Ruby’s primitives (Strings, Hashes, Arrays, Numbers - anything that has a literal syntax) are fine things. But that doesn’t mean you should use them everywhere. You’re often much better off wrapping them up in your own Value Objects.

Something I was working on at Railscamp this weekend threw up a great example of why it makes sense to replace primitives with more specific objects as soon as possible. Tom Morris asked me to take a look at Rena, an RDF/Semantic Web thingy.

The RDF spec describes two types of literals: a plain literal, which is a string with an optional language attribute and a typed literal, which is a string and an encoding (so the string might represent an integer, float, or anything else that your schema feels like expressing).

These literals can be output in one of (at least) two formats. We’ll start by looking at Literal#to_trix and see where that takes us:

def to_trix
  if @lang != nil && @lang != ""
    out = ""
  else
    out = ""
  end
  out += @contents
  out += ""
  return out
end

If we look over at TypedLiteral#to_trix we see a much more straightforward implementation:

def to_trix
  "#{@contents}"
end

How do we eliminate that ugly conditional at the beginning of Literal#to_trix, and analogous conditionals in Literal#to_n3 and TypedLiteral#to_n3?

My first thought was that I wanted to be able to write something like:

def to_trix
  "#{@contents}"
end

But I didn’t want every string in the world suddenly acquiring a to_trix method. So, the solution was to intoduce a Literal::Language class and coerce our language into it, so Literal#initialize became:

def initialize(contents, lang = nil)
  @contents = contents
  @lang = Language.coerce(lang)
end  

And Language would look something like:

def self.coerce(lang)
  if lang.is_a?(self)
    return lang
  end

  new(lang.to_s.downcase)
end

def initialize(lang)
  @value = lang
end

def to_trix
  if @value == ''
    ''
  else
    " xml:lang=\"#{@value}\""
  end
end    

That ugly conditional’s still there though, so we introduced the Null Object pattern, and things started to look a good deal cleaner:

class Language
  class Null
    include Singleton

    def to_trix
      ''
    end
  end

  def self.coerce(lang)
    case lang
    when self
      return lang
    when nil, ''
      return Null.instance
    else
      return new(lang.to_s.downcase)
    end
  end

  ...

  def to_trix
    " xml:lang=\"#{@value}\""
  end
end

At this point, we’re still just pushing code around. If anything, we’ve got more lines of code now than when we started, but we’re starting to move behaviour nearer to the data it relates to, and our objects are starting to look like objects rather than data structures. So, we press on and make a TypedLiteral::Encoding class and, at this point things start to look interesting. TypedLiteral is starting to look almost exactly the same as Literal, but with an Encoding rather than a language.

That strange leading space in Language#to_trix is starting bug me. Let’s rewrite like so:

class Literal 
  class Language
    def format_as_trix(literal)
      "#{literal}"
    end

    class Null
      def format_as_trix(literal)
        "#{literal}"
      end
    end
  end

  def to_trix
    @lang.format_as_trix(@contents)
  end
end

If we make analogous change to TypedLiteral and TypedLiteral::Encoding it’s obvious that TypedLiteral and Literal were essentially the same class. Renaming lang</code> and <code>encoding to language_or_encoding</code> makes this blindingly obvious, so we'll remove all of TypedLiteral's methods except initialize. All that remains is to introduce Literal.untyped@ and Literal.typed factory methods to Literal, and make Literal.new into a private method and we can remove TypedLiteral in its entireity. So we change the specs to reflect the new API (wrong way round I know). Now we have a chunk of shorter, clearer code that will hopefully be easier to extend to cope with outputting literals in other formats.

Retrospective

I realise that patterns aren’t the goal of development, but by the end of the process we have a Strategy (Language/Encoding), a couple of Factory Methods (Literal.typed, Literal.untyped) and a couple of factoryish methods (Language.coerce, Encoding.coerce).

The most important aspect of the change was the introduction of the two new value object classes. Once they were introduced, they became the obvious places in which to put the varying behaviour and eliminate the repeatition of conditional code from the to_* methods. If there were to be a third output style, I would look at introducing classes like N3Stream, TrixStream and WhateverStream and have a scheme like:

def to_n3
  print_on( N3Stream.new )
end

def print_on(stream)
  language_or_encoding.print_on(stream, value)
end

but that’s almost certainly over complicating things right now.

The other thing I like about this kind of refactoring is that it drives the code towards methods and classes which obey the single responsibility principle and, at the end of the process, not only do we have fewer lines of code in total, but the individual methods involved are all substantially shorter and closer to the left hand margin.

I really should start doing this kind of thing more in my Rails practice - I keep being put off by the fact that the composed_of helper is so annoyingly not quite right and, rather than submitting a patch or making a plugin I go “Ah well… I can live with a string for a bit longer…” and I know. From hard won experience at that, that it’s going to come and bite me. It’s already bitten Rails recently when Ruby got a new String#to_chars which doesn’t work like the ActiveSupport version.

Notes

If you want to see the gory details of how the change got made, Tom has merged this weekends changes into the github repository. It didn’t happen in quite the order I’ve described it in this post, but neither is this post a complete fabrication.

Changes

Corrected a stupid typo in the first block of code. Ugly condition is actually if lang != nil && lang != ''