I speak quite often about DSL and how to build them, but so far I did not do was explain why you need a DSL at all. After all, since you are reading this blog, you already know how to program. Can’t we just use “normal” programming languages to do the same job? We can even do with a dash of fluent interfaces and Domain Driven Design to make the code easier to read.
We need to inspect the different needs that lead the drive toward a DSL.
A Technical DSL
A technical DSL is supposed to be consumed by someone that understands development. It is meant to express matters in a more concise form, but it is still very much a programming language at heart. The main difference is that it is a language focused on solving the specific problem at hand. As such, it has all the benefits (and drawbacks) of single purpose languages. Examples for technical DSL include Rake, Binsor, Rhino ETL, etc.
The driving force around that is that you want to have richer expressiveness in specifying what you want to happen. Technical DSL are usually easier to write, because your target audience already understands programming.
In fact, the use of programming features can make a DSL very sweet indeed. We have already seen a Rake sample, so let us see a Binsor sample:
for type in Assembly.Load(“MyApplication.Web”).GetTypes():
continue unless type.IsAssignableFrom(Controller)
component type.FullName, type
So we take three lines to register all the controllers in the application. That is quite a bit of expressiveness. Of course, it assumes that you are familiar with the API and its usage, which is not always true, which leads nicely to business focused DSL
A Business DSL
A business DSL is focused on being (at least) readable to a businessperson with no background in programming.
This type of DSL is mainly expressive in the terms of the domain, and it has a lot less emphasis on the programming features that may still exists. It is also tend to be much more declarative than technical DSL in general and a lot more emphasis is placed on the nature of the DSL so those would not really be necessary.
I can’t really think of a good example of a business DSL in the open. A good example of DSL that I have run into include a cellular company that had to have some way to handle all the myriad of different contracts and benefits that it had. It also needed to handle this with a fairly rapid time to market, since the company needed to respond quickly to market conditions.
The end result was a DSL in which you could specify the different conditions and their results. For instance, to specify that you get 300 minutes free if you speak over 300 minutes a month, you would write something similar to this:
if CallMinutesForCurrentMonth > 300 and HasBenefit “300 Minutes Free!!!”:
AddCallMinutes -300, "300 Minutes Free!!!"
It was fairly simple to define a small language that could describe most of the types of benefits that the company wanted to express. The rest was a matter of naming conventions and dropping files in a specified folder, to be picked up and processed at regular intervals. The structure that surrounds a DSL is a subject that deserves quite a bit of attention on its own.
A businessperson may not always be able to write actions using a business DSL (more on that later), but they should be able to read and understand it. After all, it is their business and their domain that you are trying to describe.
Now, why shouldn’t a businessperson be able to write actions using a business DSL?
The main reason, as I see it, is of error handling. No, I don’t mean in the actual running of the DSL action, I mean when writing it.
A DSL is supposed to be read like a language, but it is still a programming language, and those have little tolerance for such thing as bad casing for keywords, for instance. Certain types of users will simply be unable to go over the first hurdle in the road they face, because of this.
It is important to know your audience, and it is even more important not to be contemptuous toward that mythical businessperson. You may not think that this person can understand programming, only to discover that there are quite a bit of automation going on in their life already, powered by VBScript and excel macros.
If you can leverage this knowledge, you have a very powerful combination in your hand, because you can provide that businessperson the tools, and he can provide the knowledge and the required perspective.
I am not quite sure about this classification, but it certainly has its place. Another name for this may be the IT DSL. This type of a DSL it often used to expose the internal of an application to the outside world.
Modern games are usually engines that are being configured using some sort of a DSL. In fact, I fondly remember building levels in Neverwinters Nights.
More serious usage for this can certainly be found, such as a DSL that lets you go into the internals of an application and mange it. Think about the ability to run a script that will take re-route all traffic from a server, wait for all current work to complete, and then take the server down, update it and bring it up again.
Right now, it is possible to do this with shell scripts of various kinds, but most enterprise application can certainly have more visibility into them than already exist, and a DSL that will allow me to inspect and modify the internal state would be very welcome.
I can certainly think of a few administrators who would be grateful to have more power in their hands.
To conclude, can you think of other types of DSL in use?