Make NewCommonAttributes public

README.md

@@ -1,23 +1,26 @@
 # atom
 
-An extensible Atom feed generator library that aims to be very close to
-RFC4287. It diligently checks for compliance with the standard and provides
-functions for easy creation and extension of elements.
+An extensible Atom feed generator library that aims to be very close to RFC4287.
+It diligently checks for compliance with the standard and provides functions for
+easy creation, extension and deletion of elements.
 
 ## Installation
 
 To install the latest version of the module, use the following command:
 
-```
+```sh
 go get git.streifling.com/jason/atom@latest
 ```
 
 ## Usage
 
-This library provides easy to use functions to create and extend elements of an
-Atom feed. The intended way of using it entails using these functions.
+### Basic Feed
 
-```
+This library provides convenient functions to safely create, extend and delete
+elements and attributes of Atom feeds. It also provides checks for all
+constructs' adherence to RFC4287.
+
+```go
 package main
 
 import (
@@ -28,31 +31,26 @@ import (
 )
 
 func main() {
- feed, err := atom.NewFeed("Example Feed")
- if err != nil {
+    feed := atom.NewFeed("Example Feed")
+    if err := feed.Check(); err != nil {
         log.Fatalln(err)
     }
 
     author := atom.NewPerson("John Doe")
     author.Email = "john.doe@example.com"
+    if err := author.Check(); err != nil {
+        log.Fatalln(err)
+    }
     feed.AddAuthor(author)
 
- entry, err := atom.NewEntry("First Entry")
- if err != nil {
+    entry := atom.NewEntry("First Entry")
+    entry.Content = atom.NewContent(atom.InlineText, "text", "This is the content of the first entry.")
+    if err := entry.Check(); err != nil {
         log.Fatalln(err)
     }
- content, err := atom.NewContent(atom.InlineText, "text", "This is the content of the first entry.")
- if err != nil {
-  log.Fatalln(err)
- }
- entry.Content = content
     feed.AddEntry(entry)
 
- if err := feed.Check(); err != nil {
-  log.Fatalln(err)
- }
-
- feedString, err := feed.ToXML("utf-8")
+    feedString, err := feed.ToXML("UTF-8")
     if err != nil {
         log.Fatalln(err)
     }
@@ -60,10 +58,10 @@ func main() {
 }
 ```
 
-It is also possible to use this library in a way similar to what other
-libraries would provide.
+It is also possible to use this library in a way similar to what other libraries
+provide.
 
-```
+```go
 package main
 
 import (
@@ -72,6 +70,7 @@ import (
     "time"
 
     "git.streifling.com/jason/atom"
+    "github.com/google/uuid"
 )
 
 func main() {
@@ -82,7 +81,7 @@ func main() {
             Type: "text",
             Text: "Example Feed",
         },
-  ID:      &atom.ID{URI: "urn:uuid:60a76c80-d399-11d9-b93C-0003939e0af6"},
+        ID:      &atom.ID{URI: fmt.Sprint("urn:uuid:", uuid.New())},
         Updated: &atom.Date{DateTime: atom.DateTime(now)},
         Authors: []*atom.Person{
             {
@@ -96,7 +95,7 @@ func main() {
                     Type: "text",
                     Text: "First Entry",
                 },
-    ID:      &atom.ID{URI: "urn:uuid:1225c695-cfb8-4ebb-aaaa-80da344efa6a"},
+                ID:      &atom.ID{URI: fmt.Sprint("urn:uuid:", uuid.New())},
                 Updated: &atom.Date{DateTime: atom.DateTime(now)},
                 Content: &atom.InlineTextContent{
                     Type: "text",
@@ -106,7 +105,7 @@ func main() {
         },
     }
 
- feedString, err := feed.ToXML("utf-8")
+    feedString, err := feed.ToXML("UTF-8")
     if err != nil {
         log.Fatalln(err)
     }
@@ -114,10 +113,10 @@ func main() {
 }
 ```
 
-The output of both ways of using it being an RFC4287 compliant Atom feed:
+The output of both ways of using it is an RFC4287 compliant Atom feed.
 
-```
-<?xml version="1.0" encoding="utf-8"?>
+```xml
+<?xml version="1.0" encoding="UTF-8"?>
 <feed xmlns="http://www.w3.org/2005/Atom">
     <author>
         <name>John Doe</name>
@@ -134,3 +133,109 @@ The output of both ways of using it being an RFC4287 compliant Atom feed:
     </entry>
 </feed>
 ```
+
+### Compliance and Error Checking
+
+All Atom constructs have their own ```construct.Check()``` method. It checks for
+compliance with RFC4287 and errors. This allows one to be certain that only
+compliant constructs are added to their respective parent construct. It also
+means that not the entire feed has to be checked every time a new construct is
+added. Instead one only checks the current construct and all of its
+sub-constructs with a single ```construct.Check()``` call.
+
+```go
+package main
+
+import (
+    "fmt"
+    "log"
+
+    "git.streifling.com/jason/atom"
+)
+
+func main() {
+    feed := atom.NewFeed("Example Feed")
+    if err := feed.Check(); err != nil {
+        log.Fatalln(err)
+    }
+
+    entry := atom.NewEntry("One")
+    entry.Content = atom.NewContent(atom.InlineText, "text", "Entry One")
+    entry.AddAuthor(atom.NewPerson("John Doe"))
+    if err := entry.Check(); err != nil {
+        log.Fatalln(err)
+    }
+    feed.AddEntry(entry)
+
+    feedString, err := feed.ToXML("UTF-8")
+    if err != nil {
+        log.Fatalln(err)
+    }
+    fmt.Println(feedString)
+}
+```
+
+### Adding and Deleting Items
+
+To add elements to any slice one calls the appropriate
+```someone.AddSomething(toBeAdded)``` method. It returns the item's index
+number. To delete the item one calls the Delete method.
+
+```go
+package main
+
+import (
+    "fmt"
+    "log"
+
+    "git.streifling.com/jason/atom"
+)
+
+func main() {
+    feed := atom.NewFeed("Feed")
+    if err := feed.Check(); err != nil {
+        log.Fatalln(err)
+    }
+
+    e1 := atom.NewEntry("One")
+    e1.Content = atom.NewContent(atom.InlineText, "text", "Entry One")
+    if err := e1.Check(); err != nil {
+        log.Fatalln(err)
+    }
+    i1 := feed.AddEntry(e1)
+
+    e2 := atom.NewEntry("Two")
+    e2.Content = atom.NewContent(atom.InlineText, "text", "Entry Two")
+    if err := e2.Check(); err != nil {
+        log.Fatalln(err)
+    }
+    feed.AddEntry(e2)
+
+    if err := feed.DeleteEntry(i1); err != nil {
+        log.Fatalln(err)
+    }
+
+    feedString, err := feed.ToXML("UTF-8")
+    if err != nil {
+        log.Fatalln(err)
+    }
+    fmt.Println(feedString)
+}
+```
+
+The output of this example looks like this. It only shows the second entry.
+
+```xml
+<?xml version="1.0" encoding="UTF-8"?>
+<feed xmlns="http://www.w3.org/2005/Atom">
+    <id>urn:uuid:ae89d343-b535-447d-ac14-4b80d3e02a2f</id>
+    <title type="text">Example Feed</title>
+    <updated>2024-10-20T14:57:02+02:00</updated>
+    <entry>
+        <content type="text">Entry Two</content>
+        <id>urn:uuid:620c6f73-ee1d-4c1e-be98-b0b1ad7a053f</id>
+        <title type="text">Two</title>
+        <updated>2024-10-20T14:57:02+02:00</updated>
+    </entry>
+</feed>
+```

atom.go

@@ -1,7 +1,9 @@
 package atom
 
 import (
+	"encoding/xml"
 	"fmt"
+	"html"
 	"mime"
 	"regexp"
 	"strings"
@@ -10,18 +12,37 @@ import (
 	"golang.org/x/text/language"
 )
 
-type (
-	EmailAddress string
-	LanguageTag  string
-	MediaType    string
-	IRI          string
-)
+type Countable interface {
+	*xml.Attr | *Person | *Category | *Link | *ExtensionElement | *Entry
+}
+
+// addToSlice adds a Countable to to a *[]Countable. It returns an int.
+func addToSlice[C Countable](slice *[]C, countable C) int {
+	if *slice == nil {
+		*slice = make([]C, 0)
+	}
+
+	*slice = append(*slice, countable)
+	return len(*slice) - 1
+}
+
+// deleteFromSlice deletes the Countable with the index from the *[]Countable.
+// It return an error.
+func deleteFromSlice[C Countable](slice *[]C, index int) error {
+	length := len(*slice)
+	if index > length {
+		return fmt.Errorf("id %v out of range %v", index, length)
+	}
+
+	*slice = append((*slice)[:index], (*slice)[index+1:]...)
+	return nil
+}
 
