Paul Ingles

During the most recent ThoughtWorks away-day (a chance for the office to get together, catch-up, drink etc.), George and I presented on a number of Ruby and Rails lessons we learned from our (now previous) project. One of the most interesting sections (to us anyway) was on declarative programming, specifically, refactoring to a declarative design.

I guess much like DSLs, its easier to feel when you’re achieving something declarative as opposed to necessarily defining what makes it. But, I’ll try my clumsy best to define something.

Almost every language I’ve turned my hand to (save for Erlang) are imperative languages - where programs are written as sequences of operations, with changes of state. You determine the what and the how of the system - what to do and how to do it.

Declarative programming is an alternative paradigm, whereby code is expressed as what’s. How the system executes is someone else’s responsibility.

So, for the purposes of this discussion let’s consider that application code can be split into two groups

1. Logic- the rules, the guts of things- that what’s.
2. Control- statements about execution flow.

Interestingly, one of the principles listed in Kent Beck’s most recent book (Implementation Patterns) includes “Declarative Expression” - that you should be able to read what your code is doing, without having to understand the wider execution context.

Declarative languages are all around us, with most developers I’m guessing using them almost daily.

Think of SQL, when you write a statement such like SELECT [Name], [Age] FROM [Person] WHERE [Age] > 15 you’re making a statement about what you’d like, not how you get there- that’s up for the database engine to figure out. And a good thing to! Have you ever taken a look at an execution plan for modestly complex queries?

Closter to home - think of .NET and Java Annotations- where you can decorate constructs with additional behaviours. They look and feel like core extensions to the language, but are programmable and can be used to adapt the runtime behaviour of the system.

Before working for ThoughtWorks, I worked on a system where we used attributes to allow us to add validations to properties, allowing us to re-use code and extend easily.

[LengthMustBeAtLeast(6)]
public property string FirstName
{
  get { ... }
  set { ... }
}

Everything was nicely decoupled, read well, and reduced the amount of clutter in our code. More importantly, our validation code was not spread throughout every setter of every property. We could isolate responsibilities making code easier to digest and understand, and test!

Onto Ruby. A substantial part of our project involved reading lots of CSV files from different sources to update our deal information. The answer was a kind of anticorruption layer (borrowing heavily from domain-driven design) for each different feed.

Quickly, we ended up with a few concrete classes and a base class that co-ordinated effort. Dependencies were shared both ways, imagine a number of template methods that are called in sequence to accomplish their work. Over time it grew complex, and with Ruby it’s a little tougher (than with languages like Java or .NET) to navigate and browse around the code without strong IDE support.

It was starting to get a little too complex and we felt we needed to change things, so we did (bolstered somewhat by having Jay with us).

Onto the code.

So imagine we have a class representing a Feed of information (read from a CSV file), and we want to be able to ask that Feed to provide us with a number of Deals. Internally, it will iterate over the items in the Feed, creating a Deal for each one (if possible).

Our first solution looked a little like this, firstly in the ‘abstract’ base class:

def create_deal
  Deal.create(:network_name => network_name)
end

and in the feed’s concrete implementation:

FIELD_INDICES = {:name => 1, :network_name => 2}

def network_name
  read_cell(FIELD_INDICES[:network_name])
end

Our main feed class asks our implementation class for the network_name - a template method. Not bad, now build that up to tens of attributes, a bit longer. Since we’ve defined column indices in a constant, we also now have to navigate up and down lots to determine where we’re reading from. Add in a few other tables with look-ups for other bits that we need to do the mapping and it can get a little complex pretty quick.

Our code is not only made up of the stuff determining what it is to translate between a CSV representation of our Deals to an object model one, but also all of the code necessary to find out which CSV column we’re in, how we map that column etc. They were essentially just reading values from cells, no translation needed. Most of the code we had was infrastructural, and the logic (the what of our application) was hidden amongst the noise.

This complexity, combined with the split of flow between abstract and concrete classes, made it difficult to follow and understand. Our goal was to try and reduce each concrete implementation to a single page on our screens.

These little ‘mapping’ translation methods were our first target - reduce the amount of code for each of these to one line that described the mapping, rather than how we get it all.

Firstly, we introduced a convention - every attribute of a Deal could be retrieved by calling a deal_attribute_my_attribute style method. So, we renamed all our methods, ran the tests, and then started to make steps towards having each deal_attribute_blah method defined dynamically.

We went from:

def network
  ...
end

to

def deal_attribute_network
  ...
end

to … nothing.

