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Martin Fowler on Oslo

by Jonathan Allen on Nov 08, 2008 |

For many years Martin Fowler has been in the forefront of software engineering. He is often given credit for popularizing techniques such as refactoring and dependency injection. Lately he has been evangelizing domain specific languages, so of course Oslo piqued his interest.

On the plus side, Martin remarks on his preference for file-based versioning and runtime compilation.

One of the big issues with storing stuff like this in a repository is handling version control. The notion that we can all collaborate on a single shared database (the moral equivalent of a team editing one copy of its code on a shared drive) strikes me as close to irresponsible. As a result I tend to look askance at any vendors who suggest this approach. The Oslo team suggests, wisely, that you treat the text files as the authoritative source which allows you to use regular version control tools. Of course the bad news for many Microsoft shops would be that this tool is TFS (or, god-forbid, VSS), but the great advantage of using plain text files as your source is that you can use any of the multitude of version control systems to store it.

A general thing I liked was most of the tools leant towards run-time interpretation rather than code generation and compilation. Traditionally parser generators and many language workbenches assume you are going to generate code from your models rather than interpreting them. Code generation is all very well, but it always has this messy feel to it - and tends to lead to all sorts of ways to trip you up. So I do prefer the run-time emphasis.

While Martin does not offer a formal definition for the term, he does have a rather lengthy article on language workbenches. Below is the list of characteristics he expects to find in a language workbench.

  • Users can freely define new languages which are fully integrated with each other.
  • The primary source of information is a persistent abstract representation.
  • Language designers define a DSL in three main parts: schema, editor(s), and generator(s).
  • Language users manipulate a DSL through a projectional editor.
  • A language workbench can persist incomplete or contradictory information in its abstract representation.

Martin does have some very real concerns about the future of Oslo, an uneasiness that reflected in the recent LINQ to SQL debacle.

One particularly interesting point in this comparison is comparing Oslo with Microsoft's DSL tools. They are different tools with a lot of overlap, which makes you wonder if there's a place for both them. I've heard vague "they fit together" phrases, but am yet to be convinced. It could be one of those situations (common in big companies) where multiple semi-competing projects are developed. Eventually this could lead to one being shelved. But it's hard to speculate about this as much depends on corporate politics and it's thus almost impossible to get a straight answer out of anyone (and even if you do, it's even harder to tell if it is a straight answer).

He concludes his commentary on Olso with,

It was only a couple of hours, so I can't make any far-reaching judgements about Oslo. I can, however, say it looks like some very interesting technology. What I like about it is that it seems to provide a good pathway to using language workbenches. Having Microsoft behind it would be a big deal although we do need to remember that all sorts of things were promised about Longhorn that never came to pass. But all in all I think this is an interesting addition to the Language Workbench scene and a tool that could make DSLs much more prevalent.

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