 // isValidIRI checks whether an IRI is valid or not. It returns a bool.
 // https://www.w3.org/2011/04/XMLSchema/TypeLibrary-IRI-RFC3987.xsd
-func isValidIRI(iri IRI) bool {
+func isValidIRI(iri string) bool {
 	pattern := `((([A-Za-z])[A-Za-z0-9+\-\.]*):((//(((([A-Za-z0-9\-\._~ -퟿豈-﷏ﷰ-￯𐀀-🿽𠀀-𯿽𰀀-𿿽񀀀-񏿽񐀀-񟿽񠀀-񯿽񰀀-񿿽򀀀-򏿽򐀀-򟿽򠀀-򯿽򰀀-򿿽󀀀-󏿽󐀀-󟿽󡀀-󯿽!$&'()*+,;=:]|(%[0-9A-Fa-f][0-9A-Fa-f]))*@))?((\[((((([0-9A-Fa-f]{0,4}:)){6}(([0-9A-Fa-f]{0,4}:[0-9A-Fa-f]{0,4})|(([0-9]|([1-9][0-9])|(1([0-9]){2})|(2[0-4][0-9])|(25[0-5]))\.([0-9]|([1-9][0-9])|(1([0-9]){2})|(2[0-4][0-9])|(25[0-5]))\.([0-9]|([1-9][0-9])|(1([0-9]){2})|(2[0-4][0-9])|(25[0-5]))\.([0-9]|([1-9][0-9])|(1([0-9]){2})|(2[0-4][0-9])|(25[0-5])))))|(::(([0-9A-Fa-f]{0,4}:)){5}(([0-9A-Fa-f]{0,4}:[0-9A-Fa-f]{0,4})|(([0-9]|([1-9][0-9])|(1([0-9]){2})|(2[0-4][0-9])|(25[0-5]))\.([0-9]|([1-9][0-9])|(1([0-9]){2})|(2[0-4][0-9])|(25[0-5]))\.([0-9]|([1-9][0-9])|(1([0-9]){2})|(2[0-4][0-9])|(25[0-5]))\.([0-9]|([1-9][0-9])|(1([0-9]){2})|(2[0-4][0-9])|(25[0-5])))))|(([0-9A-Fa-f]{0,4})?::(([0-9A-Fa-f]{0,4}:)){4}(([0-9A-Fa-f]{0,4}:[0-9A-Fa-f]{0,4})|(([0-9]|([1-9][0-9])|(1([0-9]){2})|(2[0-4][0-9])|(25[0-5]))\.([0-9]|([1-9][0-9])|(1([0-9]){2})|(2[0-4][0-9])|(25[0-5]))\.([0-9]|([1-9][0-9])|(1([0-9]){2})|(2[0-4][0-9])|(25[0-5]))\.([0-9]|([1-9][0-9])|(1([0-9]){2})|(2[0-4][0-9])|(25[0-5])))))|((((([0-9A-Fa-f]{0,4}:))?[0-9A-Fa-f]{0,4}))?::(([0-9A-Fa-f]{0,4}:)){3}(([0-9A-Fa-f]{0,4}:[0-9A-Fa-f]{0,4})|(([0-9]|([1-9][0-9])|(1([0-9]){2})|(2[0-4][0-9])|(25[0-5]))\.([0-9]|([1-9][0-9])|(1([0-9]){2})|(2[0-4][0-9])|(25[0-5]))\.([0-9]|([1-9][0-9])|(1([0-9]){2})|(2[0-4][0-9])|(25[0-5]))\.([0-9]|([1-9][0-9])|(1([0-9]){2})|(2[0-4][0-9])|(25[0-5])))))|((((([0-9A-Fa-f]{0,4}:)){0,2}[0-9A-Fa-f]{0,4}))?::(([0-9A-Fa-f]{0,4}:)){2}(([0-9A-Fa-f]{0,4}:[0-9A-Fa-f]{0,4})|(([0-9]|([1-9][0-9])|(1([0-9]){2})|(2[0-4][0-9])|(25[0-5]))\.([0-9]|([1-9][0-9])|(1([0-9]){2})|(2[0-4][0-9])|(25[0-5]))\.([0-9]|([1-9][0-9])|(1([0-9]){2})|(2[0-4][0-9])|(25[0-5]))\.([0-9]|([1-9][0-9])|(1([0-9]){2})|(2[0-4][0-9])|(25[0-5])))))|((((([0-9A-Fa-f]{0,4}:)){0,3}[0-9A-Fa-f]{0,4}))?::[0-9A-Fa-f]{0,4}:(([0-9A-Fa-f]{0,4}:[0-9A-Fa-f]{0,4})|(([0-9]|([1-9][0-9])|(1([0-9]){2})|(2[0-4][0-9])|(25[0-5]))\.([0-9]|([1-9][0-9])|(1([0-9]){2})|(2[0-4][0-9])|(25[0-5]))\.([0-9]|([1-9][0-9])|(1([0-9]){2})|(2[0-4][0-9])|(25[0-5]))\.([0-9]|([1-9][0-9])|(1([0-9]){2})|(2[0-4][0-9])|(25[0-5])))))|((((([0-9A-Fa-f]{0,4}:)){0,4}[0-9A-Fa-f]{0,4}))?::(([0-9A-Fa-f]{0,4}:[0-9A-Fa-f]{0,4})|(([0-9]|([1-9][0-9])|(1([0-9]){2})|(2[0-4][0-9])|(25[0-5]))\.([0-9]|([1-9][0-9])|(1([0-9]){2})|(2[0-4][0-9])|(25[0-5]))\.([0-9]|([1-9][0-9])|(1([0-9]){2})|(2[0-4][0-9])|(25[0-5]))\.([0-9]|([1-9][0-9])|(1([0-9]){2})|(2[0-4][0-9])|(25[0-5])))))|((((([0-9A-Fa-f]{0,4}:)){0,5}[0-9A-Fa-f]{0,4}))?::[0-9A-Fa-f]{0,4})|((((([0-9A-Fa-f]{0,4}:)){0,6}[0-9A-Fa-f]{0,4}))?::))|(v[0-9A-Fa-f]+\.[A-Za-z0-9\-\._~!$&'()*+,;=:]+))\])|(([0-9]|([1-9][0-9])|(1([0-9]){2})|(2[0-4][0-9])|(25[0-5]))\.([0-9]|([1-9][0-9])|(1([0-9]){2})|(2[0-4][0-9])|(25[0-5]))\.([0-9]|([1-9][0-9])|(1([0-9]){2})|(2[0-4][0-9])|(25[0-5]))\.([0-9]|([1-9][0-9])|(1([0-9]){2})|(2[0-4][0-9])|(25[0-5])))|(([A-Za-z0-9\-\._~ -퟿豈-﷏ﷰ-￯𐀀-🿽𠀀-𯿽𰀀-𿿽񀀀-񏿽񐀀-񟿽񠀀-񯿽񰀀-񿿽򀀀-򏿽򐀀-򟿽򠀀-򯿽򰀀-򿿽󀀀-󏿽󐀀-󟿽󡀀-󯿽]|(%[0-9A-Fa-f][0-9A-Fa-f])|[!$&'()*+,;=]))*)((:[0-9]*))?)((/(([A-Za-z0-9\-\._~ -퟿豈-﷏ﷰ-￯𐀀-🿽𠀀-𯿽𰀀-𿿽񀀀-񏿽񐀀-񟿽񠀀-񯿽񰀀-񿿽򀀀-򏿽򐀀-򟿽򠀀-򯿽򰀀-򿿽󀀀-󏿽󐀀-󟿽󡀀-󯿽]|(%[0-9A-Fa-f][0-9A-Fa-f])|[!$&'()*+,;=:@]))*))*)|(/(((([A-Za-z0-9\-\._~ -퟿豈-﷏ﷰ-￯𐀀-🿽𠀀-𯿽𰀀-𿿽񀀀-񏿽񐀀-񟿽񠀀-񯿽񰀀-񿿽򀀀-򏿽򐀀-򟿽򠀀-򯿽򰀀-򿿽󀀀-󏿽󐀀-󟿽󡀀-󯿽]|(%[0-9A-Fa-f][0-9A-Fa-f])|[!$&'()*+,;=:@]))+((/(([A-Za-z0-9\-\._~ -퟿豈-﷏ﷰ-￯𐀀-🿽𠀀-𯿽𰀀-𿿽񀀀-񏿽񐀀-񟿽񠀀-񯿽񰀀-񿿽򀀀-򏿽򐀀-򟿽򠀀-򯿽򰀀-򿿽󀀀-󏿽󐀀-󟿽󡀀-󯿽]|(%[0-9A-Fa-f][0-9A-Fa-f])|[!$&'()*+,;=:@]))*))*))?)|((([A-Za-z0-9\-\._~ -퟿豈-﷏ﷰ-￯𐀀-🿽𠀀-𯿽𰀀-𿿽񀀀-񏿽񐀀-񟿽񠀀-񯿽񰀀-񿿽򀀀-򏿽򐀀-򟿽򠀀-򯿽򰀀-򿿽󀀀-󏿽󐀀-󟿽󡀀-󯿽]|(%[0-9A-Fa-f][0-9A-Fa-f])|[!$&'()*+,;=:@]))+((/(([A-Za-z0-9\-\._~ -퟿豈-﷏ﷰ-￯𐀀-🿽𠀀-𯿽𰀀-𿿽񀀀-񏿽񐀀-񟿽񠀀-񯿽񰀀-񿿽򀀀-򏿽򐀀-򟿽򠀀-򯿽򰀀-򿿽󀀀-󏿽󐀀-󟿽󡀀-󯿽]|(%[0-9A-Fa-f][0-9A-Fa-f])|[!$&'()*+,;=:@]))*))*)|)((\?(([A-Za-z0-9\-\._~ -퟿豈-﷏ﷰ-￯𐀀-🿽𠀀-𯿽𰀀-𿿽񀀀-񏿽񐀀-񟿽񠀀-񯿽񰀀-񿿽򀀀-򏿽򐀀-򟿽򠀀-򯿽򰀀-򿿽󀀀-󏿽󐀀-󟿽󡀀-󯿽]|(%[0-9A-Fa-f][0-9A-Fa-f])|[!$&'()*+,;=:@])|[-󰀀-󿿽􀀀-􏿽/?])*))?((#((([A-Za-z0-9\-\._~ -퟿豈-﷏ﷰ-￯𐀀-🿽𠀀-𯿽𰀀-𿿽񀀀-񏿽񐀀-񟿽񠀀-񯿽񰀀-񿿽򀀀-򏿽򐀀-򟿽򠀀-򯿽򰀀-򿿽󀀀-󏿽󐀀-󟿽󡀀-󯿽]|(%[0-9A-Fa-f][0-9A-Fa-f])|[!$&'()*+,;=:@])|/|\?))*))?)`
-	return regexp.MustCompile(pattern).MatchString(string(iri))
+	return regexp.MustCompile(pattern).MatchString(iri)
 }
 
 // isCorrectlyEscaped checks whether a string is correctly escaped as per
@@ -40,8 +61,8 @@ func isCorrectlyEscaped(text string) bool {
 
