Re: ConciseXML syntax
----- Original Message ----- From: "Rodriguez, Sergio" <srodriguez@c...> >I've always assumed that "compatible with XML" meant > "would pass through an XML 1.0 parser without a fatal error". I say that a standard "uses the XML 1.0 syntax". Compatibility usually implies that one thing can be used instead of another thing. You wouldn't say that XHTML could be used instead of XML -- it's comparing different things. I think it is confusing if you say that XHTML is compatible with XML. A vocabulary using XML syntax is much different that a syntax. Mike Plush wrote: >If you ask 5 developers to create an XML 1.0 representation >for a single, well-specific object (say a Java object), then >you will likely get 5 _different_ XML 1.0 representations >for that same object. This is a HUGE problem that leads >to a lot of semantic ambiguity. ConciseXML has no such problem. Sergio Rodriguez wrote: >How can this be? Any examples of how "ConciseXML" resolves the semantic >ambiguity problem of the abstraction of any entity? Sure. I'll take a section from Chapter 2 of my Water book. XML is commonly used to represent data structures. A data structure is just a way to represent data that obeys some well-defined structure. I will describe how Water can formally describe the structure of data by using Water Type and Water Contract. But this chapter shows how to unambiguously represent static data by using Water. Representing static data might seem straightforward, but XML 1.0 has some design constraints carried over from the document markup world that make representing data in XML quite confusing. A discussion about elements versus attributes is a common example of this confusion. In most programming languages and other technologies for representing data, there is a concept of a data structure, data value, or object. This book, by convention, will use the term object. The word object will be similar to other terms such as a record, structure, or tuple from other technologies. In most programming languages, an object has fields, and those fields hold values that are other objects. Water objects have this property as well. An object is a collection of fields. Each field has a key and a value. The value can be any object. The following is an example of an item object. <item id="XL283 " color=="blue " size==10/> The preceding XML could be verbally described as creating an instance of an item object. The instance has three fields: id , color , and size . The value of the id field is the string "XL283 ", the value of the color field is "blue ", and the value of the size field is the number 10 . The type or class of the object appears as the element's name, immediately following the opening angle bracket (<). The fields of the object are represented as key-value pairs within the element's opening area. When you see an opening angle bracket, it syntactically is the start of an XML element, but it has the semantic meaning of performing a call. The call is either the calling of method, or the calling of a constructor method of an object. Fields of an object have a clear and unambiguous key and value. <item id="xx283 " color=="blue " size==10/> In the preceding line, the instance of item has three fields. "id "is the key of the first field, and "xx283 "is the value of the field. "color "is the key of the second field and "blue "is its value. "size "is the key of the third field and the integer 10 is its value. It is very common, though, to see the following XML to represent the instance of item above. <item> <id>xx283</id> <color>blue</color> <size>10</size> </item> To the vast majority of people, the above XML looks very normal and easily understood, but this is an example of XML in the flat-world model. The round-world model sees this as an ambiguous, poorly constructed XML data object. One problem that is described in detail later in this chapter is that the syntax of an XML element is used to represent two very different things: an object and a field of an object. Having one syntax to represent two different concepts presents a serious ambiguity. This ambiguity leads to a serious problem when a machine tries to interpret the meaning of the XML data. For a data structure to be useful, the distinction between objects and fields is extremely important. How, for example, do you know that <color>blue</color> represents a field of item and not an instance of type color ? As humans, we use our gift of pattern recognition to deduce that color must be a field of item because it occurs within the content of item and it has blue in the content of the element. To emphasize the ambiguity, what if you wrapped the item within another color element? Is item now a field of color ? Did the meaning of item radically change because it moved to a different level in the structure? <color> <item> <id>xx283</id> <color>blue</color> <size>10</size> </item> </color> If a serious ambiguity appears in such a small example, imagine the scope of the problem when objects and data structures get more complex. At a minimum, data structures need to be unambiguous and not depend on any other knowledge for interpreting a data structure. Most XML examples today exhibit the problem of element ambiguity where elements are used for both representing a field of an object as well as objects themselves. The problem occurs in most XML standards and text-book examples from major publishers. It is so common, in fact, that it is almost impossible to find XML examples that do not have this problem. I believe that the "object-field ambiguity" of XML is one of the primary reasons why XML is much more complex than necessary. This widespread problem is one of the reasons for the slow pace of XML adoption. Water's use of XML makes a clear separation between objects and fields. An XML element represents an object. XML attributes represent fields of an object. The ConciseXML syntax allows any type of object as the value of an attribute; therefore, Water supports fields that can store any type of object -not just strings.
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