Make comments more useful

README.md

@@ -2,7 +2,7 @@
 
 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.
+easy creation, extension and deletion of elements.
 
 ## Installation
 
@@ -14,124 +14,228 @@ go get git.streifling.com/jason/atom@latest
 
 ## Usage
 
-This library provides convenient functions to safely create and extend elements
+### Basic Feed
-and attributes of an Atom feed. This is because it can be hard to know all
+
-pitfalls of RFC4287. The intended way of using atom is with these functions.
+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 (
- "fmt"
+    "fmt"
- "log"
+    "log"
 
- "git.streifling.com/jason/atom"
+    "git.streifling.com/jason/atom"
 )
 
 func main() {
- feed, err := atom.NewFeed("Example Feed")
+    feed := atom.NewFeed("Example Feed")
- if err != nil {
+    if err := feed.Check(); err != nil {
-  log.Fatalln(err)
+        log.Fatalln(err)
- }
+    }
 
- author := atom.NewPerson("John Doe")
+    author := atom.NewPerson("John Doe")
- author.Email = "john.doe@example.com"
+    author.Email = "john.doe@example.com"
- feed.AddAuthor(author)
+    if err := author.Check(); err != nil {
+        log.Fatalln(err)
+    }
+    feed.AddAuthor(author)
 
- entry, err := atom.NewEntry("First Entry")
+    entry := atom.NewEntry("First Entry")
- if err != nil {
+    entry.Content = atom.NewContent(atom.InlineText, "text", "This is the content of the first entry.")
-  log.Fatalln(err)
+    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 {
+    feed.AddEntry(entry)
-  log.Fatalln(err)
- }
- entry.Content = content
- feed.AddEntry(entry)
 
- if err := feed.Check(); err != nil {
+    feedString, err := feed.ToXML("UTF-8")
-  log.Fatalln(err)
+    if err != nil {
- }
+        log.Fatalln(err)
-
+    }
- feedString, err := feed.ToXML("utf-8")
+    fmt.Println(feedString)
- if err != nil {
-  log.Fatalln(err)
- }
- fmt.Println(feedString)
 }
 ```
 
 It is also possible to use this library in a way similar to what other libraries
-would provide. This is, of course, making it easier to make mistakes.
+provide.
 
 ```go
 package main
 
 import (
- "fmt"
+    "fmt"
- "log"
+    "log"
- "time"
+    "time"
 
- "git.streifling.com/jason/atom"
+    "git.streifling.com/jason/atom"
+    "github.com/google/uuid"
 )
 
 func main() {
- now := time.Now()
+    now := time.Now()
 
- feed := &atom.Feed{
+    feed := &atom.Feed{
-  Title: &atom.PlainText{
+        Title: &atom.PlainText{
-   Type: "text",
+            Type: "text",
-   Text: "Example Feed",
+            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)},
+        Updated: &atom.Date{DateTime: atom.DateTime(now)},
-  Authors: []*atom.Person{
+        Authors: []*atom.Person{
-   {
+            {
-    Name:  "John Doe",
+                Name:  "John Doe",
-    Email: "john.doe@example.com",
+                Email: "john.doe@example.com",
-   },
+            },
-  },
+        },
-  Entries: []*atom.Entry{
+        Entries: []*atom.Entry{
-   {
+            {
-    Title: &atom.PlainText{
+                Title: &atom.PlainText{
-     Type: "text",
+                    Type: "text",
-     Text: "First Entry",
+                    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)},
+                Updated: &atom.Date{DateTime: atom.DateTime(now)},
-    Content: &atom.InlineTextContent{
+                Content: &atom.InlineTextContent{
-     Type: "text",
+                    Type: "text",
-     Text: "This is the content of the first entry.",
+                    Text: "This is the content of the first entry.",
-    },
+                },
-   },
+            },
-  },
+        },
- }
+    }
 
- feedString, err := feed.ToXML("utf-8")
+    feedString, err := feed.ToXML("UTF-8")
- if err != nil {
+    if err != nil {
-  log.Fatalln(err)
+        log.Fatalln(err)
- }
+    }
- fmt.Println(feedString)
+    fmt.Println(feedString)
 }
 ```
 
-The output of both ways of using it is an RFC4287 compliant Atom feed:
+The output of both ways of using it is an RFC4287 compliant Atom feed.
 