 // isCompositeMediaType checks whether a string is a composite media type. It
 // returns a bool.
-func isCompositeMediaType(mediaType string) bool {
-	mediaType, _, err := mime.ParseMediaType(mediaType)
+func isCompositeMediaType(m string) bool {
+	mediaType, _, err := mime.ParseMediaType(m)
 	if err != nil {
 		return false
 	}
@@ -51,8 +72,8 @@ func isCompositeMediaType(mediaType string) bool {
 
 // isXMLMediaType checks whether a string is an xml media type. It returns a
 // bool.
-func isXMLMediaType(mediaType string) bool {
-	mediaType, _, err := mime.ParseMediaType(mediaType)
+func isXMLMediaType(m string) bool {
+	mediaType, _, err := mime.ParseMediaType(m)
 	if err != nil {
 		return false
 	}
@@ -62,8 +83,8 @@ func isXMLMediaType(mediaType string) bool {
 
 // isValidMediaType checks whether a string is a valid media type. It returns a
 // bool.
-func isValidMediaType(mediaType string) bool {
-	mediaType, _, err := mime.ParseMediaType(mediaType)
+func isValidMediaType(m string) bool {
+	mediaType, _, err := mime.ParseMediaType(m)
 	if err != nil {
 		return false
 	}
@@ -77,8 +98,8 @@ func isValidMediaType(mediaType string) bool {
 }
 
 // isValidLanguageTag checks whether a LanguageTag is valid. It returns a bool.
-func isValidLanguageTag(tag LanguageTag) bool {
-	_, err := language.Parse(string(tag))
+func isValidLanguageTag(languageTag string) bool {
+	_, err := language.Parse(languageTag)
 	return err == nil
 }
 
@@ -89,6 +110,11 @@ func isValidAttribute(attribute string) bool {
 }
 
 // NewURN generates an new valid IRI based on a UUID. It returns an IRI.
-func NewURN() IRI {
-	return IRI(fmt.Sprint("urn:uuid:", uuid.New()))
+func NewURN() string {
+	return fmt.Sprint("urn:uuid:", uuid.New())
+}
+
+// Unescape unescapes a string. It returns an IRI.
+func Unescape(s string) string {
+	return html.UnescapeString(s)
 }

category.go

@@ -2,44 +2,40 @@ package atom
 
 import (
 	"encoding/xml"
-	"errors"
 	"fmt"
-	"html"
 )
 
 type Category struct {
 	XMLName xml.Name `xml:"category"`
 	*CommonAttributes
 	Term   string `xml:"term,attr"`
-	Scheme IRI    `xml:"scheme,attr,omitempty"`
-	Label  string `xml:"label,attr,omitempty"`
+	Scheme string `xml:"scheme,attr,omitempty"` // IRI
+	Label  string `xml:"label,attr,omitempty"`  // Must be unescaped
 }
 
 // NewCategory creates a new Category. It returns a *Category.
 func NewCategory(term string) *Category {
-	return &Category{Term: term}
+	return &Category{
+		CommonAttributes: NewCommonAttributes(),
+		Term:             term,
 	}
-
-// SetLabel sets the label of the Category.
-func (c *Category) SetLabel(label string) {
-	c.Label = html.UnescapeString(label)
 }
 
 // Check checks the Category for incompatibilities with RFC4287. It returns an
 // error.
 func (c *Category) Check() error {
 	if c.Term == "" {
-		return errors.New("term attribute of category empty")
+		return fmt.Errorf("term attribute of category %v empty", c)
 	}
 
 	if c.Scheme != "" {
 		if !isValidIRI(c.Scheme) {
-			return fmt.Errorf("scheme attribute %v of category not correctly formatted", c.Scheme)
+			return fmt.Errorf("scheme attribute of category %v not correctly formatted", c)
 		}
 	}
 
 	if !isCorrectlyEscaped(c.Label) {
-		return fmt.Errorf("label attribute %v of category not correctly escaped", c.Label)
+		return fmt.Errorf("label attribute of category %v not correctly escaped", c)
 	}
 
 	return nil

commonAttributes.go

@@ -2,11 +2,12 @@ package atom
 
 import (
 	"encoding/xml"
+	"fmt"
 )
 
 type CommonAttributes struct {
-	Base                IRI         `xml:"base,attr,omitempty"`
-	Lang                LanguageTag `xml:"lang,attr,omitempty"`
+	Base                string      `xml:"base,attr,omitempty"` // IRI
+	Lang                string      `xml:"lang,attr,omitempty"` // LanguageTag
 	UndefinedAttributes []*xml.Attr `xml:",attr,omitempty"`
 }
 
@@ -16,12 +17,32 @@ func NewCommonAttributes() *CommonAttributes {
 	return new(CommonAttributes)
 }
 
-// AddExtensionAttribute adds the ExtensionAttribute to the CommonAttributes.
-func (c *CommonAttributes) AddExtensionAttribute(name, value string) {
-	if c.UndefinedAttributes == nil {
-		c.UndefinedAttributes = make([]*xml.Attr, 1)
-		c.UndefinedAttributes[0] = &xml.Attr{Name: xml.Name{Local: name}, Value: value}
-	} else {
-		c.UndefinedAttributes = append(c.UndefinedAttributes, &xml.Attr{Name: xml.Name{Local: name}, Value: value})
+// AddAttribute adds the attribute to the CommonAttributes. It returns its index
+// as an int.
+func (c *CommonAttributes) AddAttribute(name, value string) int {
+	return addToSlice(&c.UndefinedAttributes, &xml.Attr{Name: xml.Name{Local: name}, Value: value})
+}
+
+// DeleteAttribute deletes the attribute at index from the CommonAttributes. It
+// return an error.
+func (c *CommonAttributes) DeleteAttribute(index int) error {
+	if err := deleteFromSlice(&c.UndefinedAttributes, index); err != nil {
+		return fmt.Errorf("error deleting undefined attribute %v from common attributes %v: %v", index, c, err)
+	}
+	return nil
+}
+
+// Check checks the CommonAttributes for incompatibilities with RFC4287. It
+// returns an error.
+func (c *CommonAttributes) Check() error {
+	for i, u := range c.UndefinedAttributes {
+		if u.Name.Local == "" {
+			return fmt.Errorf("xml name of undefined attribute %v empty", i)
+		}
+		if u.Value == "" {
+			return fmt.Errorf("value of undefined attribute %v empty", i)
 		}
 	}
+
+	return nil
+}

content.go

@@ -1,9 +1,5 @@
 package atom
 
-import (
-	"fmt"
-)
-
 const (
 	InlineText = iota
 	InlineXHTML
@@ -19,17 +15,17 @@ type Content interface {
 }
 
 // NewContent creates a new Content. It returns a Content and an error.
-func NewContent(contentType int, mediaType string, content any) (Content, error) {
+func NewContent(contentType int, mediaType string, content any) Content {
 	switch contentType {
 	case 0:
-		return newInlineTextContent(mediaType, content)
+		return newInlineTextContent(mediaType, content.(string))
 	case 1:
-		return newInlineXHTMLContent(mediaType, content)
+		return newInlineXHTMLContent(mediaType, content.(*XHTMLDiv))
 	case 2:
 		return newInlineOtherContent(mediaType, content)
 	case 3:
-		return newOutOfLineContent(mediaType, content)
+		return newOutOfLineContent(mediaType, content.(string))
 	default:
-		return nil, fmt.Errorf("%v is not a valid text type", contentType)
+		return nil
 	}
 }

date.go

@@ -13,12 +13,15 @@ type Date struct {
 // DateTime formats a time.Time to string formated as defined by RFC3339. It
 // returns a string.
 func DateTime(t time.Time) string {
-	return string(t.Format(time.RFC3339))
+	return t.Format(time.RFC3339)
 }
 
 // NewDate creates a new Date. It returns a *Date.
 func NewDate(t time.Time) *Date {
-	return &Date{DateTime: DateTime(t)}
+	return &Date{
+		CommonAttributes: NewCommonAttributes(),
+		DateTime:         DateTime(t),
+	}
 }
 
 // Check checks the Date for incompatibilities with RFC4287. It returns an

entry.go

@@ -2,7 +2,6 @@ package atom
 
 import (
 	"encoding/xml"
-	"errors"
 	"fmt"
 	"strings"
 	"time"
@@ -40,16 +39,16 @@ func (e *Entry) checkAuthors(authorInFeed bool) error {
 	if e.Authors == nil {
 		if !authorInFeed {
 			if e.Source == nil {
-				return errors.New("no authors set in entry")
+				return fmt.Errorf("no authors set in entry %v", e.ID.URI)
 			}
 			if e.Source.Authors == nil {
-				return errors.New("no authors set in entry")
+				return fmt.Errorf("no authors set in entry %v", e.ID.URI)
 			}
 		}
 	} else {
 		for i, a := range e.Authors {
 			if err := a.Check(); err != nil {
-				return fmt.Errorf("author element %v of entry: %v", i, err)
+				return fmt.Errorf("author element %v of entry %v: %v", i, e.ID.URI, err)
 			}
 		}
 	}
@@ -57,93 +56,160 @@ func (e *Entry) checkAuthors(authorInFeed bool) error {
 	return nil
 }
 
-// NewEntry creates a new Entry. It returns a *Entry and an error.
-func NewEntry(title string) (*Entry, error) {
-	text, err := NewText("text", title)
-	if err != nil {
-		return nil, fmt.Errorf("error creating new entry: %v", err)
-	}
-
-	id, err := NewID(NewURN())
-	if err != nil {
-		return nil, fmt.Errorf("error creating new entry: %v", err)
-	}
-
+// NewEntry creates a new Entry. It returns a *Entry.
+func NewEntry(title string) *Entry {
 	return &Entry{
-		ID:      id,
-		Title:   text,
+		CommonAttributes: NewCommonAttributes(),
+		ID:               NewID(NewURN()),
+		Title:            NewText("text", title),
 		Updated:          NewDate(time.Now()),
-	}, nil
+	}
 }
 
