IfcXML list-of-values serialization for string derived simple types

As you're probably aware in IFC4 we introduced several shortcuts in the serialization of step-xml by means of the tagless list-of-values configuration option.

### ifc2x3

~~~xml








~~~

### ifc4

~~~xml








~~~

Small difference in XSD, but big difference in XML because the xs:list here is merely a space delimited value of an attribute and xs:sequence would give you a full tag-decorated sequence of child nodes.

I'm somewhat surprised though to see this also applied in string-based attributes such as `MiddleNames` on `IfcPerson`.

~~~xml









~~~

The part 28 text is of course a bit vague on this:

> By definition, all representations of the data types BOOLEAN, INTEGER, LOGICAL, NUMBER, and REAL do not
> contain whitespace, and the XML list form – tokens separated by whitespace – is an unambiguous
> representation.

> NOTE 2 When the base-type of the EXPRESS aggregation data type is STRING or BINARY, or a defined data
> type whose fundamental type is STRING or BINARY, 8.2.2.2 generally applies, but this subclause applies when
> specified by Table 3.

So my summary don't use this representation form on strings unless specified in Table 3. The note under Table 3 says:

> NOTE 4 As specified in 10.2.8, tagless="true" shall not be specified unless the base-type is one of the following:
>
> — a simple type as defined above,
> — a STRING data type whose values **will not contain whitespace**,

I didn't really understand when it is sufficiently established when a *STRING data type will not contain whitespace*. Maybe whitespace in a middle name is unlikely, but IfcLabel in itself will not prevent this so we end up with an ambiguous serialization.

My proposal. Never use list-of-values on any string-based type. Not even IfcIdentifier.