 ```xml
-<?xml version="1.0" encoding="utf-8"?>
+<?xml version="1.0" encoding="UTF-8"?>
 <feed xmlns="http://www.w3.org/2005/Atom">
-  <author>
+    <author>
-    <name>John Doe</name>
+        <name>John Doe</name>
-    <email>john.doe@example.com</email>
+        <email>john.doe@example.com</email>
-  </author>
+    </author>
-  <id>urn:uuid:60a76c80-d399-11d9-b93C-0003939e0af6</id>
+    <id>urn:uuid:60a76c80-d399-11d9-b93C-0003939e0af6</id>
-  <title type="text">Example Feed</title>
+    <title type="text">Example Feed</title>
-  <updated>2024-10-18T05:49:08+02:00</updated>
+    <updated>2024-10-18T05:49:08+02:00</updated>
-  <entry>
+    <entry>
-    <content type="text">This is the content of the first entry.</content>
+        <content type="text">This is the content of the first entry.</content>
-    <id>urn:uuid:1225c695-cfb8-4ebb-aaaa-80da344efa6a</id>
+        <id>urn:uuid:1225c695-cfb8-4ebb-aaaa-80da344efa6a</id>
-    <title type="text">First Entry</title>
+        <title type="text">First Entry</title>
-    <updated>2006-01-02T15:04:05+07:00</updated>
+        <updated>2006-01-02T15:04:05+07:00</updated>
-  </entry>
+    </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"
@@ -10,6 +12,33 @@ import (
 	"golang.org/x/text/language"
 )
 
+type Countable interface {
+	*xml.Attr | *Person | *Category | *Link | *ExtensionElement | *Entry
+}
+
+// addToSlice adds a Countable countable to to a *[]Countable slice. It returns
+// the index as 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 at index from the *[]Countable slice.
+// It returns 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 string) bool {
@@ -17,7 +46,7 @@ func isValidIRI(iri string) bool {
 	return regexp.MustCompile(pattern).MatchString(iri)
 }
 
-// isCorrectlyEscaped checks whether a string is correctly escaped as per
+// isCorrectlyEscaped checks whether the text is correctly escaped as per
 // RFC4287. It returns a bool.
 func isCorrectlyEscaped(text string) bool {
 	relevantEntities := []string{"&", "<", ">", """, "'"}
@@ -31,8 +60,8 @@ func isCorrectlyEscaped(text string) bool {
 	return true
 }
 
-// isCompositeMediaType checks whether a string is a composite media type. It
+// isCompositeMediaType checks whether the string m is a composite media type.
-// returns a bool.
+// It returns a bool.
 func isCompositeMediaType(m string) bool {
 	mediaType, _, err := mime.ParseMediaType(m)
 	if err != nil {
@@ -42,7 +71,7 @@ func isCompositeMediaType(m string) bool {
 	return strings.HasPrefix(mediaType, "multipart/") || strings.HasPrefix(mediaType, "message/")
 }
 
-// isXMLMediaType checks whether a string is an xml media type. It returns a
+// isXMLMediaType checks whether the string m is an xml media type. It returns a
 // bool.
 func isXMLMediaType(m string) bool {
 	mediaType, _, err := mime.ParseMediaType(m)
@@ -53,8 +82,8 @@ func isXMLMediaType(m string) bool {
 	return strings.HasSuffix(mediaType, "/xml") || strings.HasSuffix(mediaType, "+xml")
 }
 