-// AddAuthor adds the Person as an author to the Entry.
-func (e *Entry) AddAuthor(p *Person) {
-	if e.Authors == nil {
-		e.Authors = make([]*Person, 1)
-		e.Authors[0] = p
+// AddAuthor adds the Person as an author to the Entry. It returns its index as
+// an int.
+func (e *Entry) AddAuthor(p *Person) int {
+	if e.Updated == nil {
+		e.Updated = NewDate(time.Now())
 	} else {
-		e.Authors = append(e.Authors, p)
-	}
-
 		e.Updated.DateTime = DateTime(time.Now())
 	}
 
-// AddCategory adds the Category to the Entry.
-func (e *Entry) AddCategory(c *Category) {
-	if e.Categories == nil {
-		e.Categories = make([]*Category, 1)
-		e.Categories[0] = c
-	} else {
-		e.Categories = append(e.Categories, c)
+	return addToSlice(&e.Authors, p)
 }
 
+// DeleteAuthor deletes the Person at index from the Entry. It return an error.
+func (e *Entry) DeleteAuthor(index int) error {
+	if err := deleteFromSlice(&e.Authors, index); err != nil {
+		return fmt.Errorf("error deleting author %v from entry %v: %v", index, e.ID.URI, err)
+	}
+
+	if e.Updated == nil {
+		e.Updated = NewDate(time.Now())
+	} else {
 		e.Updated.DateTime = DateTime(time.Now())
 	}
 
-// AddContributor adds the Person as a contributor to the Entry.
-func (e *Entry) AddContributor(c *Person) {
-	if e.Contributors == nil {
-		e.Contributors = make([]*Person, 1)
-		e.Contributors[0] = c
-	} else {
-		e.Contributors = append(e.Contributors, c)
+	return nil
 }
 
+// AddCategory adds the Category to the Entry. It returns ts index as an int.
+func (e *Entry) AddCategory(c *Category) int {
+	if e.Updated == nil {
+		e.Updated = NewDate(time.Now())
+	} else {
 		e.Updated.DateTime = DateTime(time.Now())
 	}
 
-// AddLink adds the Link to the Entry.
-func (e *Entry) AddLink(l *Link) {
-	if e.Links == nil {
-		e.Links = make([]*Link, 1)
-		e.Links[0] = l
-	} else {
-		e.Links = append(e.Links, l)
+	return addToSlice(&e.Categories, c)
 }
 
+// DeleteCategory deletes the Category at index from the Entry. It return an
+// error.
+func (e *Entry) DeleteCategory(index int) error {
+	if err := deleteFromSlice(&e.Categories, index); err != nil {
+		return fmt.Errorf("error deleting category %v from entry %v: %v", index, e.ID.URI, err)
+	}
+
+	if e.Updated == nil {
+		e.Updated = NewDate(time.Now())
+	} else {
 		e.Updated.DateTime = DateTime(time.Now())
 	}
 
-// AddExtension adds the ExtensionElement to the Entry.
-func (e *Entry) AddExtension(x *ExtensionElement) {
-	if e.Extensions == nil {
-		e.Extensions = make([]*ExtensionElement, 1)
-		e.Extensions[0] = x
-	} else {
-		e.Extensions = append(e.Extensions, x)
+	return nil
 }
 
+// AddContributor adds the Person as a contributor to the Entry. It returns its
+// index as an int.
+func (e *Entry) AddContributor(c *Person) int {
+	if e.Updated == nil {
+		e.Updated = NewDate(time.Now())
+	} else {
 		e.Updated.DateTime = DateTime(time.Now())
 	}
 
+	return addToSlice(&e.Contributors, c)
+}
+
+// DeleteContributor deletes the Person at index from the Entry. It return an
+// error.
+func (e *Entry) DeleteContributor(index int) error {
+	if err := deleteFromSlice(&e.Contributors, index); err != nil {
+		return fmt.Errorf("error deleting contributor %v from entry %v: %v", index, e.ID.URI, err)
+	}
+
+	if e.Updated == nil {
+		e.Updated = NewDate(time.Now())
+	} else {
+		e.Updated.DateTime = DateTime(time.Now())
+	}
+
+	return nil
+}
+
+// AddLink adds the Link to the Entry. It returns its index as an int.
+func (e *Entry) AddLink(l *Link) int {
+	if e.Updated == nil {
+		e.Updated = NewDate(time.Now())
+	} else {
+		e.Updated.DateTime = DateTime(time.Now())
+	}
+
+	return addToSlice(&e.Links, l)
+}
+
+// DeleteLink deletes the Link at index from the Entry. It return an error.
+func (e *Entry) DeleteLink(index int) error {
+	if err := deleteFromSlice(&e.Links, index); err != nil {
+		return fmt.Errorf("error deleting link %v from entry %v: %v", index, e.ID.URI, err)
+	}
+
+	if e.Updated == nil {
+		e.Updated = NewDate(time.Now())
+	} else {
+		e.Updated.DateTime = DateTime(time.Now())
+	}
+
+	return nil
+}
+
+// AddExtension adds the ExtensionElement to the Entry. It returns its index as
+// an int.
+func (e *Entry) AddExtension(x *ExtensionElement) int {
+	if e.Updated == nil {
+		e.Updated = NewDate(time.Now())
+	} else {
+		e.Updated.DateTime = DateTime(time.Now())
+	}
+
+	return addToSlice(&e.Extensions, x)
+}
+
+// DeleteExtension deletes the Extension at index from the Entry. It return an
+// error.
+func (e *Entry) DeleteExtension(index int) error {
+	if err := deleteFromSlice(&e.Extensions, index); err != nil {
+		return fmt.Errorf("error deleting extension %v from entry %v: %v", index, e.ID.URI, err)
+	}
+
+	if e.Updated == nil {
+		e.Updated = NewDate(time.Now())
+	} else {
+		e.Updated.DateTime = DateTime(time.Now())
+	}
+
+	return nil
+}
+
 // Check checks the Entry for incompatibilities with RFC4287. It returns an
 // error.
 func (e *Entry) Check() error {
 	if e.ID == nil {
-		return errors.New("no id element of entry")
+		return fmt.Errorf("no id element of entry %v", e)
 	} else {
 		if err := e.ID.Check(); err != nil {
-			return fmt.Errorf("id element of entry: %v", err)
+			return fmt.Errorf("id element of entry %v: %v", e, err)
 		}
 	}
 
@@ -166,7 +232,7 @@ func (e *Entry) Check() error {
 		// contain at least one atom:link element with a rel attribute value of
 		// "alternate".
 		if !alternateRelExists(e.Links) {
-			return errors.New("no content element of entry %v and no link element with rel \"alternate\"")
+			return fmt.Errorf("no content element of entry %v and no link element with rel \"alternate\"", e.ID.URI)
 		}
 	}
 
@@ -182,7 +248,7 @@ func (e *Entry) Check() error {
 		}
 	}
 	if hasAlternateDuplicateLinks(e.Links) {
-		return fmt.Errorf("links with with a rel attribute value of \"alternate\" and duplicate type and hreflang attribute values found in entry %v", e.ID.URI)
+		return fmt.Errorf("links with a rel attribute value of \"alternate\" and duplicate type and hreflang attribute values found in entry %v", e.ID.URI)
 	}
 
 	if e.Published != nil {
@@ -254,7 +320,7 @@ func (e *Entry) Check() error {
 func (e *Entry) ToXML(encoding string) (string, error) {
 	xml, err := xml.MarshalIndent(e, "", "  ")
 	if err != nil {
-		return "", fmt.Errorf("error xml encoding entry: %v", err)
+		return "", fmt.Errorf("error xml encoding entry %v: %v", e.ID.URI, err)
 	}
 
 	return fmt.Sprintln(`<?xml version="1.0" encoding="`+encoding+`"?>`) + string(xml), nil

extensionElement.go

@@ -2,7 +2,7 @@ package atom
 
 import (
 	"encoding/xml"
-	"errors"
+	"fmt"
 )
 
 type ExtensionElement struct {
@@ -19,8 +19,12 @@ func NewExtensionElement(name string, value any) *ExtensionElement {
 // Check checks the ExtensionElement for incompatibilities with RFC4287. It
 // returns an error.
 func (e *ExtensionElement) Check() error {
+	if e.XMLName.Local == "" {
+		return fmt.Errorf("xml name of extension %v empty", e)
+	}
+
 	if e.Value == nil {
-		return errors.New("value element of extension element empty")
+		return fmt.Errorf("value of extension %v empty", e)
 	}
 
 	return nil

feed.go

@@ -2,7 +2,6 @@ package atom
 
 import (
 	"encoding/xml"
-	"errors"
 	"fmt"
 	"time"
 )
@@ -26,105 +25,213 @@ type Feed struct {
 	Entries      []*Entry            `xml:",omitempty"`
 }
 
-// NewFeed creates a new Feed. It returns a *Feed and an error.
-func NewFeed(title string) (*Feed, error) {
-	text, err := NewText("text", title)
-	if err != nil {
-		return nil, fmt.Errorf("error creating new feed: %v", err)
-	}
-
-	id, err := NewID(NewURN())
-	if err != nil {
-		return nil, fmt.Errorf("error creating new feed: %v", err)
-	}
-
+// NewFeed creates a new Feed. It returns a *Feed.
+func NewFeed(title string) *Feed {
 	return &Feed{
-		ID:      id,
-		Title:   text,
+		CommonAttributes: NewCommonAttributes(),
+		ID:               NewID(NewURN()),
+		Title:            NewText("text", title),
 		Updated:          NewDate(time.Now()),
-	}, nil
+	}
 }
 
-// AddAuthor adds the Person as an author to the Feed.
-func (f *Feed) AddAuthor(p *Person) {
-	if f.Authors == nil {
-		f.Authors = make([]*Person, 1)
-		f.Authors[0] = p
+// AddAuthor adds the Person as an author to the Feed. It returns its index as
+// an int.
+func (f *Feed) AddAuthor(p *Person) int {
+	if f.Updated == nil {
+		f.Updated = NewDate(time.Now())
 	} else {
-		f.Authors = append(f.Authors, p)
-	}
-
 		f.Updated.DateTime = DateTime(time.Now())
 	}
 