Well, not quite. Instead, what we wanted was to have a method constructed by adding a class method to a module that we could mix-in. Then, we could just define the mapping and Ruby would wire up the rest. We settled on the following syntax

deal_attribute :name => 'NAME'

Neat. A little classeval and instanceeval magic later we were able to push our infrastructural code out of our concrete feed class and into the co-ordinator.

Our code is now more declarative. We’re stating what we need to do our work, rather than worrying about how we get at it. Not only that, for the common case (where we may be just moving values from one place to another) there’s no need to do anything more than describe that relationship. Declarative programming makes it much easier to express important relationships. It’s now much easier to see the relationship between the name attribute of a Deal and the NAME column for this CSV feed.

Notice also how we’re pushing our dependencies up to our caller- the coupling is now one-way. We state what we need from our caller (the main deal feed class) - our caller is able to then pass the information on. We’re just answering questions, rather than answering questions and asking questions (of our caller).

The syntax also lends itself to explaining a dependency, that an attribute of our deal is read from ‘NAME’ (for example).

That’s great, next step was to tidy up some of the slightly more complex examples where we do some additional translation. For example, where we take the name of something and we need to pull back an object from the database instead. Let’s say we keep track of the Phone that a Deal is for.

So, from

def deal_attribute_phone
  Phone.find(deal_attribute_brand, deal_attribute_model)
end

to

deal_attribute(:phone => ['MAKE', 'MODELNAME']) do |brand, model|
  Phone.find(brand, model)
end

We’ve extended the syntax to reveal that this translation needs values from both the ‘MAKE’ and ‘MODELNAME’ columns. From our perspective, we’re pushing responsibility up and keeping our code focused on what we need to map this attribute.

This is a little more complex to achieve since we’re also passing arguments across (and our deal_attribute translator methods also sometimes need to access instance variables) so we need to use instance_exec instead of the standard instance_eval.

The end result was feed classes that looked as follows

class MySpecialFeed
  deal_attribute :name => 'NAME'
  deal_attribute(:description => 'DESC') {|desc| cleanse_description(desc) }
  deal_attribute(:phone => ['MAKE', 'MODELNAME']) do |brand, model|
    Phone.find(brand, model)
  end
  ...
end

In total, it took us probably just over a day to refactor the code for all of our classes. Most of which we managed to get down to some 40 or 50 lines total. We didn’t refactor all the code, so there’s still potential for exploiting it further but it was definitely a very exciting thing to see happen.

From across the desk, George asks “can you copy classes in Ruby?”. We talk about it quickly and reason that since everything’s an Object (even classes), you probably can. Since the constant isn’t changed or duplicated (you’re essentially assigning a new one) then it ought to be possible.

Turns out it is!

class First
  def initialize
    @value = 99
  end

  def say_value
    @value
  end
end
First.new.say_value # => 99

Second = First.clone
Second.class_eval do
  define_method :say_value do
    @value + 100
  end
end
Second.new.say_value # => 199

Neat.

I’m not sure quite why you would want to clone a class to take advantage of re-use - rather than extract to a module (and share the implementation that way) or, if there’s a strong relationship that doesn’t violate the LSP etc. then look for some kind of inheritance-based design.

But, I guess you could work some kind of cool ultra-dynamic super-meta system from it. Perhaps someone with way more of a Ruby-thinking brain than me could offer some thoughts?

Rails makes heavy use of a declarative style around it’s codebase- for example the has_one and belongs_to declarations inside ActiveRecord (amongst others). These are just class methods defined on modules, letting Rails wire up relationships, but they read like fully-fledged statements within a mini-language:

class Student < ActiveRecord::Base
  has_many :tutorials

We can take advantage of the same in our code- letting us write in a declarative-style (hopefully revealing stronger intent) and reduce the amount of code we write (by using declarations to meta-program for us). I posted a little while ago that we’d used such an approach for marking attributes as immutable.

Anyway, back to the title of the post- #instance_exec, it’s a method defined in Ruby Facets. Like it’s documentation says, it’s equivalent to instance_eval but also lets you pass parameters- roll on the meta magic.

Let’s say we’re writing a system to calculate the monthly salary payment for an employee. We want to be able to say what the payment is rather than how it’s calculated.

class FullTimeEmployee
  include Employee

  bill_at {|hours| 50.pounds_sterling * hours}

In the snippet above, we’re making a declaration - defining the relationship between an hourly rate and the number of hours the employee works. We can implement bill_at in Employee as follows:

module Employee
  module ClassMethods
    def bill_at &block
      define_method(:calculate_bill) do |hours|
        instance_exec hours, &block
      end
  ...

Our assertion could be:

assert_equal 500.pounds_sterling, employee.calculate_bill(5.hours)