97 Things Every Programmer Should Know

Chapter 23. Domain-Specific Languages

Michael Hunger

WHENEVER YOU LISTEN TO A DISCUSSION BY EXPERTS in any domain, be it chess players, kindergarten teachers, or insurance agents, you’ll notice that their vocabulary is quite different from everyday language. That’s part of what domain-specific languages (DSLs) are about: a specific domain has a specialized vocabulary to describe the things that are particular to that domain.

In the world of software, DSLs are about executable expressions in a language specific to a domain, employing a limited vocabulary and grammar that is readable, understandable, and—hopefully—writable by domain experts. DSLs targeted at software developers or scientists have been around for a long time. The Unix “little languages” found in configuration files and the languages created with the power of LISP macros are some of the older examples.

DSLs are commonly classified as either internal or external:

Internal DSLs

Are written in a general-purpose programming language whose syntax has been bent to look much more like natural language. This is easier for languages that offer more syntactic sugar and formatting possibilities (e.g., Ruby and Scala) than it is for others that do not (e.g., Java). Most internal DSLs wrap existing APIs, libraries, or business code and provide a wrapper for less mind-bending access to the functionality. They are directly ...