-// isValidMediaType checks whether a string is a valid media type. It returns a
+// isValidMediaType checks whether the string m is a valid media type. It
-// bool.
+// returns a bool.
 func isValidMediaType(m string) bool {
 	mediaType, _, err := mime.ParseMediaType(m)
 	if err != nil {
@@ -62,26 +91,28 @@ func isValidMediaType(m string) bool {
 	}
 
 	typeParts := strings.Split(mediaType, "/")
-	if len(typeParts) != 2 || typeParts[0] == "" || typeParts[1] == "" {
+	return len(typeParts) == 2 && typeParts[0] != "" && typeParts[1] != ""
-		return false
-	}
-
-	return true
 }
 
-// isValidLanguageTag checks whether a LanguageTag is valid. It returns a bool.
+// isValidLanguageTag checks whether the string languageTag is valid. It returns
+// a bool.
 func isValidLanguageTag(languageTag string) bool {
 	_, err := language.Parse(languageTag)
 	return err == nil
 }
 
-// isValidAttribute checks whether an Attribute is valid. It returns a bool.
+// isValidAttribute checks whether the string attribute is valid. It returns a
+// bool.
 func isValidAttribute(attribute string) bool {
-	regex := regexp.MustCompile(`^[a-zA-Z0-9_]+="[^"]*"$`)
+	return regexp.MustCompile(`^[a-zA-Z0-9_]+="[^"]*"$`).MatchString(attribute)
-	return regex.MatchString(attribute)
 }
 
 // NewURN generates an new valid IRI based on a UUID. It returns an IRI.
 func NewURN() string {
 	return fmt.Sprint("urn:uuid:", uuid.New())
 }
+
+// Unescape unescapes the string s. It returns an IRI.
+func Unescape(s string) string {
+	return html.UnescapeString(s)
+}

category.go

@@ -2,9 +2,7 @@ package atom
 
 import (
 	"encoding/xml"
-	"errors"
 	"fmt"
-	"html"
 )
 
 type Category struct {
@@ -12,46 +10,16 @@ type Category struct {
 	*CommonAttributes
 	Term   string `xml:"term,attr"`
 	Scheme string `xml:"scheme,attr,omitempty"` // IRI
-	Label  string `xml:"label,attr,omitempty"`
+	Label  string `xml:"label,attr,omitempty"`  // Must be unescaped
 }
 
-// NewCategory creates a new Category. It returns a *Category and an error.
+// NewCategory creates a new Category. It takes in a string term and returns a
-func NewCategory(term string) (*Category, error) {
+// *Category.
-	if term == "" {
+func NewCategory(term string) *Category {
-		return nil, errors.New("error creating new category: term string empty")
+	return &Category{
+		CommonAttributes: NewCommonAttributes(),
+		Term:             term,
 	}
-
-	return &Category{Term: term}, nil
-}
-
-// SetTerm sets the Term attribute of the Category. It returns an error.
-func (c *Category) SetTerm(t string) error {
-	if t == "" {
-		return errors.New("error setting term of category: t string empty")
-	}
-
-	c.Term = t
-	return nil
-}
-
-// SetScheme sets the Scheme attribute of the Category. It returns an error.
-func (c *Category) SetScheme(s string) error {
-	if !isValidIRI(s) {
-		return fmt.Errorf("scheme %v not correctly formatted", s)
-	}
-
-	c.Scheme = s
-	return nil
-}
-
-// SetLabel sets the Label attribute of the Category. It returns an error.
-func (c *Category) SetLabel(label string) error {
-	if label == "" {
-		return errors.New("error setting label of category: label string empty")
-	}
-
-	c.Label = html.UnescapeString(label)
-	return nil
 }
 
 // Check checks the Category for incompatibilities with RFC4287. It returns an

commonAttributes.go

@@ -2,7 +2,7 @@ package atom
 
 import (
 	"encoding/xml"
-	"errors"
+	"fmt"
 )
 
 type CommonAttributes struct {
@@ -17,20 +17,31 @@ func NewCommonAttributes() *CommonAttributes {
 	return new(CommonAttributes)
 }
 
