How to Marshal Maps to XML in Go: Beyond Structs with `xml.Marshaler`

Marshall Maps to XML in Go: Beyond structs with xml.Marshaler

Encountering the error "xml: unsupported type: map[string]int" while attempting to marshal a map to XML data is not uncommon. While maps marshal well into JSON, understanding their behavior with XML requires a deeper dive.

The key difference lies in the nature of XML itself. Unlike JSON, which treats data as a nested hierarchy of key-value pairs, XML structures data as a hierarchy of elements and attributes. This distinction prohibits the direct conversion of maps.

To overcome this challenge, a custom implementation is necessary. Here, we employ the xml.Marshaler interface, which allows for tailored marshaling logic. By implementing MarshalXML, we define the specific way the map should be serialized.

For instance, we can create a custom type called StringMap that embeds the map[string]string data structure. Within MarshalXML, we construct XML tokens, including start and end elements, to represent the map. For each key-value pair, we generate start and end elements with the key as the element name and the value as character data.

By implementing this MarshalXML method, StringMap gains the ability to marshal itself into XML format. Now, marshalling maps becomes straightforward, involving only a call to xml.MarshalIndent. This custom approach provides a viable solution beyond relying solely on structs for XML marshaling.

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