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Sat, 15 Jul 2006

def_init'ly DRYer Ruby

I'm leading a study group at my current client. Nine of us are reading Ruby for Rails and get together every week with a projector to answer questions and hack away at some examples. I read a few chapters ahead so that I can provide exercises for the group to prepare for our meetings. I was reading chapter 5 today, it's fairly basic Ruby stuff, introducing the convenient and self-documenting attr_* methods. The following class is the example from the book, your typical brainless data holder...
class Ticket
  def initialize(venue, date)
    @venue = venue
    @date = date

  def price=(price)
    @price = price

  def venue

  def date

  def price
Using the attr_* methods, the mind-numbingly boring getters and setters can be collapsed into this more readable version...
class Ticket
  attr_reader :venue, :date, :price
  attr_writer :price

  def initialize(venue, date)
    @venue = venue
    @date = date
Now, I've written constructors like that one more times than I care to remember in all sorts of languages. And everytime I do it, I feel a twinge of pain, it just feels silly to have to do that mapping of parameters to instance variables manually. When I'm writing Java, IDEA reduces this pain by generating most of it after a few ALT-ENTERs. But when I'm writing Ruby, repetitive constructs like these hurt even more, since that sort of repetition goes against one of the Ruby culture's most fundamental values: DRYness.

And thus the cousin to the attr_* siblings was born: def_init. By using def_init, the constuctor in the example could be reduced to...

class Ticket
  attr_reader :venue, :date, :price
  attr_writer :price
  def_init :venue, :date
I'm guessing that this functionality is already out there somewhere, so let me know if I've missed it. Here's the code that makes it happen...
class Class
  def def_init(*attrs)
    constructor = "def initialize("
    constructor <<{|a| a.to_s}.join(",")
    constructor << ")\n"
    attrs.each do |attribute|
      constructor << "@#{attribute} = #{attribute}\n"
    constructor << "end"
It's interesting to note how Ruby and Java solve these sort of repetition problems differently. Java tends to solve repetition with excellent tools that generate code at development time, while Ruby tends to solve it through giving developers the power to generate code at runtime.

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