Criticism
Although XML Schema is successful in that it has been widely adopted and largely achieves what it set out to, it has been the subject of a great deal of severe criticism, perhaps more so than any other W3C Recommendation.
A good summary of the criticisms is provided by James Clark (who promotes his own alternative, RELAX NG):
- There are many surprises in the language, for example that restriction of elements works differently from restriction of attributes.
- The W3C Recommendation itself is extremely difficult to read. Most users find W3Cs XML Schema Primer much easier to understand
- XSD lacks any formal mathematical specification. (This makes it difficult to reason about schemas, for example to prove that a modification to a schema is backwards compatible.)
- XSD 1.0 provided no facilities to state that the value or presence of one attribute is dependent on the values or presence of other attributes (so-called co-occurrence constraints). This has been fixed in XSD 1.1.
- XSD offers very weak support for unordered content.
- The set of XSD datatypes on offer is highly arbitrary.
- The two tasks of validation and augmentation (adding type information and default values) should be kept separate.
Read more about this topic: XML Schema (W3C)
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