Re: ruby and DSL

On Feb 19, 2008, at 9:30 PM, P Rajmohan Banavi-A17190 wrote:

> Are there any ruby book(s) that explains more in detail about  
> DSL(Domain
> Specific Language) implementation in ruby? Any pointers to online
> tutorials will also be of help.

I don't know if there are any books that directly address this, but a  
good way to learn about DSLs is to pick some apart and figure out what  
about the language work for you and how the authors implemented it.  
rSpec is a great example because it uses the flexibility of Ruby to  
create an expressive language that models its problem domain. So, for  
example, you can write:

result.should eql(3)

This reads like English, but it is actually a clever bit of  
programming. There is a method called "should" that is mixed into  
Object by rSpec. Because everything inherits from Object, all  
descendent objects gain the "should" method. Simplifying a bit, the  
"should" method calls a matcher called "eql" with the argument 3. If  
"eql" returns true, then the spec passes, otherwise the spec fails.  
Again, this oversimplifies what's going on, but you can see that by  
placing method calls carefully, a very readable language for  
expressing specifications has been created.

Another example from the rSpec bag of tricks has to do with  
expectations on mock objects:

@cash_machine.should_receive(:withdraw).twice.and_return(true)

This kind of method chaining is very nice in DSLs, but it relies on  
you to create a structure in which the expected return value of each  
method is an object on which any of the chained methods can be  
invoked. So, should_receive must return an object that responds to  
twice. twice must return an object that responds to and_return. As it  
turns out, you can write:

@cash_machine.should_receive(:withdraw).and_return(false)
@cash_machine.should_receive(:deposit).once.and_return(true)
@cash_machine 
.should_receive(:balance).once.with_any_args.and_return(150)

So you can see that rSpec expectations are crafted such that you can  
really chain an expressive set of constraints on an object that is  
being mocked.

Jamis Buck writes in an older blog post (http://weblog.jamisbuck.org/2006/3/9/integration-testing-in-rails-1-1 
) about integration testing in Rails. In the process of the post,  
Jamis extracts a DSL out of the testing scenario so that it becomes  
quite natural to use.

Of course, Rails is a huge example of DSLs. I don't consider Rails one  
DSL, but several. Obviously, ActiveRecord is an implementation of a  
pattern, but where the DSL part comes into play is in how  
relationships and validations and such are handled:

class CashMachine < ActiveRecord::Base
   has_many   :customers
   belongs_to :bank
   validates_presence_of :security_camera
   # ...
end

Well, you can see I'm not in the banking business, but no matter. The  
point is that even the casual reader could see that any given cash  
machine has many (zero or more) customers and that that machine  
belongs uniquely to a bank. Further, a valid cash machine has to have  
a security camera. It's totally readable (unlike my typical cryptic  
one-liners with a couple of regex'es thrown in). The trick is to  
evolve a language that is possible to speak.

Creating a DSL has to do with having a problem domain and factoring  
out common operations within that domain. While it's possible to write  
about a very real DSL such as rSpec, or an almost-real one like Jamis'  
integration testing scenario, it's not possible to give you a manual  
and say "here's how to write your very own DSL."

I hope this is useful info.