Unfortunately I can't test this with IBM DFDL, as one of our limitations is that you can't specify assert or discriminator on a global element or global type, and by definition a prefix length type must be global
. However, although the DFDL spec does not explicitly say that assert or discriminator
can be used on such a simple type, that's not the way the spec usually works; it usually says when you
can't do something. So, given that there is no wording to say you can't, both are allowed.
I believe the DFDL spec does not specify whether the prefixLengthType for an element of lengthKind 'prefixed' can have assertions, and
if those assertions are evaluated, and when.
Consider this example below. We really want the assert on the bufferPayload type to be evaluated before the length computed from the prefix is used as a length because we'd like good diagnostic behavior that points out that the prefix value is way out
of realistic range.
In many formats we see 4-byte length prefixes, but the range of values for specific fields is relatively tiny, often even just 1 to 100. Enforcing this via the assert expressed on the prefixLengthType's simpleType definition makes this easy.
I could go either way on whether a prefixLengthType can carry a discriminator. But an assert should definitely be allowed.
...
<complexType>
<sequence>
<element name="payload" type="xs:hexBinary"
dfdl:lengthKind="prefixed"
dfdl:prefixIncludesPrefixLength="no"
dfdl:prefixLengthType="p8:bufferPayload"/>
</sequence>
</complexType>
...
<simpleType name="bufferPayload">
<annotation>
<appinfo source="http:>
<dfdl:assert>{ dfdl:checkConstraints(.) }</dfdl:assert>
</appinfo>
</annotation>
<restriction base="p8:unsignedLong">
<minInclusive value="0"/>
<maxInclusive value="32768"/>
</restriction>
</simpleType>