Content can also be empty

atom.go

@@ -1,6 +1,7 @@
 package atom
 
 import (
+	"encoding/xml"
 	"fmt"
 	"mime"
 	"regexp"
@@ -86,3 +87,9 @@ func isValidLanguageTag(tag LanguageTag) bool {
 func NewURN() IRI {
 	return IRI(fmt.Sprint("urn:uuid:", uuid.New()))
 }
+
+// isValidXML checks whether a string is valid XML. It returns a bool.
+func isValidXML(input string) bool {
+	var v interface{}
+	return xml.Unmarshal([]byte(input), &v) == nil
+}

category.go

@@ -16,6 +16,12 @@ type Category struct {
 
 // NewCategory creates a new Category. It returns a *Category and an error.
 func NewCategory(term, content string) (*Category, error) {
+	if content != "" {
+		if !isValidXML(content) {
+			return nil, fmt.Errorf("%v not valid XML", content)
+		}
+	}
+
 	return &Category{Term: term, Content: content}, nil
 }
 
@@ -41,8 +47,10 @@ func (c *Category) Check() error {
 		return fmt.Errorf("label attribute %v of category not correctly escaped", c.Label)
 	}
 
-	if c.Content == "" {
-		return errors.New("content element of category empty")
+	if c.Content != "" {
+		if !isValidXML(c.Content) {
+			return fmt.Errorf("content element %v of category not valid XML", c.Content)
+		}
 	}
 
 	return nil

link.go

@@ -19,6 +19,12 @@ type Link struct {
 
 // NewLink creates a new Link. It returns a *Link and an error.
 func NewLink(href, content string) (*Link, error) {
+	if content != "" {
+		if !isValidXML(content) {
+			return nil, fmt.Errorf("%v not valid XML", content)
+		}
+	}
+
 	return &Link{Href: IRI(href), Content: content}, nil
 }
 
@@ -51,8 +57,10 @@ func (l *Link) Check() error {
 		}
 	}
 
-	if l.Content == "" {
-		return errors.New("content element of link empty")
+	if l.Content != "" {
+		if !isValidXML(l.Content) {
+			return fmt.Errorf("content element %v of link not valid XML", l.Content)
+		}
 	}
 
 	return nil