-// AddCategory adds the Category to the Feed.
-func (f *Feed) AddCategory(c *Category) {
-	if f.Categories == nil {
-		f.Categories = make([]*Category, 1)
-		f.Categories[0] = c
-	} else {
-		f.Categories = append(f.Categories, c)
+	return addToSlice(&f.Authors, p)
 }
 
+// DeleteAuthor deletes the Person at index from the Feed. It return an error.
+func (f *Feed) DeleteAuthor(index int) error {
+	if err := deleteFromSlice(&f.Authors, index); err != nil {
+		return fmt.Errorf("error deleting author %v from entry %v: %v", index, f.ID.URI, err)
+	}
+
+	if f.Updated == nil {
+		f.Updated = NewDate(time.Now())
+	} else {
 		f.Updated.DateTime = DateTime(time.Now())
 	}
 
-// AddContributor adds the Person as a contributor to the Feed.
-func (f *Feed) AddContributor(c *Person) {
-	if f.Contributors == nil {
-		f.Contributors = make([]*Person, 1)
-		f.Contributors[0] = c
-	} else {
-		f.Contributors = append(f.Contributors, c)
+	return nil
 }
 
+// AddCategory adds the Category to the Feed. It returns its index as an int.
+func (f *Feed) AddCategory(c *Category) int {
+	if f.Updated == nil {
+		f.Updated = NewDate(time.Now())
+	} else {
 		f.Updated.DateTime = DateTime(time.Now())
 	}
 
+	return addToSlice(&f.Categories, c)
+}
+
+// DeleteCategory deletes the Category at index from the Feed. It return an
+// error.
+func (f *Feed) DeleteCategory(index int) error {
+	if err := deleteFromSlice(&f.Categories, index); err != nil {
+		return fmt.Errorf("error deleting category %v from entry %v: %v", index, f.ID.URI, err)
+	}
+
+	if f.Updated == nil {
+		f.Updated = NewDate(time.Now())
+	} else {
+		f.Updated.DateTime = DateTime(time.Now())
+	}
+
+	return nil
+}
+
+// AddContributor adds the Person as a contributor to the Feed. It returns its
+// index as an int.
+func (f *Feed) AddContributor(c *Person) int {
+	if f.Updated == nil {
+		f.Updated = NewDate(time.Now())
+	} else {
+		f.Updated.DateTime = DateTime(time.Now())
+	}
+
+	return addToSlice(&f.Contributors, c)
+}
+
+// DeleteContributor deletes the Person at index from the Feed. It return an
+// error.
+func (f *Feed) DeleteContributor(index int) error {
+	if err := deleteFromSlice(&f.Contributors, index); err != nil {
+		return fmt.Errorf("error deleting contributor %v from entry %v: %v", index, f.ID.URI, err)
+	}
+
+	if f.Updated == nil {
+		f.Updated = NewDate(time.Now())
+	} else {
+		f.Updated.DateTime = DateTime(time.Now())
+	}
+
+	return nil
+}
+
 // AddLink adds the Link to the Feed. There should be one Link with Rel "self".
-func (f *Feed) AddLink(l *Link) {
-	if f.Links == nil {
-		f.Links = make([]*Link, 1)
-		f.Links[0] = l
+// It returns its index as an int.
+func (f *Feed) AddLink(l *Link) int {
+	if f.Updated == nil {
+		f.Updated = NewDate(time.Now())
 	} else {
-		f.Links = append(f.Links, l)
-	}
-
 		f.Updated.DateTime = DateTime(time.Now())
 	}
 
-// AddExtension adds the Extension to the Feed.
-func (f *Feed) AddExtension(e *ExtensionElement) {
-	if f.Extensions == nil {
-		f.Extensions = make([]*ExtensionElement, 1)
-		f.Extensions[0] = e
-	} else {
-		f.Extensions = append(f.Extensions, e)
+	return addToSlice(&f.Links, l)
 }
 
+// DeleteLink deletes the Link at index from the Feed. It return an error.
+func (f *Feed) DeleteLink(index int) error {
+	if err := deleteFromSlice(&f.Links, index); err != nil {
+		return fmt.Errorf("error deleting link %v from entry %v: %v", index, f.ID.URI, err)
+	}
+
+	if f.Updated == nil {
+		f.Updated = NewDate(time.Now())
+	} else {
 		f.Updated.DateTime = DateTime(time.Now())
 	}
 
-// AddEntry adds the Entry to the Feed.
-func (f *Feed) AddEntry(e *Entry) {
-	if f.Entries == nil {
-		f.Entries = make([]*Entry, 1)
-		f.Entries[0] = e
-	} else {
-		f.Entries = append(f.Entries, e)
+	return nil
 }
 
+// AddExtension adds the Extension to the Feed. It returns its index as an int.
+func (f *Feed) AddExtension(e *ExtensionElement) int {
+	if f.Updated == nil {
+		f.Updated = NewDate(time.Now())
+	} else {
 		f.Updated.DateTime = DateTime(time.Now())
 	}
 
+	return addToSlice(&f.Extensions, e)
+}
+
+// DeleteExtension deletes the Extension at index from the Feed. It return an
+// error.
+func (f *Feed) DeleteExtension(index int) error {
+	if err := deleteFromSlice(&f.Extensions, index); err != nil {
+		return fmt.Errorf("error deleting extension %v from entry %v: %v", index, f.ID.URI, err)
+	}
+
+	if f.Updated == nil {
+		f.Updated = NewDate(time.Now())
+	} else {
+		f.Updated.DateTime = DateTime(time.Now())
+	}
+
+	return nil
+}
+
+// AddEntry adds the Entry to the Feed. It returns its index as an int.
+func (f *Feed) AddEntry(e *Entry) int {
+	if f.Updated == nil {
+		f.Updated = NewDate(time.Now())
+	} else {
+		f.Updated.DateTime = DateTime(time.Now())
+	}
+
+	return addToSlice(&f.Entries, e)
+}
+
+// DeleteEntry deletes the Entry at index from the Feed. It return an error.
+func (f *Feed) DeleteEntry(index int) error {
+	if err := deleteFromSlice(&f.Entries, index); err != nil {
+		return fmt.Errorf("error deleting entry %v from entry %v: %v", index, f.ID.URI, err)
+	}
+
+	if f.Updated == nil {
+		f.Updated = NewDate(time.Now())
+	} else {
+		f.Updated.DateTime = DateTime(time.Now())
+	}
+
+	return nil
+}
+
+// DeleteEntryByURI deletes the Entry from the Feed. It return an error.
+func (f *Feed) DeleteEntryByURI(uri string) error {
+	if !isValidIRI(uri) {
+		return fmt.Errorf("error deleting entry from feed %v: uri %v invalid", f.ID.URI, uri)
+	}
+
+	index := -1
+	for i, e := range f.Entries {
+		if e.ID.URI == uri {
+			index = i
+			break
+		}
+	}
+	if index < 0 {
+		return fmt.Errorf("error deleting entry from feed %v: id %v not found", f.ID.URI, uri)
+	}
+
+	f.Entries = append(f.Entries[:index], f.Entries[index+1:]...)
+	if f.Updated == nil {
+		f.Updated = NewDate(time.Now())
+	} else {
+		f.Updated.DateTime = DateTime(time.Now())
+	}
+
+	return nil
+}
+
 // Check checks the Feed for incompatibilities with RFC4287. It returns an
 // error.
 func (f *Feed) Check() error {
 	if f.ID == nil {
-		return errors.New("no id element of feed")
+		return fmt.Errorf("no id element of feed %v", f)
 	} else {
 		if err := f.ID.Check(); err != nil {
-			return fmt.Errorf("id element of feed: %v", err)
+			return fmt.Errorf("id element of feed %v: %v", f, err)
 		}
 	}
 
@@ -175,7 +282,7 @@ func (f *Feed) Check() error {
 		}
 	}
 	if hasAlternateDuplicateLinks(f.Links) {
-		return fmt.Errorf("links with with a rel attribute value of \"alternate\" and duplicate type and hreflang attribute values found in feed %v", f.ID.URI)
+		return fmt.Errorf("links with a rel attribute value of \"alternate\" and duplicate type and hreflang attribute values found in feed %v", f.ID.URI)
 	}
 
 	if f.Logo != nil {
@@ -205,7 +312,7 @@ func (f *Feed) Check() error {
 	}
 
 	if f.Updated == nil {
-		return fmt.Errorf("no updated element of feed %v", f.ID)
+		return fmt.Errorf("no updated element of feed %v", f.ID.URI)
 	} else {
 		if err := f.Updated.Check(); err != nil {
 			return fmt.Errorf("updated element of feed %v: %v", f.ID.URI, err)
@@ -231,7 +338,7 @@ func (f *Feed) Check() error {
 func (f *Feed) ToXML(encoding string) (string, error) {
 	xml, err := xml.MarshalIndent(f, "", "  ")
 	if err != nil {
-		return "", fmt.Errorf("error xml encoding feed: %v", err)
+		return "", fmt.Errorf("error xml encoding feed %v: %v", f.ID.URI, err)
 	}
 
 	return fmt.Sprintln(`<?xml version="1.0" encoding="`+encoding+`"?>`) + string(xml), nil

generator.go

@@ -2,7 +2,6 @@ package atom
 
 import (
 	"encoding/xml"
-	"errors"
 	"fmt"
 	"html"
 )
@@ -10,14 +9,17 @@ import (
 type Generator struct {
 	XMLName xml.Name `xml:"generator"`
 	*CommonAttributes
-	URI     IRI    `xml:"uri,attr,omitempty"`
+	URI     string `xml:"uri,attr,omitempty"` // IRI
 	Version string `xml:"version,attr,omitempty"`
 	Text    string `xml:",chardata"`
 }
 
 // NewGenerator creates a new Generator. It returns a *Generator.
 func NewGenerator(text string) *Generator {
-	return &Generator{Text: html.UnescapeString(text)}
+	return &Generator{
+		CommonAttributes: NewCommonAttributes(),
+		Text:             html.UnescapeString(text),
+	}
 }
 