-// AddAttribute adds the Attribute to the CommonAttributes. It returns an error.
+// AddAttribute adds an attribute to the CommonAttributes. It takes in the
-func (c *CommonAttributes) AddAttribute(name, value string) error {
+// strings name and value and returns the index as an int.
-	if name == "" {
+func (c *CommonAttributes) AddAttribute(name, value string) int {
-		return errors.New("error adding attribute: name string empty")
+	return addToSlice(&c.UndefinedAttributes, &xml.Attr{Name: xml.Name{Local: name}, Value: value})
-	}
+}
-	if value == "" {
-		return errors.New("error adding attribute: value string empty")
-	}
 
-	if c.UndefinedAttributes == nil {
+// DeleteAttribute deletes the attribute at index from the CommonAttributes. It
-		c.UndefinedAttributes = make([]*xml.Attr, 1)
+// returns an error.
-		c.UndefinedAttributes[0] = &xml.Attr{Name: xml.Name{Local: name}, Value: value}
+func (c *CommonAttributes) DeleteAttribute(index int) error {
-	} else {
+	if err := deleteFromSlice(&c.UndefinedAttributes, index); err != nil {
-		c.UndefinedAttributes = append(c.UndefinedAttributes, &xml.Attr{Name: xml.Name{Local: name}, Value: value})
+		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

@@ -1,7 +1,5 @@
 package atom
 
-import "fmt"
-
 const (
 	InlineText = iota
 	InlineXHTML
@@ -16,18 +14,21 @@ type Content interface {
 	Check() error
 }
 
-// NewContent creates a new Content. It returns a Content and an error.
+// NewContent creates a new Content. It takes in an int contentType, a string
-func NewContent(contentType int, mediaType string, content any) (Content, error) {
+// mediaType and an any content and returns a Content.
+//
+// If contentType is invalid, it returns nil.
+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("error creating new content: %v is not a valid text type", contentType)
+		return nil
 	}
 }

date.go

@@ -10,15 +10,18 @@ type Date struct {
 	DateTime string `xml:",chardata"`
 }
 
-// DateTime formats a time.Time to string formated as defined by RFC3339. It
+// DateTime formats the time.Time t to a string as defined by RFC3339. It
 // returns a string.
 func DateTime(t time.Time) string {
 	return t.Format(time.RFC3339)
 }
 
-// NewDate creates a new Date. It returns a *Date.
+// NewDate creates a new Date. It takes in a time.Time t and 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"
@@ -31,25 +30,26 @@ type Entry struct {
 }
 
 // checkAuthors checks the entry's authors for incompatibilities with RFC4287.
-// It returns an errors.
+// It takes in a bool authorIsInFeed and returns an errors.
+//
 // atom:entry elements MUST contain one or more atom:author elements, unless
 // the atom:entry contains an atom:source element that contains an atom:author
 // element or, in an Atom Feed Document, the atom:feed element contains an
 // atom:author element itself.
-func (e *Entry) checkAuthors(authorInFeed bool) error {
+func (e *Entry) checkAuthors(authorIsInFeed bool) error {
 	if e.Authors == nil {
-		if !authorInFeed {
+		if !authorIsInFeed {
 			if e.Source == nil {
-				return fmt.Errorf("no authors set in entry %v", e)
+				return fmt.Errorf("no authors set in entry %v", e.ID.URI)
 			}
 			if e.Source.Authors == nil {
-				return fmt.Errorf("no authors set in entry %v", e)
+				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: %v", i, e, err)
+				return fmt.Errorf("author element %v of entry %v: %v", i, e.ID.URI, err)
 			}
 		}
 	}
@@ -57,108 +57,109 @@ func (e *Entry) checkAuthors(authorInFeed bool) error {
 	return nil
 }
 
-// NewEntry creates a new Entry. It returns a *Entry and an error.
+// update sets the Updated time to time.Now.
-func NewEntry(title string) (*Entry, error) {
+func (e *Entry) update() {
-	text, err := NewText("text", title)
+	if e.Updated == nil {
-	if err != nil {
+		e.Updated = NewDate(time.Now())
-		return nil, fmt.Errorf("error creating new entry: %v", err)
+	} else {
-	}
+		e.Updated.DateTime = DateTime(time.Now())
-
-	id, err := NewID(NewURN())
-	if err != nil {
-		return nil, fmt.Errorf("error creating new entry: %v", err)
 	}
+}
 
