Skip to main content

The true role of domain specific languages

It is easy to be confused by the term domain specific language. It sounds like a fancy term for jargon. It is often interpreted to mean some form of specialized language. I would like to explore another role for them: as vehicles for policy statements.

In mathematics there are many examples of instances where it is easier to attack a problem by solving a more general, more uniform, problem and then specializing the result to get the desired answer.

It is very similar in programming: most programs take the form of a general mechanism paired with a policy that controls the machine. Taken seriously, you can see this effect down to the smallest example:
fact(n) where n>0 is n*fact(n-1);
fact(0) is 1

is a general machine for computing factorial; and the expression:fact(10) is a policy 'assertion' that specifies which particular use of the factorial machine is intended.

One important aspect of policies is that they need to be intelligible to the owner of the machine: unlike the machine itself which only needs to be trusted by the owner.

So, one critical role for a DSL is to be the policy language for the user of a mechanism.

Starting from this light leads to interesting conclusions. In particular, DSLs should be ubiquitous not rare; in particular, DSLs support the role that abstractions play in programming: by layering an appropriate syntax on top of the expression of the abstraction. It also means that programming languages should be designed to make it easy to construct and use DSLs within systems as well as externally.

A simple example: the query notation in Star, as well as in formalisms such as LINQ, may be better viewed as simple DSLs where the user is the programmer. The difference between these and more traditional DSLs is that the DSL expressions are embedded in the program rather than being separated from the code.

I think that embracing the DSL in this way should make it easier for a programming language to survive the evolution of programming itself. With an effective DSL mechanism a language 'extension' encoding a new language concept (for example, queries over C# or objects over C) and be done without invalidating the existing language. (The mechanisms in Star go further: it is possible to construct a DSL in Star that either augments the base language or even replaces it. We have used both approaches.)

It also explains why LISP's macro facilities have allowed it to survive today more-or-less unchanged (nearly 60 years after being invented) whereas languages like Java and C++ have had to undergo painful transitions in order to stay relevant.

Popular posts from this blog

Comments Should be Meaningless

This is something of a counterintuitive idea: Comments should be meaningless What, I hear you ask, are you talking about? Comments should communicate to the reader! At least that is the received conventional wisdom handed does over the last few centuries (decades at least). Well, certainly, if you are programming in Assembler, or C, then yes, comments should convey meaning because the programming language cannot So, conversely, as a comment on the programming language itself, anytime the programmer feels the imperative to write a meaningful comment it is because the language is not able to convey the intent of the programmer. I have already noticed that I write far fewer comments in my Java programs than in my C programs.  That is because Java is able to capture more of my meaning and comments would be superfluous. So, if a language were able to capture all of my intentions, I would never need to write a comment. Hence the title of this blog.

In Praise of Crappy Code

Not all code needs to be perfect! This is pretty heretical thinking for a software engineer. The issue is simple: how do you go about developing software for a small fixed budget. Imagine that you have $500 to implement a solution to a problem. If you spend more than that you will never recoup the extra that you spent. This comes up a lot in systems integration scenarios and also in customization efforts. Someone wants you to 'tweak' an application that they are using; you know that no-one else would want that feature and that if you spend more than what the customer will pay you will end up losing money. From the customer's perspective, the common 'time and materials' approach to quoting for software development is a nightmare. Being able to offer a fixed price contract for a task is a big benefit for the customer. But, how much do you quote for? Too much and you scare the customer away. Too little and you lose money. This is where 'crappy code' com...

Action at a distance

We are currently writing our first draft of the SOA Reference Architecture. Everyone is very busy doing their bit. My current section is on the Real World Effect of using a service. The RA is really an abstract architecture: we are not focusing on things like SOAP, or any of the other 60+ Web service specifications out there. We are trying to get at the essence of makes SOA special and how it can be made to work. It is a pretty basic aspect of services that we are trying to get something to happen: buy a book, get the weather forecast whatever. In other words: its action at a distance. I am communicating with you in the hope that we can get some mutual benefit. This already distinguishes SOA from the Web, whose basic abstraction is to acquire a representation of a resource will be rendered locally for human consumption. Actions are not inherently about representations, they are about changing the world - one book at a time. Action itself is a very difficult concept to get hold of. It ...