Ruby on Steroids(DSLs): The Powerful Spell Called DSL

Software engineering is all about trade-offs. There rarely is a “best” or “correct” solution to programming problems. The typical general-purpose programming language is good at solving a huge range of problems. Unfortunately, there is a price to be paid for being general purpose. A language that tries to do everything can’t afford to be great at any one thing. Domain specific languages, or DSLs for short, strike a different sort of bargain.

What are DSLs

According to Martin Fowler:

The basic idea of a domain specific language (DSL) is a computer language that’s targeted to a particular kind of problem, rather than a general purpose language that’s aimed at any kind of software problem. Domain specific languages have been talked about, and used for almost as long as computing has been done.

Basically, DSLs are mini-programming language of limited expressiveness focused on solving a particular problem. Since DSLs are very specific, it solves any problem in it’s domain very efficiently.

Types of DSLs

There are two major types of DSL: Internal and External DSL. External DSL’s exist independently from any other language, SQL, SASS, Cucumber, HAML, Jade are good examples of external DSLs. Internal DSL’s on the other hand live inside another programming language for example RSpec is an internal DSL which is hosted within the Ruby programming language.

Typically internal DSLs are easier to create but are not as flexible as external DSL’s. Internal DSL’s need not worry about parsing or grammars but must conform to valid syntax within the host language (eg. all RSpec code is valid Ruby syntax), conversely an external DSL can have any syntax its creator wishes at the cost of more work to build a parser and grammar. Ruby, with its “the programmer is always right” feature set and very flexible syntax, makes a great platform for building internal DSLs.

DSLs In the wild

What makes rails very flexible and magical is that it uses internal DSLs in a lot of places. Let’s take a look at the different internal DSLs used in rails.

1. Rails Routing

2. Rake Tasks

3. DB Migrations

4. ActiveRecord Validations

5. RSpec

Not really part of rails but used a lot of times to test rails applications.

As you can see, Rails uses internal DSLs to solve very specific problems within the framework. Infact you can say that rails is one massive DSL used for building web applications that contains smaller DSLs.

Creating our own DSL: mark_ruby

mark_ruby is a Ruby DSL that generates html. Below is the syntax.

Which generates something like this.

Below is the actual DSL code. There are a lot metaprogramming magic happening here. Check out the Ruby on Steroidsseries if you need a refresher on metaprogramming. Most Ruby Internal DSLs are powered by metaprogramming and blocks. If you can master both, then you have all the tools you need to build internal DSL. Check out Ruby on Steroids series on the magic of metaprogramming

In the MarkRuby class, html_tag method is used to generate any tag we need, it accepts as it’s first argument the name of the tag. So if we want to create a div tag, we do it like this html_tag(:div)and this will return <div></div>, however we want to call the div method directly, instead of creating methods for all the available html tags like this:

We can take advantage of ruby’s method_missing to DRY things up. For a refresher on method_missing, check out this blog post. Also notice that instance_eval is been used in html_tag method. Check out this blog post for a refresher on instance_eval.

Finally I created a private method in the kernel module. This is where methods like puts exist. This makes it possible to call the method from virtually anywhere. In essence, we are creating a global method. Check out this blog post for more information on ruby kernel.

Final Thoughts

DSLs are very powerful spells. They are are best-suited:

  • When tasks require a lot of repeated, abstractable functionality, such as handling migrations or creating a rake task
  • In situations where source should be readable by people besides the developers (Chef, Cucumber, RSpec to a degree)
  • When we need a language that makes it easier to get stuff done as a programmer, and not more complex (all of the DSLs we have seen so far)
  • In situations where we want to create a highly modular API with focus on DRYness and flexibility.
  • When we want to tackle specific problems with an expressive language.

Ruby’s flexibility encourages the creation of DSLs. Ruby has a lot of inbuilt DSL and so do rails. You should take advantage of ruby’s flexibility and build your own DSL when the situation presents itself.

Ikem Okonkwo

About Ikem Okonkwo

Ruby Evangelist, .NET Advocate. Trainer at @andela. Passionate about education and lifelong learning. Loves good food and soccer.