+// NewEntry creates a new Entry. It takes in a string title and returns a
+// *Entry.
+func NewEntry(title string) *Entry {
 	return &Entry{
-		ID:      id,
+		CommonAttributes: NewCommonAttributes(),
-		Title:   text,
+		ID:               NewID(NewURN()),
-		Updated: NewDate(time.Now()),
+		Title:            NewText("text", title),
-	}, nil
+		Updated:          NewDate(time.Now()),
+	}
 }
 
-// AddAuthor adds the Person as an author to the Entry. It returns an error.
+// AddAuthor adds the Person a as an author to the Entry. It returns the index
-func (e *Entry) AddAuthor(p *Person) error {
+// as an int.
-	if p == nil {
+func (e *Entry) AddAuthor(a *Person) int {
-		return errors.New("error adding author element to entry: *Person is nil")
+	e.update()
+	return addToSlice(&e.Authors, a)
+}
+
+// DeleteAuthor deletes the Person at index from the Entry. It returns 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.Authors == nil {
+	e.update()
-		e.Authors = make([]*Person, 1)
-		e.Authors[0] = p
-	} else {
-		e.Authors = append(e.Authors, p)
-	}
-
-	e.Updated.DateTime = DateTime(time.Now())
 	return nil
 }
 
-// AddCategory adds the Category to the Entry. It returns an error.
+// AddCategory adds the Category c to the Entry. It returns the index as an int.
-func (e *Entry) AddCategory(c *Category) error {
+func (e *Entry) AddCategory(c *Category) int {
-	if c == nil {
+	e.update()
-		return errors.New("error adding category element to entry: *Category is nil")
+	return addToSlice(&e.Categories, c)
-	}
-
-	if e.Categories == nil {
-		e.Categories = make([]*Category, 1)
-		e.Categories[0] = c
-	} else {
-		e.Categories = append(e.Categories, c)
-	}
-
-	e.Updated.DateTime = DateTime(time.Now())
-	return nil
 }
 
-// AddContributor adds the Person as a contributor to the Entry. It returns an
+// DeleteCategory deletes the Category at index from the Entry. It returns an
 // error.
-func (e *Entry) AddContributor(c *Person) error {
+func (e *Entry) DeleteCategory(index int) error {
-	if c == nil {
+	if err := deleteFromSlice(&e.Categories, index); err != nil {
-		return errors.New("error adding contributor element to entry: *Person is nil")
+		return fmt.Errorf("error deleting category %v from entry %v: %v", index, e.ID.URI, err)
 	}
 
-	if e.Contributors == nil {
+	e.update()
-		e.Contributors = make([]*Person, 1)
-		e.Contributors[0] = c
-	} else {
-		e.Contributors = append(e.Contributors, c)
-	}
-
-	e.Updated.DateTime = DateTime(time.Now())
 	return nil
 }
 
-// AddLink adds the Link to the Entry. It returns an error.
+// AddContributor adds the Person c as a contributor to the Entry. It returns
-func (e *Entry) AddLink(l *Link) error {
+// the index as an int.
-	if l == nil {
+func (e *Entry) AddContributor(c *Person) int {
-		return errors.New("error adding link element to entry: *Link is nil")
+	e.update()
+	return addToSlice(&e.Contributors, c)
+}
+
+// DeleteContributor deletes the Person at index from the Entry. It returns 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.Links == nil {
+	e.update()
-		e.Links = make([]*Link, 1)
-		e.Links[0] = l
-	} else {
-		e.Links = append(e.Links, l)
-	}
-
-	e.Updated.DateTime = DateTime(time.Now())
 	return nil
 }
 
