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copied!<p><strong>Here are part of <a href="http://blog.jclark.com/2008/11/some-thoughts-on-oslo-modeling-language.html" rel="nofollow noreferrer">James Clark's thoughts on M</a></strong>:</p> <p>" I see several major things missing in M, whose absence might be acceptable for a database application of M, but which would be a significant barrier for other applications of M. Most fundamental is order. M has two types of compound value, collections and entities, and they are both unordered. In XML, unordered is the poor relation of ordered. Attributes are unordered, but attributes cannot have structured values. Elements have structure but there's no way in the instance to say that the order of child elements is not significant. The lack of support for unordered data is clearly a weakness of XML for many applications. On the other hand, order is equally crucial for other applications. Obviously, you can fake order in M by having index fields in entities and such like. But it's still faking it. A good modeling language needs to support both ordered and unordered data in a first class way. This issue is perhaps the most fundamental because it affects the data model.</p> <p>Another area where M seems weak is identity. In the abstract data model, entities have identity independently of the values of their fields. But the type system forces me to talk about identity in an SQL-like way by creating artificial fields that duplicate the inherent identity of the entity. Worse, scopes for identity are extents, which are flat tables. Related to this is support for hierarchy. A graph is a more general data model than a tree, so I am happy to have graphs rather than trees. But when I am dealing with trees, I want to be able to say that the graph is a tree (which amounts to specifying constraints on the identity of nodes in the graph), and I want to be able to operate on it as a tree, in particular I want hierarchical paths.</p> <p>One of the strengths of XML is that it handles both documents and data. This is important because the world doesn't neatly divide into documents and data. You have data that contains documents and document that contain data. The key thing you need to model documents cleanly is mixed text. How are you going to support documents in M? The lack of support for order is a major problem here, because ordered is the norm for documents.</p> <p>A related issue is how M and XML fit together. I believe there's a canonical way to represent an M value as an XML document. But if you have data that's in XML how do you express it in M? In many cases, you will want to translate your XML structure into an M structure that cleanly models your data. But you might not always want to take the time to do that, and if your XML has document-like content, it is going to get ugly. You might be better off representing chunks of XML as simple values in M (just as in the JSON world, you often get strings containing chunks of HTML). M should make this easy. You could solve this elegantly with RELAX NG (I know this isn't going to happen given Microsoft's commitment to XSD, but it's an interesting thought experiment): provide a function that allows you to constrain a simple value to match a RELAX NG pattern expressed in the compact syntax (with the compact syntax perhaps tweaked to harmonize with the rest of M's syntax) and use M's repertoire of simple types as a RELAX NG datatype library.</p> <p>Finally, there's the issue of standardization. The achievement of XML in my mind isn't primarily a technical one. It's a social one: getting a huge range of communities to agree to use a common format. Standardization was the critical factor in getting that agreement. XML would not have gone anywhere as a single vendor format. It was striking that the talks about Oslo at the PDC made several mentions of open source, and how Microsoft was putting the spec under its Open Specification Promise so as to enable open source implementations, but no mentions of standardization. I can understand this: if I was Microsoft, I certainly wouldn't be keen to repeat the XSD or OOXML experience. But open source is not a substitute for standardization.</p> <p> "</p> <p>Read <a href="http://blog.jclark.com/2008/11/some-thoughts-on-oslo-modeling-language.html" rel="nofollow noreferrer"><strong>here</strong></a> James Clark's blog article on the <a href="http://msdn.microsoft.com/en-us/library/dd285282.aspx" rel="nofollow noreferrer">Oslo Modelling language</a>.</p>

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