Одна из сессий на нынешней PDC посвящена Oslo. Фаулер однако уже успел познакомиться с превью этой сессии и выложил Небольшой обзор.

Oslo is a project at Microsoft, of which various things have been heard but with little details until this week's PDC conference. What we have known is that it has something to do with ModelDrivenSoftwareDevelopment and DomainSpecificLanguages.

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Oslo has three main components:

* a modeling language (currently code-named M) for textual DSLs
* a design surface (named Quadrant) for graphical DSLs
* a repository (without a name) that stores semantic models in a relational database.

(All of these names are current code names. The marketing department will still use the same smarts that replaced "Avalon and Indigo" with "WPF and WCF". I'm just hoping they'll rename "Windows" to "Windows Technology Foundation".)

The textual language environment is bootstrapped and provides three base languages:

* MGrammar: defines grammars for Syntax Directed Translation.
* MSchema: defines schemas for a Semantic Model
* MGraph: is a textual language for representing the population of a Semantic Model. So while MSchema represents types, MGraph represents instances. Lispers might think of MGraph as s-expressions with a ugly syntax.

You can represent any model in MGraph, but the syntax is often not too good. With MGrammar you can define a grammar for your own DSL which allows you to write scripts in your own DSL and build a parser to translate them into something more useful.

Using the state machine example from my book introduction, you could define a state machine semantic model with MSchema. You could then populate it (in an ugly way) with MGraph. You can build a decent DSL to populate it using MGrammar to define the syntax and to drive a parser.

There is a grammar compiler (called mg) that will take an input file in MGrammar and compile it into what they call an image file, or .mgx file. This is different to most parser generator tools. Most parser generators tools take the grammar and generate code which has to be compiled into a parser. Instead Oslo's tools compile the grammar into a binary form of the parse rules. There's then a separate tool (mgx) that can take an input script and a compiled grammar and outputs the MGraph representation of the syntax tree of the input script.

More likely you can take the compiled grammar and add it to your own code as a resource. With this you can call a general parser mechanism that Oslo provides as a .NET framework, supply the reference to the compiled grammar file, and generate an in-memory syntax tree. You can then walk this syntax tree and use it to do whatever you will — the parsing strategy I refer to as Tree Construction.

The parser gives you a syntax tree, but that's often not the same as a semantic model. So usually you'll write code to walk the tree and populate a semantic model defined with MSchema. Once you've done this you can easily take that model and store it in the repository so that it can accessed via SQL tools. Their demo showed entering some data via a DSL and accessing corresponding tables in the repository, although we didn't go into complicated structures.

И еще

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).