NXDL: should primitiveType be an enumeration instead of a union?

[woutdenolf]

The NXDL schema defines a complexType called fieldType with this attribute

<xs:attribute name="type" type="nx:primitiveType" default="NX_CHAR">

and the type of the type is defined as

<xs:simpleType name="primitiveType">
	<xs:annotation>
		<xs:documentation>any valid NeXus field or attribute type</xs:documentation>
	</xs:annotation>
	<xs:union memberTypes="
		nxdl:ISO8601
		nxdl:NX_BINARY
		nxdl:NX_BOOLEAN
		nxdl:NX_CCOMPLEX
		nxdl:NX_CHAR
		nxdl:NX_COMPLEX
		nxdl:NX_DATE_TIME
		nxdl:NX_FLOAT
		nxdl:NX_INT
		nxdl:NX_NUMBER
		nxdl:NX_PCOMPLEX
		nxdl:NX_POSINT
		nxdl:NX_QUATERNION
		nxdl:NX_UINT
		nxdl:NX_CHAR_OR_NUMBER
		"/>
</xs:simpleType>

and NX_CHAR, NX_INT, ... etc are simple types

<xs:simpleType name="NX_CHAR">
	<xs:annotation>
		<xs:documentation>
			A string of characters. The preferred string encoding is :index:`UTF-8`. 
			This is the default field type.
		</xs:documentation>
	</xs:annotation>
	<xs:restriction base="xs:string" />
</xs:simpleType>

<xs:simpleType name="NX_INT">
	<xs:annotation>
		<xs:documentation>any representation of an integer number</xs:documentation>
	</xs:annotation>
	<xs:restriction base="xs:integer" />
</xs:simpleType>

Some values of the field's type attribute that are valid

XML	Why
type="NX_FLOAT"	It's a valid string (NX_CHAR).
type="NX_INT"	It's a valid string (NX_CHAR).
type="banana"	It's also a valid string (NX_CHAR).
type="42"	Valid as NX_INT and NX_CHAR.
type="3.14"	Valid as NX_FLOAT and NX_CHAR.

I think what NXDL actually intended is this

<xs:simpleType name="primitiveType">
    <xs:restriction base="xs:string">
        <xs:enumeration value="NX_BINARY"/>
        <xs:enumeration value="NX_BOOLEAN"/>
        <xs:enumeration value="NX_CCOMPLEX"/>
        <xs:enumeration value="NX_CHAR"/>
        <xs:enumeration value="NX_COMPLEX"/>
        <xs:enumeration value="NX_DATE_TIME"/>
        <xs:enumeration value="NX_FLOAT"/>
        <xs:enumeration value="NX_INT"/>
        <xs:enumeration value="NX_NUMBER"/>
        <xs:enumeration value="NX_PCOMPLEX"/>
        <xs:enumeration value="NX_POSINT"/>
        <xs:enumeration value="NX_QUATERNION"/>
        <xs:enumeration value="NX_UINT"/>
        <xs:enumeration value="NX_CHAR_OR_NUMBER"/>
        <xs:enumeration value="ISO8601"/>
    </xs:restriction>
</xs:simpleType>

Which essentially means that the simple type definitions NX_CHAR, NX_INT, ... are not actually used.