A common phrase that's bandied about when talking about DomainSpecificLanguages (or indeed any computer language) is that of noisy syntax. People may say that Ruby is less noisy than Java, or that external DSLs are less noisy than internal DSLs. By Syntactic Noise, what people mean is extraneous characters that aren't part of what we really need to say, but are there to satisfy the language definition. Noise characters are bad because they obscure the meaning of our program, forcing us to puzzle out what it's doing.

Like many concepts, syntactic noise is both loose and subjective, which makes it hard to talk about. A while ago Gilhad Braha tried to illustrate his perception of syntactic noise during a talk at JAOO. Here I'm going to have a go at a similar approach and apply it to several formulations of a DSL that I'm using in my current introduction in my DSL book. (I'm using a subset of the example state machine, to keep the text a reasonable size.)

In his talk he illustrated noise by coloring what he considered to be noise characters. A problem with this, of course, is this requires us to define what we mean by noise characters. I'm going to side-step that and make a different distinction. I'll distinguish between what I'll call domain text and punctuation. The DSL scripts I'm looking at define a state machine, and thus talk about states, events, and commands. Anything that describes information about my particular state machine - such as the names of states - I'll define as domain text. Anything else is punctuation and I'll highlight the latter in red.

I'll start with the custom syntax of an external DSL.

events doorClosed D1CL drawOpened D2OP lightOn L1ON end commands unlockDoor D1UL lockPanel PNLK end state idle actions { unlockDoor lockPanel } doorClosed => active end state active drawOpened => waitingForLight lightOn => waitingForDraw end

A custom syntax tends to minimize noise, so as a result you see relatively small amount of punctuation here. This text also makes clear that we need some punctuation. Both events and commands are defined by giving their name and their code - you need the punctuation in order to tell them apart. So punctuation isn't the same as noise, I would say that the wrong kind of punctuation is noise, or too much punctuation is noise. In particular I don't think it's a good idea to try to reduce punctuation to the absolute minimum, too little punctuation also makes a DSL harder to comprehend.

Let's now look at an internal DSL for the same domain information in Ruby.

event : doorClosed , " D1CL " event : drawOpened , " D2OP " event : lightOn , " L1ON " command : lockPanel , " PNLK " command : unlockDoor , " D1UL " state : idle do actions : unlockDoor , : lockPanel transitions : doorClosed => : active end state : active do transitions : drawOpened => : waitingForLight , : lightOn => : waitingForDraw end

Now we see a lot more punctuation. Certainly I could have made some choices in my DSL to reduce punctuation, but I think most people would still agree that a ruby DSL has more punctuation than a custom one. The noise here, at least for me, is the little things: the ":" to mark a symbol, the "," to separate arguments, the '"' to quote strings.

One of the main themes in my DSL thinking is that a DSL is a way to populate a framework. In this case the framework is one that describes state machines. As well as populating a framework with a DSL you can also do it with a regular push-button API. Let's color the punctuation on that.

Event doorClosed = new Event(" doorClosed ", " D1CL "); Event drawOpened = new Event(" drawOpened ", " D2OP "); Event lightOn = new Event(" lightOn ", " L1ON "); Command lockPanelCmd = new Command(" lockPanel ", " PNLK "); Command unlockDoorCmd = new Command(" unlockDoo r", " D1UL "); State idle = new State(" idle "); State activeState = new State(" active "); StateMachine machine = new StateMachine( idle ); idle .addTransition( doorClosed , activeState ); idle .addCommand( unlockDoorCmd ); idle .addCommand( lockPanelCmd ); activeState .addTransition( drawOpened , waitingForLightState ); activeState .addTransition( lightOn , waitingForDrawState );

Here's a lot more punctuation. All sorts of quotes and brackets as well as method keywords and local variable declarations. The latter present an interesting classification question. I've counted the declaring of a local variable as punctuation (as it duplicates the name) but it's later use as domain text.

Java can also be written in a fluent way, so here's the fluent version from the book.

public class BasicStateMachine extends StateMachineBuilder { Events doorClosed , drawOpened , lightOn ; Commands lockPanel , unlockDoor ; States idle , active ; protected void defineStateMachine() { doorClosed . code(" D1CL "); drawOpened . code(" D2OP "); lightOn . code(" L1ON "); lockPanel . code(" PNLK "); unlockDoor . code(" D1UL "); idle .actions( unlockDoor , lockPanel ) .transition( doorClosed ).to( active ) ; active .transition( drawOpened ).to( waitingForLight ) .transition( lightOn ). to( waitingForDraw ) ; }

Whenever two or three are gathered together to talk about syntactic noise, XML is bound to come up.

<stateMachine start = " idle "> <event name=" doorClosed " code=" D1CL "/> <event name=" drawOpened " code=" D2OP "/> <event name=" lightOn " code=" L1ON "/> <command name=" lockPanel " code=" PNLK "/> <command name=" unlockDoor " code=" D1UL "/> <state name=" idle "> <transition event=" doorClosed " target=" active "/> <action command=" unlockDoor "/> <action command=" lockPanel "/> </state> <state name=" active "> <transition event=" drawOpened " target=" waitingForLight "/> <transition event=" lightOn " target=" waitingForDraw "/> </state> </stateMachine>

I don't think we can read too much into this particular example, but it does provide some food for thought. Although I don't think we can make a rigorous separation between useful punctuation and noise, the distinction between domain text and punctuation can help us focus on the punctuation and consider what punctuation serves us best. And I might add that having more characters of punctuation than you do of domain text in a DSL is a smell.

(Mikael Jansson has put out a lisp version of this example. Mihailo Lalevic did one in JavaScript.)