’s a curry restaurant that sells Vindaloo and Tarka Daal and there’s a customer that likes to order curry from the restaurant… bear with me :). Upon receiving their meal, the customer will respond with a comment if the spiceyness isn’t to their taste. Unfortunately the restuarant isn’t consistent with the spices it uses when cooking.

With the above code we can do things like:

spicey_customer = SpiceyCustomer.new

If any of the meals are mild then the customer will respond with “That’s too mild!”.

A new greedy customer

Then along comes another customer, a greedy one. They also like to order curry and comment on the meal. However, rather than comment on the spiceyness, they like to comment on the size of the portion. Unfortunately the restuarant is also inconsistent with its porton sizes, but the greedy customer doesn’t care what type of curry it is, they just want it large!

Already we have a bit of duplication between the two customers but it’s not a massive problem at this point.

A new menu item - Jalfrezi

The restaurant is doing really well and introduces a new Jalfrezi curry to the menu. The greedy customer responds the same to it as for any curry, but the spicey customer responds to the Jalfrezi, the same as when recieving a Vindaloo - they want it hot!

We’ve added order_jalfrezi to both customers and added expect_hot_curry to SpiceyCustomer because they respond the same for jalfrezi and vindaloo. Now we really do have duplication between the two customers, so we’ll look at refactoring that next.

Refactor - Dining Out

We decide that the two customers have enough shared behaviour to introduce a new module called DiningOut. This means both customers can order all three currys and then respond in a way that is specific to them. With the custom response defined in each Customer in the respond_to_meal method.

You might be thinking it would be better to make Customer a class and have GreedyCustomer and SpiceyCustomer as subclasses that inherit from it, but for this example it doesn’t matter. This is a version of the Template method pattern.

We have our new DiningOut module and we’ve defined a respond_to_meal in both customers. This code is definitely cleaner but there is still a bit of a smell.

If the restaurant expands with lots of new items on the menu and these customers need to respond differently, then the respond_to_meal method is going to get pretty big and complicated pretty quickly. We’re also very reliant on the name of the meal not changing.

In addition the DiningOut module will bloat quickly with each new menu item. That isn’t a massive problem, but there’s already a fair bit of obvious duplication going on where the name of the order method closely matches the argument sent to order_meal.

Now you might be thinking, why can’t we just expose order_meal as a public method and call SpiceyCustomer.new.order_meal(:jalfrezi) instead of SpiceyCustomer.new.order_jalfrezi. However…

A new fussy customer

Along comes the Fussy customer who will only eat tarka daal and the newest item on the menu, chips. The fussy customer isn’t interested in responding to the meal and just eats in silence. In addition, the Greedy customer will order chips, but the Spicey customer would never order chips. Sounds complicated doesn’t it?

The simplest thing to do is to add an order_chips method to DiningOut and an empty respond_to_meal to the new FussyCustomer.

Now we have some complicated rules and they’re not properly enforced. There’s nothing stopping us calling FussyCustomer.new.order_vindaloo or SpiceyCustomer.new.order_chips even though neither of those customers would order those things.

We could start putting some logic into the order_* methods to check the type of customer before ordering the meal, but that would get ugly fast. Ideally we want a solution where we can tell by looking at the code which customer likes to eat which food and each customer wont even know how to order food they don’t want to order.

Introducing define_method and instance_exec

Before we get into the final solution, the end result is that we can now do things like the following:

spicey_customer = SpiceyCustomer.new
spicey_customer.order_chips # returns an undefined method error

greedy_customer = GreedyCustomer.new

fussy_customer = FussyCustomer.new
fussy_customer.order_tarka_daal # returns an undefined method error

Also when we look at each customer we can see very clearly what they like to eat and how they like to respond for each menu item.

The nice thing is that if the restaurant adds a new menu item, we don’t need to touch the DiningOut module at all. We just need to decide which customers would like to order the new menu item and how they might optionally respond.

It works by defining a new class method eats. This method takes the name of the menu item and a block to be called with the meal so the customer can inspect the meal to determine their response.

The eats class method in DiningOut uses define_method to dynamically create the order_* instance method on the customer. This means if the customer declares eats :chips then they will respond to the method order_chips (and conversely if they don’t define it then they will return undefined method error). This makes it very obvious what menu items the customer will eat and prevents mistakes.

The order method uses instance_exec to execute the response block passed in. This is required because we want to ensure the block is called on the customer instance and not in the context of the class method - if we just did block.call(meal) then an undefined method (for whatever method the response block called) would be raised.

By accepting a block and calling it with instance_exec gives the nice benefit of that block being able to call any method it likes on the instance. In this scenario each Customer can call a different response method, for each type of menu item, if they want. This is important because, for example, the Chips menu item does not respond to the method spiceyness - so if there was a single assumed respond_to_meal method each Customer implemented it would have to handle every type of meal (checking if it responds to methods such as spiceyness and all sorts of other logic).


Obviously this is a ridiculously contrived and long example to demonstrate two metaprogramming methods in Ruby but hopefully if you’ve read this far, you’ve got something out of it.

As I mentioned in the intro, these metaprogramming methods should be used with caution. Powerful and concise code does not necessarily make for easy to understand/debug/maintain code.