Understanding Ruby Metaprogramming and DSLS

What is a DSL?

A DSL, or Domain Specific Language, is a language that has a specific purpose rather than a general purpose like C or Java. One of the most popular DSLs is SQL because it’s the standard for querying a relational database and it’s syntax is specific to activities such as sorting, filtering and displaying data.

SQL falls under the category of external DSLs which means it requires its own parser and implementation of all of the necessary components. Other examples of external DSLs are Gherkin, for writing feature files, Make Files, for building C and C++ applications and HTML, for declaring webpages).

What is a Ruby DSL?

Ruby is a powerful and expressive language that can be leveraged to create an internal DSL. Instead of requiring a full-syntax parser and an underlying implementation, metaprogramming techniques can be used to create a DSL that leverages the Ruby language.

Anyone that has used a popular Ruby framework has most likely seen a Ruby DSL. Frameworks that leverage these include:

Rake
RSpec
Active Record
Sinatra

Before RSpec came along, TestUnit was the most popular testing framework in Ruby. A test looked a lot like a typical Ruby class:

require "test/unit" class TestCalculator < Test::Unit::TestCase def test_simple assert_equal(4, Calculator.add(2, 2) ) end end

RSpec changed the way programmers write test cases by creating an internal DSL that is much more expressive.

describe TestCalculator do it "should return the sum of the two numbers" do expect(Calculator.add(2, 2)).to eq(4) end end

Creating a simple Ruby DSL:

In this tutorial we’ll create a DSL for querying a CSV file.

The full file can be found here.

The end result will be a Ruby DSL that reads like this:

Step 1: Create a standard Ruby class

Look at the plain old Ruby code from the following branch here.

The class is as follows:

require "csv" class WorldCupDSL attr_reader :conditions def initialize file_path @conditions = {} @data = CSV.read(file_path, headers: true, header_converters: :symbol, converters: :all).collect do |row| Hash[row.collect { |c, r| [c, r] }] end end def where property, expected @conditions[property] = expected end def data results = @data.dup @conditions.each do |key, value| results = results.find_all do |row| row[key].to_s == value end end results end def flush @conditions = {} end

This can be used just like any other Ruby object:

wc_dsl.where(:country, "Argentina") Wc_dsl.data #returns all of the rows where the country is Argentina

Step 2: Implementing method_missing

Method missing is the first technique we will use to build our DSL,so paste the following into a IRB console:

class MyClass end MyClass.new.does_not_exist You will see the following error: NoMethodError: undefined method `does_not_exist' for #<MyClass:0x007fe48e873618> from (irb):6

We’ll now override the default behaviour by implementing method_missing to bring the error up on the screen:

class MyClass def method_missing(m, *args, &block) puts "called method #, but it does not exist" end end MyClass.new.does_not_exist

Now that we’ve changed the default behaviour we can leverage this to create our Ruby DSL. Add the following to the ç class in the world_cup_dsl.rb file:

def method_missing(m, *args, &block) where(m, args.first) end We can now run the WorldCupDSL as such: wc_dsl = WorldCupDSL.new 'data.csv' wc_dsl.country "Argentina" wc_dsl.data #returns all of the rows where the country is Argentina

As you can see, we can now use a method called country, or any other row heading in the CSV such as position or last_name,to filter the the data by column value.

The full code for this part can be seen here.

Step 3: Implementing instance_eval

A Proc in Ruby is Ruby code that is declared in a block and assigned to a variable to be used at a later time. Try the following in IRB:

p= Proc.new {|arg|puts arg} p.call “input”

Now let us change the implementation to use a method call foo instead of puts

p= Proc.new {|arg|foo arg} p.call “input”

You will get an error: NoMethodError: undefined method `foo' for main:Object

This is where our second metaprogramming technique will be used. We will use instance_eval to call this proc but in a different way with an object that has the foo method.

class MyClass def foo arg puts 'foo ' + arg end end p = Proc.new do foo 'Something' end MyClass.new.instance_eval &p

By using instance_eval the code block is run in a different way where the foo method exists.

def query &block instance_eval(&block) data end

With this extra functionality we can now execute WorldCupDSL in a way that is similar to RSpec:

wc_dsl = WorldCupDSL.new 'data.csv' wc_dsl.query do country "Argentina" height '175' end

You can see the final product here.