 // Check checks the Generator for incompatibilities with RFC4287. It returns an
@@ -25,16 +27,16 @@ func NewGenerator(text string) *Generator {
 func (g *Generator) Check() error {
 	if g.URI != "" {
 		if !isValidIRI(g.URI) {
-			return fmt.Errorf("uri attribute %v of generator not correctly formatted", g.URI)
+			return fmt.Errorf("uri attribute of generator %v not correctly formatted", g)
 		}
 	}
 
 	if g.Text == "" {
-		return errors.New("text element of generator empty")
+		return fmt.Errorf("text element of generator %v empty", g)
 	}
 
 	if !isCorrectlyEscaped(g.Text) {
-		return fmt.Errorf("text element %v of generator not correctly escaped", g.Text)
+		return fmt.Errorf("text element of generator %v not correctly escaped", g)
 	}
 
 	return nil

icon.go

@@ -9,16 +9,15 @@ import (
 type Icon struct {
 	XMLName xml.Name `xml:"icon"`
 	*CommonAttributes
-	URI IRI `xml:",chardata"`
+	URI string `xml:",chardata"` // IRI
 }
 
-// NewIcon creates a new Icon. It returns a *Icon and an error.
-func NewIcon(uri IRI) (*Icon, error) {
-	if !isValidIRI(uri) {
-		return nil, fmt.Errorf("uri %v not correctly formatted", uri)
+// NewIcon creates a new Icon. It returns a *Icon.
+func NewIcon(uri string) *Icon {
+	return &Icon{
+		CommonAttributes: NewCommonAttributes(),
+		URI:              uri,
 	}
-
-	return &Icon{URI: uri}, nil
 }
 
 // Check checks the Icon for incompatibilities with RFC4287. It returns an
@@ -28,7 +27,7 @@ func (i *Icon) Check() error {
 		return errors.New("uri element of icon empty")
 	} else {
 		if !isValidIRI(i.URI) {
-			return fmt.Errorf("uri attribute %v of icon not correctly formatted", i.URI)
+			return fmt.Errorf("uri attribute of icon %v not correctly formatted", i)
 		}
 	}
 

id.go

@@ -9,16 +9,15 @@ import (
 type ID struct {
 	XMLName xml.Name `xml:"id"`
 	*CommonAttributes
-	URI IRI `xml:",chardata"`
+	URI string `xml:",chardata"` // IRI
 }
 
-// NewID creates a new ID. It returns a *ID and an error.
-func NewID(uri IRI) (*ID, error) {
-	if !isValidIRI(uri) {
-		return nil, fmt.Errorf("uri %v not correctly formatted", uri)
+// NewID creates a new ID. It returns a *ID.
+func NewID(uri string) *ID {
+	return &ID{
+		CommonAttributes: NewCommonAttributes(),
+		URI:              uri,
 	}
-
-	return &ID{URI: IRI(uri)}, nil
 }
 
 // Check checks the ID for incompatibilities with RFC4287. It returns an error.
@@ -27,7 +26,7 @@ func (i *ID) Check() error {
 		return errors.New("uri element of id empty")
 	} else {
 		if !isValidIRI(i.URI) {
-			return fmt.Errorf("uri element %v of id not correctly formatted", i.URI)
+			return fmt.Errorf("uri element of id %v not correctly formatted", i)
 		}
 	}
 

inlineOtherContent.go

@@ -2,7 +2,6 @@ package atom
 
 import (
 	"encoding/xml"
-	"errors"
 	"fmt"
 	"mime"
 )
@@ -11,17 +10,19 @@ type InlineOtherContent struct {
 	XMLName xml.Name `xml:"content"`
 	*CommonAttributes
 	AnyElement any    `xml:",chardata"`
-	Type       MediaType `xml:"type,attr,omitempty"`
+	Type       string `xml:"type,attr,omitempty"` // MediaType
 }
 
 // newInlineOtherContent creates a new InlineOtherContent. It returns a
 // *InlineOtherContent and an error.
-func newInlineOtherContent(mediaType string, content any) (*InlineOtherContent, error) {
-	if mediaType, _, err := mime.ParseMediaType(mediaType); err != nil {
-		return nil, fmt.Errorf("media type %v incompatible with inline other content", mediaType)
-	}
+func newInlineOtherContent(mediaType string, content any) *InlineOtherContent {
+	mediaType, _, _ = mime.ParseMediaType(mediaType)
 
-	return &InlineOtherContent{Type: MediaType(mediaType), AnyElement: content}, nil
+	return &InlineOtherContent{
+		CommonAttributes: NewCommonAttributes(),
+		Type:             mediaType,
+		AnyElement:       content,
+	}
 }
 
 // isContent checks whether the InlineOtherContent is a Content. It returns a
@@ -33,19 +34,19 @@ func (i *InlineOtherContent) isContent() bool { return true }
 func (i *InlineOtherContent) hasSRC() bool { return false }
 
 // getType returns the Type of the InlineOtherContent as a string.
-func (i *InlineOtherContent) getType() string { return string(i.Type) }
+func (i *InlineOtherContent) getType() string { return i.Type }
 
 // Check checks the InlineOtherContent for incompatibilities with RFC4287. It
 // returns an error.
 func (i *InlineOtherContent) Check() error {
 	mediaType := i.getType()
 
-	if mediaType, _, err := mime.ParseMediaType(mediaType); err != nil {
-		return fmt.Errorf("type attribute %v incompatible with inline other content", mediaType)
+	if !isValidMediaType(mediaType) {
+		return fmt.Errorf("type attribute of inline other content %v invalid media type", i)
 	}
 
 	if isCompositeMediaType(mediaType) {
-		return errors.New("type attribute of inline other content must not be a composite type")
+		return fmt.Errorf("type attribute of inline other content %v must not be a composite type", i)
 	}
 
 	return nil

inlineTextContent.go

@@ -2,7 +2,6 @@ package atom
 
 import (
 	"encoding/xml"
-	"errors"
 	"fmt"
 )
 
@@ -14,18 +13,13 @@ type InlineTextContent struct {
 }
 
 // newInlineTextContent creates a new InlineTextContent. It returns a
-// *InlineTextContent and an error.
-func newInlineTextContent(mediaType string, content any) (*InlineTextContent, error) {
-	if mediaType != "text" && mediaType != "html" && mediaType != "" {
-		return nil, fmt.Errorf("media type %v incompatible with inline text content", mediaType)
+// *InlineTextContent.
+func newInlineTextContent(mediaType, text string) *InlineTextContent {
+	return &InlineTextContent{
+		CommonAttributes: NewCommonAttributes(),
+		Type:             mediaType,
+		Text:             text,
 	}
-
-	text, ok := content.(string)
-	if !ok {
-		return nil, fmt.Errorf("content type %T incompatible with inline text content", content)
-	}
-
-	return &InlineTextContent{Type: mediaType, Text: text}, nil
 }
 
 // isContent checks whether the InlineTextContent is a Content. It returns a
@@ -43,7 +37,7 @@ func (i *InlineTextContent) getType() string { return i.Type }
 // returns an error.
 func (i *InlineTextContent) Check() error {
 	if i.Type != "" && i.Type != "text" && i.Type != "html" {
-		return errors.New("type attribute of inline text content must be text or html if not omitted")
+		return fmt.Errorf("type attribute of inline text content %v must be text or html if not omitted", i)
 	}
 
 	return nil

inlineXHTMLContent.go

@@ -2,7 +2,6 @@ package atom
 
 import (
 	"encoding/xml"
-	"errors"
 	"fmt"
 )
 
@@ -14,18 +13,13 @@ type InlineXHTMLContent struct {
 }
 
 // newInlineXHTMLContent creates a new InlineXHTMLContent. It returns a
-// *InlineXHTMLContent and an error.
-func newInlineXHTMLContent(mediaType string, content any) (*InlineXHTMLContent, error) {
-	if mediaType != "xhtml" {
-		return nil, fmt.Errorf("media type %v incompatible with inline xhtml content", mediaType)
+// *InlineXHTMLContent.
+func newInlineXHTMLContent(mediaType string, div *XHTMLDiv) *InlineXHTMLContent {
+	return &InlineXHTMLContent{
+		CommonAttributes: NewCommonAttributes(),
+		Type:             mediaType,
+		XHTMLDiv:         div,
 	}
-
-	xhtmlDiv, ok := content.(*XHTMLDiv)
-	if !ok {
-		return nil, fmt.Errorf("content type %T incompatible with inline xhtml content", content)
-	}
-
-	return &InlineXHTMLContent{Type: mediaType, XHTMLDiv: xhtmlDiv}, nil
 }
 
 // isContent checks whether the InlineXHTMLContent is a Content. It returns a
@@ -43,11 +37,11 @@ func (i *InlineXHTMLContent) getType() string { return i.Type }
 // returns an error.
 func (i *InlineXHTMLContent) Check() error {
 	if i.Type != "xhtml" {
-		return errors.New("type attribute of inline xhtml content must be xhtml")
+		return fmt.Errorf("type attribute of inline xhtml content %v must be xhtml", i)
 	}
 
 	if err := i.XHTMLDiv.Check(); err != nil {
-		return fmt.Errorf("xhtml div element %v of inline xhtml content %v: %v", i.XHTMLDiv, i, err)
+		return fmt.Errorf("xhtml div element of inline xhtml content %v: %v", i, err)
 	}
 
 	return nil

link.go

@@ -2,7 +2,6 @@ package atom
 
 import (
 	"encoding/xml"
-	"errors"
 	"fmt"
 	"strings"
 )
@@ -11,44 +10,47 @@ type Link struct {
 	XMLName xml.Name `xml:"link"`
 	*CommonAttributes
 	Title    string `xml:"title,attr,omitempty"`
-	Href     IRI         `xml:"href,attr"`
+	Href     string `xml:"href,attr"` // IRI
 	Rel      string `xml:"rel,attr,omitempty"`
-	Type     MediaType   `xml:"type,attr,omitempty"`
-	HrefLang LanguageTag `xml:"hreflang,attr,omitempty"`
+	Type     string `xml:"type,attr,omitempty"`     // MediaType
+	HrefLang string `xml:"hreflang,attr,omitempty"` // LanguageTag
 	Length   uint   `xml:"length,attr,omitempty"`
 }
 
