What is a domain specific language? Anybody using it? And in what way?

(addressing the crux of the question)

I think the first time I saw DSL somewhere and its definition as "domain specific language" I also thought it was a particular, concrete language that I just hadn't heard about -- but, no, its a general term for languages that are tailored to a particular application area.

Ironically, if you had just heard about TCL as a "tool command language" you might think, like DSLs, that there would be lots of TCLs for various tools -- but, no, its the specific name of a particular scripting language.

You can think of DSLs as complex arguments for functions written in a more general programming language. The real programming language parses the DSL code and does something with it, typically, the DSL code only focuses on the what you want to do, and the larger system figures out the how.

Examples of DSL include all query languages (SQL, XPath, ...), all template languages (Django, Smarty, ...), shell scripts, especially including stuff like twill, a command driven web browser (mostly used for automated test), data storage and exchange languages (XML, YAML, ...), and document languages like LaTex, HTML or CSS.

Some languages with very flexible syntax like TCL and Lisp build their DSL directly into the language... when possible. The majority of languages use strings, usually loaded from external files.

Are there any particular advantages of using them? Using them for their intended purposes is very advantageous to the point you will turn to them without knowing, just like you have been using (I presume) SQL or HTML without thinking of them as DSLs.

I'll dare saying there are enough DSLs out there for any sort of application you may need; you almost certainly don't need to learn how to write your own one.

A domain specific language is a language that's written to deal with a specific domain or set of concerns. There are a lot of them around, like make, ant, and rake for describing software builds, or lexx and yacc for language construction. In recent years, they've become popular as some things have combined to make them easier to build. Big among those things has been the increasing popularity of Ruby, which has several features that make it easy to build new DSLs.

Martin Fowler is a big proponent of the idea, as here.

Everything is a DSL...

Assembler: MOV R1 to R2
Compilers: Assignment Statements -- A = A + 1, Conditionals -- IF (TRUE) ..., Branch -- RETURN
HTML: ... describe a nested structure
TCP/IP: describe to/from addresses
PDF: describe text/image placement on paper
Fonts: describe characters

Any language that we use to describe a specific process is a DSL. Unfortunately there is a lack of domain specific languages to describe even our most basic processes, so we use the few languages we do have to describe everything we do. "Zip all html files in my web site" requires 300 lines of 3 or 4 different Languages to complete.

To build a DSL determine the minimum number of characters needed to describe a process that you can remember and does not require documentation. Remember that speed and ease of use are the primary design criteria. Parsing is so fast that any syntax you use is fine, I prefer natural language as my syntax in most cases, "Pay Employees at the first of the month", but domain specific is just that, domain specific, you determine the syntax that best fits the problem.

I would stay away from using other solutions that might be convenient but do not fit the problem such as HTML that was used to define Data (XML). CSV is very useful, it fits most problems. JSON does not fit the ease of use portion, it is overkill that adds unnecessary complications were CSV works for most problem. We use EXCEL a lot for DSL, it works great for describing small problems, under 65K to 1M rows, such as a tree structures or menus, column A is the level, other columns are icons, colors, labels and such (EXCEL is an editable CSV).

I found that HTML did not really solve the problem of page layout, so I got rid of it and defined a DSL that does fit. I defined 6 regions on the page, HEADER, BODY, FOOTER, LEFT/RIGHT MARGINS, and LEFT/RIGHT FULL MARGINS. I could then tell the page generator to add a TITLE BAR, STATUS BAR, MENUS, TABLE, FORMS,..., to specific cells. Each of these Cells could then be split into Rows and Columns to any depth. Page layout takes seconds for any style.

BODY contains a Table of my Employees
HEADER contains a Title Bar caption 'Hello World' with login to Collins Software

A Menu DSL don't fit the page layout DSL, so I built a unique DSL for menus.

Resource My Main Menu
*define:menu,m,Level,Label,Icon,Action;
m,0,file;
m,1,open,open.gif,Dialog Open File;

Each problem is unique, the computer can use any format, it is the human that DSLs are designed for, so make it understandable by humans, something they can type in, and make the language out of real words; for it is real people, places, and things that we are describing.