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To me, the most fundamental reason for datatypes is to allow them to be
part of the contract between producers and consumers of data. Suppose we
have a number of systems that work with data about people. Simon has
suggested that we should specify only the names and nesting of elements and
attributes. Consider the following example:
<person>
<ssn>123456789</ssn>
<name>Jonathan Robie</name>
<children>3</children>
</person>
Using an XML 1.0 validator, if the above example is valid, so is the one below:
<person>
<ssn></ssn>
<name>Jonathan Robie</name>
<children>Hickory dickory dock</children>
</person>
If the applications that use this data require data of the appropriate
type, and we want validation to be able to determine whether the contract
is being followed, then we have to allow data types to be declared.
Jonathan
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