 // NewLink creates a new Link. It returns a *Link.
 func NewLink(href string) *Link {
-	return &Link{Href: IRI(href)}
+	return &Link{
+		CommonAttributes: NewCommonAttributes(),
+		Href:             href,
+	}
 }
 
 // Check checks the Link for incompatibilities with RFC4287. It returns an
 // error.
 func (l *Link) Check() error {
 	if l.Href == "" {
-		return errors.New("href attribute of link empty")
+		return fmt.Errorf("href attribute of link %v empty", l)
 	} else {
 		if !isValidIRI(l.Href) {
-			return fmt.Errorf("href attribute %v of link not correctly formatted", l.Href)
+			return fmt.Errorf("href attribute of link %v not correctly formatted", l)
 		}
 	}
 
 	if l.Rel != "" {
-		if strings.Contains(l.Rel, ":") && !isValidIRI(IRI(l.Rel)) {
-			return fmt.Errorf("rel attribute %v of link %v not correctly formatted", l.Rel, l.Href)
+		if strings.Contains(l.Rel, ":") && !isValidIRI(l.Rel) {
+			return fmt.Errorf("rel attribute of link %v not correctly formatted", l)
 		}
 	}
 
 	if l.Type != "" {
-		if !isValidMediaType(string(l.Type)) {
-			return fmt.Errorf("type attribute %v of link %v invalid media type", l.Type, l.Href)
+		if !isValidMediaType(l.Type) {
+			return fmt.Errorf("type attribute of link %v invalid media type", l)
 		}
 	}
 
 	if l.HrefLang != "" {
 		if !isValidLanguageTag(l.HrefLang) {
-			return fmt.Errorf("hreflang attribute %v of link %v invalid language tag", l.Type, l.HrefLang)
+			return fmt.Errorf("hreflang attribute of link %v invalid language tag", l)
 		}
 	}
 

logo.go

@@ -2,33 +2,31 @@ package atom
 
 import (
 	"encoding/xml"
-	"errors"
 	"fmt"
 )
 
 type Logo struct {
 	XMLName xml.Name `xml:"logo"`
 	*CommonAttributes
-	URI IRI `xml:",chardata"`
+	URI string `xml:",chardata"` // IRI
 }
 
 // NewLogo creates a new Logo. It returns a *Logo.
-func NewLogo(uri IRI) (*Logo, error) {
-	if !isValidIRI(uri) {
-		return nil, fmt.Errorf("uri %v not correctly formatted", uri)
+func NewLogo(uri string) *Logo {
+	return &Logo{
+		CommonAttributes: NewCommonAttributes(),
+		URI:              uri,
 	}
-
-	return &Logo{URI: uri}, nil
 }
 
 // Check checks the Logo for incompatibilities with RFC4287. It returns an
 // error.
 func (l *Logo) Check() error {
 	if l.URI == "" {
-		return errors.New("uri element of logo empty")
+		return fmt.Errorf("uri element of logo %v empty", l)
 	} else {
 		if !isValidIRI(l.URI) {
-			return fmt.Errorf("uri element %v of logo not correctly formatted", l.URI)
+			return fmt.Errorf("uri element of logo %v not correctly formatted", l)
 		}
 	}
 

outOfLineContent.go

@@ -2,7 +2,6 @@ package atom
 
 import (
 	"encoding/xml"
-	"errors"
 	"fmt"
 	"mime"
 )
@@ -10,27 +9,20 @@ import (
 type OutOfLineContent struct {
 	XMLName xml.Name `xml:"content"`
 	*CommonAttributes
-	Type MediaType `xml:"type,attr,omitempty"`
-	SRC  IRI       `xml:"src,attr"`
+	Type string `xml:"type,attr,omitempty"` // MediaType
+	SRC  string `xml:"src,attr"`            // IRI
 }
 
 // newOutOfLineContent creates a new OutOfLineContent. It returns a
-// *OutOfLineContent and an error.
-func newOutOfLineContent(mediaType string, content any) (*OutOfLineContent, error) {
-	if mediaType, _, err := mime.ParseMediaType(mediaType); err != nil {
-		return nil, fmt.Errorf("media type %v incompatible with out of line content", mediaType)
-	}
+// *OutOfLineContent.
+func newOutOfLineContent(mediaType, src string) *OutOfLineContent {
+	mediaType, _, _ = mime.ParseMediaType(mediaType)
 
-	iri, ok := content.(IRI)
-	if !ok {
-		return nil, fmt.Errorf("content type %T incompatible with out of line content", content)
+	return &OutOfLineContent{
+		CommonAttributes: NewCommonAttributes(),
+		Type:             mediaType,
+		SRC:              src,
 	}
-
-	if !isValidIRI(iri) {
-		return nil, errors.New("content not a valid uri")
-	}
-
-	return &OutOfLineContent{Type: MediaType(mediaType), SRC: iri}, nil
 }
 
 // isContent checks whether the OutOfLineContent is a Content. It returns a
@@ -42,23 +34,23 @@ func (o *OutOfLineContent) isContent() bool { return true }
 func (o *OutOfLineContent) hasSRC() bool { return true }
 
 // getType returns the Type of the OutOfLineContent as a string.
-func (o *OutOfLineContent) getType() string { return string(o.Type) }
+func (o *OutOfLineContent) getType() string { return o.Type }
 
 // Check checks the OutOfLineContent for incompatibilities with RFC4287. It
 // returns an error.
 func (o *OutOfLineContent) Check() error {
 	mediaType := o.getType()
 
-	if mediaType, _, err := mime.ParseMediaType(mediaType); err != nil {
-		return fmt.Errorf("type attribute %v incompatible with out of line content", mediaType)
+	if !isValidMediaType(mediaType) {
+		return fmt.Errorf("type attribute of out of line content %v invalid media type", o)
 	}
 
 	if isCompositeMediaType(mediaType) {
-		return errors.New("type attribute of out of line content must not be a composite type")
+		return fmt.Errorf("type attribute of out of line content %v must not be a composite type", o)
 	}
 
 	if o.SRC == "" {
-		return errors.New("src attribute of out of line content empty")
+		return fmt.Errorf("src attribute of out of line content %v empty", o)
 	}
 
 	return nil

person.go

@@ -1,7 +1,6 @@
 package atom
 
 import (
-	"errors"
 	"fmt"
 	"net/mail"
 )
@@ -9,50 +8,56 @@ import (
 type Person struct {
 	*CommonAttributes
 	Name       string              `xml:"name"`
-	URI        IRI                 `xml:"uri,omitempty"`
-	Email      EmailAddress        `xml:"email,omitempty"`
+	URI        string              `xml:"uri,omitempty"`   // IRI
+	Email      string              `xml:"email,omitempty"` // EmailAddress
 	Extensions []*ExtensionElement `xml:",any,omitempty"`
 }
 
 // NewPerson creates a new Person. It returns a *Person.
 func NewPerson(name string) *Person {
-	return &Person{Name: name}
+	return &Person{
+		CommonAttributes: NewCommonAttributes(),
+		Name:             name,
+	}
 }
 
-// AddExtension adds the Extension to the Person.
-func (p *Person) AddExtension(e *ExtensionElement) {
-	if p.Extensions == nil {
-		p.Extensions = make([]*ExtensionElement, 1)
-		p.Extensions[0] = e
-	} else {
-		p.Extensions = append(p.Extensions, e)
+// AddExtension adds the Extension to the Person. It returns its index as an
+// int.
+func (p *Person) AddExtension(e *ExtensionElement) int {
+	return addToSlice(&p.Extensions, e)
 }
+
+// DeleteExtension deletes the Extension at index from the Person. It return an
+// error.
+func (p *Person) DeleteExtension(index int) error {
+	if err := deleteFromSlice(&p.Extensions, index); err != nil {
+		return fmt.Errorf("error deleting extension %v from person %v: %v", index, p, err)
+	}
+	return nil
 }
 
 // Check checks the Person for incompatibilities with RFC4287. It returns an
 // error.
 func (p *Person) Check() error {
 	if p.Name == "" {
-		return errors.New("name element of person element empty")
+		return fmt.Errorf("name element of person %v empty", p)
 	}
 
 	if p.URI != "" {
 		if !isValidIRI(p.URI) {
-			return fmt.Errorf("uri element of person %v not correctly formatted", p.Name)
+			return fmt.Errorf("uri element of person %v not correctly formatted", p)
 		}
 	}
 
 	if p.Email != "" {
-		if _, err := mail.ParseAddress(string(p.Email)); err != nil {
-			return fmt.Errorf("email element of person %v not correctly formatted", p.Name)
+		if _, err := mail.ParseAddress(p.Email); err != nil {
+			return fmt.Errorf("email element of person %v not correctly formatted", p)
 		}
 	}
 
-	if p.Extensions != nil {
 	for i, e := range p.Extensions {
 		if err := e.Check(); err != nil {
-				return fmt.Errorf("extension element %v of person %v: %v", i, p.Name, err)
-			}
+			return fmt.Errorf("extension element %v of person %v: %v", i, p, err)
 		}
 	}
 

plainText.go

@@ -1,6 +1,8 @@
 package atom
 
-import "errors"
+import (
+	"fmt"
+)
 
 type PlainText struct {
 	*CommonAttributes
@@ -11,19 +13,28 @@ type PlainText struct {
 // isText checks whether the PlainText is a Text. It returns a bool.
 func (p *PlainText) isText() bool { return true }
 
