I've hit a significant, if purely internal, milestone in the DSL book recently. I also find that people regularly ask me what the status is of the book is. So it seems like a good moment to post a note about where I am with the book and where I see things going.

The most common question I get is "when will the book be out?" The way things look at the moment, I'd estimate it appearing some time in 2010. That's an estimate, with all the usual hedges that apply to such things.

To help see how the current state meshes in with that estimate, I'll describe the general progress of how books work, at least for me. The first phase is writing the First Review Draft. This means writing all the material in the book, to the level that I can submit it to peer review. For my larger books (eg Refactoring, P of EAA) this takes about 12-18 months. This book is taking longer, I've already been at it for nearly two years, and I suspect there's another year to go. Once I have the First Review Draft, it goes out to review. This takes time for people to read it, get their comments to me, and for me to modify the text based on the review. I usually do two rounds of review - and it takes around 6 months for that to happen. Once I'm done with formal review the book is a Final Draft. At this point it goes off for production - which includes copy-edit, indexing, layout, printing, etc. That takes another 6 months. So I estimate that the book will appear about a year after I get a First Review Draft.

Not all authors work this way, some write a few chapters and sent them for review while they write a few more chapters. I use the approach I do because it was how I did my first book and it seemed to work very well.

To describe the current state of the book, I need to talk a bit about its structure. The final structure of the book will be a DuplexBook, but while I'm working on it I tend to think of it in terms of a set of subject areas, where each subject area will end up as a set of narratives and topics within the duplex structure.

My recent milestone was completing my first coherent pass at the internal DSL subject area. By this I mean I now have what I think is a coherent draft of the that subject area. It will still need more work before I get to the First Review Draft, but reaching a coherent draft is a big point for me because it means I have reached a point where it's looking in good shape. Here's my current list of subject areas and their status:

• Introduction: coherent (this is the first three narratives: An Introductory Example, Using Domain Specific Languages, and Implementing DSLs).
• External DSLs: incoherent
• Internal DSLs: coherent
• Code Generation: coherent, but with some important bits missing
• Alternative Computational Models: incoherent
• Error Diagnostics: open.
• Language Workbenches: open.

I've been working since March on getting the internal DSL section into a coherent state. I suspect it will take me a similar amount of time to get external DSLs into coherence, and a similar time again to get alternative computational models into coherence. This is why I think I'm at least a year off First Review Draft. These are also the next two subject areas that I'm going to work on, in that order.

As well as those two subject areas to make coherent, I also have some holes to fill and a couple of elephants to deal with. The holes are topics within subject areas that aren't really complete yet, although the overall structure is reasonable. Code generation is a good example of that. I'm pretty happy with what I have, and it is coherent. But I need to address at least one topic (the Generation Gap pattern) before I can really say that subject area is done. These holes don't take a huge amount of time to fill and I tend to work on them as the mood takes me - often I like to take a break in the middle of a major topic for something like this.

The two elephants are the open topics of error diagnostics and language workbenches. These are elephants because I don't know how I'm going to tackle them, or indeed if I'm going to put much effort into tackling them. It may be that I'll do little more than a skimpy overview chapter, or I may spend six months each on them. I'm deliberately putting off those decisions while I work on the next two subject areas.

Of course this road-map could all change. One of the reasons I don't like doing incremental review (eg sending off the internal DSL section for a formal review now) is that working on one section can cause me to completely rethink another. This has already happened. I did a substantial chunk of work on the internal DSL section late in 2007 and then worked for a while on what is now the Computational Network and Dependency Network patterns at the end of 2007. Working on those patterns made me realize my internal DSL thinking was all wrong and I thus needed to rewrite that whole section. This kind of thing is common with a topic area like this where there is no pre-existing structure. (This is different to a book on an existing technology, eg UML Distilled, where the thing I'm describing already has an understood structure before I start.)

Having said that, if you're interested in what I'm doing I think I'm at the point where you can take a good look at the introductory and internal DSL sections and it should make sense. I'd certainly be happy for feedback on those sections. Just beware that there are cross links between those and other sections that may still get worked on.

I'll update this page when I hit a major milestone or major re-plan. If you want more granular updates I have a more detailed feed. If you want to send me comments on work so far, please do so. I have a mailing list for the book, I'd prefer all comments to go to that mailing list so that there's an opportunity for discussion. So if you do send me comments, please let me know if you're happy with them going on the list or not and whether you wish to join the list.