-// AddExtension adds the ExtensionElement to the Entry. It returns an error.
+// AddLink adds the Link l to the Entry. It returns the index as an int.
-func (e *Entry) AddExtension(x *ExtensionElement) error {
+func (e *Entry) AddLink(l *Link) int {
-	if x == nil {
+	e.update()
-		return errors.New("error adding extension element to entry: *ExtensionElement is nil")
+	return addToSlice(&e.Links, l)
+}
+
+// DeleteLink deletes the Link at index from the Entry. It returns 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.Extensions == nil {
+	e.update()
-		e.Extensions = make([]*ExtensionElement, 1)
+	return nil
-		e.Extensions[0] = x
+}
-	} else {
+
-		e.Extensions = append(e.Extensions, x)
+// AddExtension adds the ExtensionElement x to the Entry. It returns the index
+// as an int.
+func (e *Entry) AddExtension(x *ExtensionElement) int {
+	e.update()
+	return addToSlice(&e.Extensions, x)
+}
+
+// DeleteExtension deletes the Extension at index from the Entry. It returns 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)
 	}
 
-	e.Updated.DateTime = DateTime(time.Now())
+	e.update()
 	return nil
 }
 
@@ -174,64 +175,64 @@ func (e *Entry) Check() error {
 	}
 
 	if err := e.checkAuthors(true); err != nil {
-		return fmt.Errorf("entry %v: %v", e, err)
+		return fmt.Errorf("entry %v: %v", e.ID.URI, err)
 	}
 
 	for i, c := range e.Categories {
 		if err := c.Check(); err != nil {
-			return fmt.Errorf("category element %v of entry %v: %v", i, e, err)
+			return fmt.Errorf("category element %v of entry %v: %v", i, e.ID.URI, err)
 		}
 	}
 
 	if e.Content != nil {
 		if err := e.Content.Check(); err != nil {
-			return fmt.Errorf("content element of entry %v: %v", e, err)
+			return fmt.Errorf("content element of entry %v: %v", e.ID.URI, err)
 		}
 	} else {
 		// atom:entry elements that contain no child atom:content element MUST
 		// contain at least one atom:link element with a rel attribute value of
 		// "alternate".
 		if !alternateRelExists(e.Links) {
-			return fmt.Errorf("no content element of entry %v and no link element with rel \"alternate\"", e)
+			return fmt.Errorf("no content element of entry %v and no link element with rel \"alternate\"", e.ID.URI)
 		}
 	}
 
 	for i, c := range e.Contributors {
 		if err := c.Check(); err != nil {
-			return fmt.Errorf("contributor element %v of entry %v: %v", i, e, err)
+			return fmt.Errorf("contributor element %v of entry %v: %v", i, e.ID.URI, err)
 		}
 	}
 
 	for i, l := range e.Links {
 		if err := l.Check(); err != nil {
-			return fmt.Errorf("link element %v of entry %v: %v", i, e, err)
+			return fmt.Errorf("link element %v of entry %v: %v", i, e.ID.URI, err)
 		}
 	}
 	if hasAlternateDuplicateLinks(e.Links) {
-		return fmt.Errorf("links with a rel attribute value of \"alternate\" and duplicate type and hreflang attribute values found in entry %v", e)
+		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 {
 		if err := e.Published.Check(); err != nil {
-			return fmt.Errorf("published element of entry %v: %v", e, err)
+			return fmt.Errorf("published element of entry %v: %v", e.ID.URI, err)
 		}
 	}
 
 	if e.Rights != nil {
 		if err := e.Rights.Check(); err != nil {
-			return fmt.Errorf("rights element of entry %v: %v", e, err)
+			return fmt.Errorf("rights element of entry %v: %v", e.ID.URI, err)
 		}
 	}
 
 	if e.Source != nil {
 		if err := e.Source.Check(); err != nil {
-			return fmt.Errorf("source element of entry %v: %v", e, err)
+			return fmt.Errorf("source element of entry %v: %v", e.ID.URI, err)
 		}
 	}
 