+// newPlainText creates a new PlainText. It returns a *PlainText.
+func newPlainText(textType, content string) *PlainText {
+	return &PlainText{
+		CommonAttributes: NewCommonAttributes(),
+		Type:             textType,
+		Text:             content,
+	}
+}
+
 // Check checks the PlainText for incompatibilities with RFC4287. It returns an
 // error.
 func (p *PlainText) Check() error {
 	if p.Type != "" && p.Type != "text" && p.Type != "html" {
-		return errors.New("type attribute of plain text must be text or html if not omitted")
+		return fmt.Errorf("type attribute of plain text %v must be text or html if not omitted", p)
 	}
 
 	if p.Type == "html" && !isCorrectlyEscaped(p.Text) {
-		return errors.New("text element of plain text not correctly escaped")
+		return fmt.Errorf("text element of plain text %v not correctly escaped", p)
 	}
 
 	if p.Text == "" {
-		return errors.New("text element of plain text empty")
+		return fmt.Errorf("text element of plain text %v empty", p)
 	}
 
 	return nil

source.go

@@ -23,84 +23,160 @@ type Source struct {
 	Extensions   []*ExtensionElement `xml:",any,omitempty"`
 }
 
+// NewSource creates a new Source. It returns a *Source.
+func NewSource() *Source {
+	return &Source{CommonAttributes: NewCommonAttributes()}
+}
+
+// AddAuthor adds the Person as an author to the Source. It returns its index as
+// an int.
+func (s *Source) AddAuthor(p *Person) int {
+	return addToSlice(&s.Authors, p)
+}
+
+// DeleteAuthor deletes the Person at index from the Source. It return an error.
+func (s *Source) DeleteAuthor(index int) error {
+	if err := deleteFromSlice(&s.Authors, index); err != nil {
+		return fmt.Errorf("error deleting author %v from source %v: %v", index, s, err)
+	}
+	return nil
+}
+
+// AddCategory adds the Category to the Source. It returns its index as an int.
+func (s *Source) AddCategory(c *Category) int {
+	return addToSlice(&s.Categories, c)
+}
+
+// DeleteCategory deletes the Category at index from the Source. It return an
+// error.
+func (s *Source) DeleteCategory(index int) error {
+	if err := deleteFromSlice(&s.Categories, index); err != nil {
+		return fmt.Errorf("error deleting category %v from source %v: %v", index, s, err)
+	}
+	return nil
+}
+
+// AddContributor adds the Person as a contributor to the Source. It returns its
+// index as an int.
+func (s *Source) AddContributor(c *Person) int {
+	return addToSlice(&s.Contributors, c)
+}
+
+// DeleteContributor deletes the Person at index from the Source. It return an
+// error.
+func (s *Source) DeleteContributor(index int) error {
+	if err := deleteFromSlice(&s.Contributors, index); err != nil {
+		return fmt.Errorf("error deleting contributor %v from source %v: %v", index, s, err)
+	}
+	return nil
+}
+
+// AddLink adds the Link to the Source. It returns its index as an int.
+func (s *Source) AddLink(l *Link) int {
+	return addToSlice(&s.Links, l)
+}
+
+// DeleteLink deletes the Link at index from the Source. It return an error.
+func (s *Source) DeleteLink(index int) error {
+	if err := deleteFromSlice(&s.Links, index); err != nil {
+		return fmt.Errorf("error deleting link %v from source %v: %v", index, s, err)
+	}
+	return nil
+}
+
+// AddExtension adds the ExtensionElement to the Source. It returns its index as
+// an int.
+func (s *Source) AddExtension(e *ExtensionElement) int {
+	return addToSlice(&s.Extensions, e)
+}
+
+// DeleteExtension deletes the Extension at index from the Source. It return an
+// error.
+func (s *Source) DeleteExtension(index int) error {
+	if err := deleteFromSlice(&s.Extensions, index); err != nil {
+		return fmt.Errorf("error deleting extension %v from source %v: %v", index, s, err)
+	}
+	return nil
+}
+
 // Check checks the Source for incompatibilities with RFC4287. It returns an
 // error.
 func (s *Source) Check() error {
 	for i, a := range s.Authors {
 		if err := a.Check(); err != nil {
-			return fmt.Errorf("author element %v of source %v: %v", i, s.ID.URI, err)
+			return fmt.Errorf("author element %v of source %v: %v", i, s, err)
 		}
 	}
 
 	for i, c := range s.Categories {
 		if err := c.Check(); err != nil {
-			return fmt.Errorf("category element %v of source %v: %v", i, s.ID.URI, err)
+			return fmt.Errorf("category element %v of source %v: %v", i, s, err)
 		}
 	}
 
 	for i, c := range s.Contributors {
 		if err := c.Check(); err != nil {
-			return fmt.Errorf("contributor element %v of source %v: %v", i, s.ID.URI, err)
+			return fmt.Errorf("contributor element %v of source %v: %v", i, s, err)
 		}
 	}
 
 	if s.Generator != nil {
 		if err := s.Generator.Check(); err != nil {
-			return fmt.Errorf("generator element of source %v: %v", s.ID.URI, err)
+			return fmt.Errorf("generator element of source %v: %v", s, err)
 		}
 	}
 
 	if s.Icon != nil {
 		if err := s.Icon.Check(); err != nil {
-			return fmt.Errorf("icon element of source %v: %v", s.ID.URI, err)
+			return fmt.Errorf("icon element of source %v: %v", s, err)
 		}
 	}
 
 	if s.ID != nil {
 		if err := s.ID.Check(); err != nil {
-			return fmt.Errorf("id element of source %v: %v", s.ID.URI, err)
+			return fmt.Errorf("id element of source %v: %v", s, err)
 		}
 	}
 
 	for i, l := range s.Links {
 		if err := l.Check(); err != nil {
-			return fmt.Errorf("link element %v of source %v: %v", i, s.ID.URI, err)
+			return fmt.Errorf("link element %v of source %v: %v", i, s, err)
 		}
 	}
 
 	if s.Logo != nil {
 		if err := s.Logo.Check(); err != nil {
-			return fmt.Errorf("logo element of source %v: %v", s.ID.URI, err)
+			return fmt.Errorf("logo element of source %v: %v", s, err)
 		}
 	}
 
 	if s.Rights != nil {
 		if err := s.Rights.Check(); err != nil {
-			return fmt.Errorf("rights element of source %v: %v", s.ID.URI, err)
+			return fmt.Errorf("rights element of source %v: %v", s, err)
 		}
 	}
 
 	if s.Subtitle != nil {
 		if err := s.Subtitle.Check(); err != nil {
-			return fmt.Errorf("subtitle element of source %v: %v", s.ID.URI, err)
+			return fmt.Errorf("subtitle element of source %v: %v", s, err)
 		}
 	}
 
 	if s.Title != nil {
 		if err := s.Title.Check(); err != nil {
-			return fmt.Errorf("title element of source %v: %v", s.ID.URI, err)
+			return fmt.Errorf("title element of source %v: %v", s, err)
 		}
 	}
 
 	if s.Updated != nil {
 		if err := s.Updated.Check(); err != nil {
-			return fmt.Errorf("updated element of source %v: %v", s.ID.URI, err)
+			return fmt.Errorf("updated element of source %v: %v", s, err)
 		}
 	}
 
 	for i, e := range s.Extensions {
 		if err := e.Check(); err != nil {
-			return fmt.Errorf("extension element %v of source %v: %v", i, s.ID.URI, err)
+			return fmt.Errorf("extension element %v of source %v: %v", i, s, err)
 		}
 	}
 

text.go

@@ -1,31 +1,22 @@
 package atom
 
-import (
-	"fmt"
-	"html"
-)
+import "html"
 
 type Text interface {
 	isText() bool
 	Check() error
 }
 
-// NewText creates a new Text. It returns a Text and an error.
-func NewText(textType, content string) (Text, error) {
+// NewText creates a new Text. It returns a Text.
+func NewText(textType, content string) Text {
 	switch textType {
 	case "text", "":
-		return &PlainText{Type: textType, Text: content}, nil
+		return newPlainText(textType, content)
 	case "html":
-		return &PlainText{Type: textType, Text: html.UnescapeString(content)}, nil
+		return newPlainText(textType, html.UnescapeString(content))
 	case "xhtml":
-		return &XHTMLText{
-			Type: textType,
-			XHTMLDiv: &XHTMLDiv{
-				XMLNS:   "http://www.w3.org/1999/xhtml",
-				Content: content,
-			},
-		}, nil
+		return newXHTMLText(textType, content)
 	default:
-		return nil, fmt.Errorf("%v is not a valid text type", textType)
+		return nil
 	}
 }

xhtmlDiv.go

@@ -2,7 +2,7 @@ package atom
 
 import (
 	"encoding/xml"
-	"errors"
+	"fmt"
 )
 
 type XHTMLDiv struct {
@@ -23,7 +23,7 @@ func NewXHTMLDiv(content string) *XHTMLDiv {
 // error.
 func (x *XHTMLDiv) Check() error {
 	if x.XMLNS != "http://www.w3.org/1999/xhtml" {
-		return errors.New("xmlns attribute of xhtml text must be http://www.w3.org/1999/xhtml")
+		return fmt.Errorf("xmlns attribute of xhtml text %v must be http://www.w3.org/1999/xhtml", x)
 	}
 
 	return nil

xhtmlText.go

@@ -1,7 +1,6 @@
 package atom
 
 import (
-	"errors"
 	"fmt"
 )
 
@@ -14,11 +13,20 @@ type XHTMLText struct {
 // isText checks whether the XHTMLText is a Text. It returns a bool.
 func (x *XHTMLText) isText() bool { return true }
 
+// newPlainText creates a new PlainText. It returns a *PlainText.
+func newXHTMLText(textType, content string) *XHTMLText {
+	return &XHTMLText{
+		CommonAttributes: NewCommonAttributes(),
+		Type:             textType,
+		XHTMLDiv:         NewXHTMLDiv(content),
+	}
+}
+
 // Check checks the XHTMLText for incompatibilities with RFC4287. It returns an
 // error.
 func (x *XHTMLText) Check() error {
 	if x.Type != "xhtml" {
-		return errors.New("type attribute of xhtml text must be xhtml")
+		return fmt.Errorf("type attribute of xhtml text %v must be xhtml", x)
 	}
 
 	if err := x.XHTMLDiv.Check(); err != nil {