How to represent alternates in a Go xml struct tag

I'm trying to write the appropriate set of struct tags to parse the XML version of UCUM. Following are two examples of the unit tag:

<unit Code="deg" CODE="DEG" isMetric="no" class="iso1000">
   <name>degree</name>
   <printSymbol>°</printSymbol>
   <property>plane angle</property>
   <value Unit="[pi].rad/360" UNIT="[PI].RAD/360" value="2">2</value>
</unit>

<unit Code="[degF]" CODE="[DEGF]" isMetric="no" isSpecial="yes" class="heat">
   <name>degree Fahrenheit</name>
   <printSymbol>°F</printSymbol>
   <property>temperature</property>
   <value Unit="degf(5 K/9)" UNIT="DEGF(5 K/9)">
      <function name="degF" value="5" Unit="K/9"/>
   </value>
</unit>

The tricky part is the contents of the value tag, which can either be a string (which I'd represent with a string attribute) or a function (which would require a struct of its own). Here's what I've got so far:

type Unit struct {
	Code             string `xml:Code,attr`
	CodeCaps         string `xml:CODE,attr`
	IsMetric         bool   `xml:isMetric,attr,omitempty`
	IsSpecial        bool   `xml:isEmptySpecial,attr,omitempty`
	Class            string `xml:class,attr`
	Name             string `xml:name`
    PrintSymbol      string `xml:printSymbol,chardata`
	DimensionTypeKey string `xml:property,chardata`
	Value            struct {
		Unit       string `xml:Unit,attr`
		UnitCaps   string `xml:UNIT,attr`
		Value      string `xml:Value,attr`
		PrintValue string `xml:,chardata`
        Function   struct { ... }
	} `xml:value`
}

How do I accurately describe this XML with struct tags?

Your code is almost working; here's the fixed version.

Basically you need the following adjustments:

• In struct tags, the values—bits after the xml: prefix—must be enclosed in double quotes, like this:

    `xml:"foo,attr"`
  

• There's no need to specify the ,chardata bit when you already just want to get whatever is between the element's tags.

• To extract the <function> element just provide a data type for is: the parser will extract it if it's present or not if it's absent.

  To tell whether there was <function> or not, you can check the value of the PrintValue field: if it's not all-whitespace, there was no <function> element; otherwise there was.

  Alternatively, define a separate struct data type for unmarshaling this element, and have the field for it defined as a pointer to that type,
  like in

    type Function struct { ... }
    ...
    Function *Function `xml:"function"`
  

  This way, if there was no <function> element the field's value will be nil, otherwise it will point to a heap-allocated Function instance.