I like to define Language-Oriented Programming (LOP) as a way to develop computer programs by creating a new language or modifying an existing one. The new language created during the process will be extremely problem-focused and efficient at solving the given problem.

Using LOP means that you will create a language. When the Hibernate team created its HQL language they did use LOP but when someone uses Hibernate he or she is not necessarily using that paradigm -you can’t say you’re using LOP just because you are using HQL.

LOP is a paradigm, it is a tool and not a goal by itself. When a developer builds a solution using Object-Orientation –other paradigm- the problem won’t be solved just by creating objects, objects are tools to be used while developing the solution. Just the same when you do LOP you use the construction of a new language as a tool to solve your problem, the language is not a goal in itself. When an engineer is working on a new programming language the goal is not solving a business problem but creating a new language, so he is not applying LOP.

Domains and Models

Martin Fowler has a more strict definition about Language-Oriented Programming tying the term to Domain-Specific Languages:

I use Language Oriented Programming to mean the general style of development which operates about the idea of building software around a set of domain specific languages.

-Martin Fowler, Language Workbenches

I think that it depends on what you call a domain. I have seen dozens of examples where LOP were applied to more basic level of abstractions than modeling business domains so the statement above looked like telling someone that object-orientation is about modeling the real world, what although not wrong is a huge oversimplification.

Let’s get an example of a classic LOP platform: Common Lisp. This language doesn’t provide a loop facility and we normally find like the for or while constructs in other languages. The LOOP construct in Common Lisp is a macro, just like any other that could be written by a user or bought from a vendor. The LOOP macro defines its own language, for example:

(loop for x across "domain" collect x)

Would return a list like the one below:

(#\d #\o #\m #\a #\i #\n)

The “for x across [string]” is not valid Common Lisp syntax, it is a language that is interpreted by the LOOP macro itself. All Lisp dialects are full of examples like this one and this is clearly a form of Language-Oriented Programming but is absolutely not related to the Domain Model of the application.

Fowler’s definition could be a weird opinion from a single author but many papers and articles put LOP and DSLs as synonyms. Dealing with lists and iterating through them, what is what LOOP does is a domain by itself but since I’m familiar with Fowler’s vocabulary I think he is using the same definition he uses on his Pattern of Enterprise Application Architecture book: “[domain is] Logic that is the real point of the system” and as a synonymous to Business Logic.

When wondering about that I remembered one of my favorite basic object-orientation texts, the one from Meilir Page-Jones called Fundamentals of Object-Oriented Design in UML and his broader definition of domains. It divides classes among four major domains: Fundamental, Architecture, Business and Application.

Fundamental Domain is the very basic aspect of a system. Classes at this level deals with concepts like iterating, dates, text strings, numbers. Those classes are generally useful in all kinds of systems since no matter what is your business about you’ll need them. I would include in this classes that although more sophisticated are still generic, like ORM tools or even very basic frameworks. The important thing about Fundamental Domain’s components is that they can’t impose anything on how your applications is structured.

The Architecture Domain comes right at this structure hole. Classes that deal with this domain will be concerned about the structure of your application. MVC frameworks are a good example.

Ideally, the Business Domain is where the application developer joins the game (we’re not supposed to be creating Fundamental or Architecture classes for each project). This is where the business concepts are modeled into classes, so you’ll find here the Domain Model of a system. The Business Domain can (and ideally will) be used by all applications in the same business developed by that group.

Then the Application Domain defines logic that is exclusive to a given application. As an example a Product class can be used in different systems like an electronic store and a logistics application. The concept of Product is the same for all applications so its behavior and data would be defined in a Business Domain components but the behavior of a store (the classes created to support it) are relative to that application, not to the whole business, and would be contained into the Application Domain.

So LOOP and the like are part of the Fundamental Domain but Fowler’s Domain Specific Languages concept would be somewhere between Business and Application’s domains.

(for more information on Page-Jones Domains applied to LOP read: the Layering Domains essay)

Why LOP?

One of the main things about Page-Jones’ domains is that they draw a very clear line on software reusability. The Fundamental Domain is designed to be reused by others, while the Application Domain is probably useful only for a single scenario.

Language-Oriented Programming is no new paradigm (maybe is a new name but the concept is very old) and I think it’s getting momentum right now because we’re not applying it only in the Fundamental Domain anymore. Experienced programmers in flexible languages like Lisp or Smalltalk are used to creating mini-languages on their daily job tasks. UNIX administrators and users are used to the dozens of mini-languages provided by the environment . The point is that current improvements in programming environments makes easy to apply DSLs even on mainstream languages like Java or Ruby.

For years mainstream development used LOP just at Fundamental Domain levels, defining new query or markup languages for example, but right now we’re seeing movement on bringing LOP to the more valuable layers: Business and even Application levels.

And that’s why big players and scientists are interested so much in this technology. We are trying to use LOP so we can reduce the complexity of developing new systems just like WYSIWYG editors like Microsoft Excel helped reducing complexity on creating new spreadsheets.

History

• 10/05/2007 - First Draft