 	if e.Summary != nil {
 		if err := e.Summary.Check(); err != nil {
-			return fmt.Errorf("summary element of entry %v: %v", e, err)
+			return fmt.Errorf("summary element of entry %v: %v", e.ID.URI, err)
 		}
 	} else {
 		// atom:entry elements MUST contain an atom:summary element in either
@@ -239,7 +240,7 @@ func (e *Entry) Check() error {
 		// the atom:entry contains an atom:content that has a "src" attribute
 		// (and is thus empty).
 		if e.Content.hasSRC() {
-			return fmt.Errorf("no summary element of entry %v but content of type out of line content", e)
+			return fmt.Errorf("no summary element of entry %v but content of type out of line content", e.ID.URI)
 		}
 		// the atom:entry contains content that is encoded in Base64; i.e., the
 		// "type" attribute of atom:content is a MIME media type [MIMEREG], but
@@ -247,40 +248,41 @@ func (e *Entry) Check() error {
 		// does not end with "/xml" or "+xml".
 		mediaType := e.Content.getType()
 		if isValidMediaType(mediaType) && !isXMLMediaType(mediaType) && !strings.HasPrefix(mediaType, "text/") {
-			return fmt.Errorf("no summary element of entry %v but media type not xml", e)
+			return fmt.Errorf("no summary element of entry %v but media type not xml", e.ID.URI)
 		}
 	}
 
 	if e.Title == nil {
-		return fmt.Errorf("no title element of entry %v", e)
+		return fmt.Errorf("no title element of entry %v", e.ID.URI)
 	} else {
 		if err := e.Title.Check(); err != nil {
-			return fmt.Errorf("title element of entry %v: %v", e, err)
+			return fmt.Errorf("title element of entry %v: %v", e.ID.URI, err)
 		}
 	}
 
 	if e.Updated == nil {
-		return fmt.Errorf("no updated element of entry %v", e)
+		return fmt.Errorf("no updated element of entry %v", e.ID.URI)
 	} else {
 		if err := e.Updated.Check(); err != nil {
-			return fmt.Errorf("updated element of entry %v: %v", e, err)
+			return fmt.Errorf("updated element of entry %v: %v", e.ID.URI, err)
 		}
 	}
 
 	for i, x := range e.Extensions {
 		if err := x.Check(); err != nil {
-			return fmt.Errorf("extension element %v of entry %v: %v", i, e, err)
+			return fmt.Errorf("extension element %v of entry %v: %v", i, e.ID.URI, err)
 		}
 	}
 
 	return nil
 }
 
-// ToXML converts the Feed to XML. It returns a string and an error.
+// ToXML converts the Feed to XML. It takes in a string encoding and returns a
+// string and an 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 {
@@ -10,28 +10,21 @@ type ExtensionElement struct {
 	XMLName xml.Name
 }
 
-// NewExtensionElement creates a new ExtensionElement. It returns a
+// NewExtensionElement creates a new ExtensionElement. It takes in a string name
-// *ExtensionElement and an error.
+// and any value and returns a *ExtensionElement.
-func NewExtensionElement(name string, value any) (*ExtensionElement, error) {
+func NewExtensionElement(name string, value any) *ExtensionElement {
-	if name == "" {
+	return &ExtensionElement{XMLName: xml.Name{Local: name}, Value: value}
-		return nil, errors.New("error adding extension attribute: name string empty")
-	}
-	if value == "" {
-		return nil, errors.New("error adding extension attribute: value string empty")
-	}
-
-	return &ExtensionElement{XMLName: xml.Name{Local: name}, Value: value}, nil
 }
 
 // Check checks the ExtensionElement for incompatibilities with RFC4287. It
 // returns an error.
 func (e *ExtensionElement) Check() error {
 	if e.XMLName.Local == "" {
-		return errors.New("xml name of extension empty")
+		return fmt.Errorf("xml name of extension %v empty", e)
 	}
 
 	if e.Value == nil {
-		return errors.New("value element of extension 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"
 )
@@ -26,126 +25,150 @@ type Feed struct {
 	Entries      []*Entry            `xml:",omitempty"`
 }
 
-// NewFeed creates a new Feed. It returns a *Feed and an error.
+// update sets the Updated time to time.Now.
-func NewFeed(title string) (*Feed, error) {
+func (f *Feed) update() {
-	text, err := NewText("text", title)
+	if f.Updated == nil {
-	if err != nil {
+		f.Updated = NewDate(time.Now())
-		return nil, fmt.Errorf("error creating new feed: %v", err)
+	} else {
-	}
+		f.Updated.DateTime = DateTime(time.Now())
-
-	id, err := NewID(NewURN())
-	if err != nil {
-		return nil, fmt.Errorf("error creating new feed: %v", err)
 	}
+}
 
+// NewFeed creates a new Feed. It takes in a string title and returns a *Feed.
+func NewFeed(title string) *Feed {
 	return &Feed{
-		ID:      id,
+		CommonAttributes: NewCommonAttributes(),
-		Title:   text,
+		ID:               NewID(NewURN()),
-		Updated: NewDate(time.Now()),
+		Title:            NewText("text", title),
-	}, nil
+		Updated:          NewDate(time.Now()),
+	}
 }
 
-// AddAuthor adds the Person as an author to the Feed. It returns an error.
+// AddAuthor adds the Person a as an author to the Feed. It returns the index as
-func (f *Feed) AddAuthor(p *Person) error {
+// an int.
-	if p == nil {
+func (f *Feed) AddAuthor(a *Person) int {
-		return errors.New("error adding author element to feed: *Person is nil")
+	f.update()
+	return addToSlice(&f.Authors, a)
+}
+
+// DeleteAuthor deletes the Person at index from the Feed. It returns 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.Authors == nil {
+	f.update()
-		f.Authors = make([]*Person, 1)
-		f.Authors[0] = p
-	} else {
-		f.Authors = append(f.Authors, p)
-	}
-
-	f.Updated.DateTime = DateTime(time.Now())
 	return nil
 }
 
-// AddCategory adds the Category to the Feed. It returns an error.
+// AddCategory adds the Category c to the Feed. It returns the index as an int.
-func (f *Feed) AddCategory(c *Category) error {
+func (f *Feed) AddCategory(c *Category) int {
-	if c == nil {
+	f.update()
-		return errors.New("error adding category element to feed: *Category is nil")
+	return addToSlice(&f.Categories, c)
-	}
-
-	if f.Categories == nil {
-		f.Categories = make([]*Category, 1)
-		f.Categories[0] = c
-	} else {
-		f.Categories = append(f.Categories, c)
-	}
-
-	f.Updated.DateTime = DateTime(time.Now())
-	return nil
 }
 
-// AddContributor adds the Person as a contributor to the Feed. It returns an
+// DeleteCategory deletes the Category at index from the Feed. It returns an
 // error.
-func (f *Feed) AddContributor(c *Person) error {
+func (f *Feed) DeleteCategory(index int) error {
-	if c == nil {
+	if err := deleteFromSlice(&f.Categories, index); err != nil {
-		return errors.New("error adding contributor element to feed: *Person is nil")
+		return fmt.Errorf("error deleting category %v from entry %v: %v", index, f.ID.URI, err)
 	}
 
-	if f.Contributors == nil {
+	f.update()
-		f.Contributors = make([]*Person, 1)
-		f.Contributors[0] = c
-	} else {
-		f.Contributors = append(f.Contributors, c)
-	}
-
-	f.Updated.DateTime = DateTime(time.Now())
 	return nil
 }
 
-// AddLink adds the Link to the Feed. It returns an error. There should be one
+// AddContributor adds the Person c as a contributor to the Feed. It returns the
-// Link with Rel "self".
+// index as an int.
-func (f *Feed) AddLink(l *Link) error {
+func (f *Feed) AddContributor(c *Person) int {
-	if l == nil {
+	f.update()
-		return errors.New("error adding link element to feed: *Link is nil")
+	return addToSlice(&f.Contributors, c)
+}
+
+// DeleteContributor deletes the Person at index from the Feed. It returns 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.Links == nil {
+	f.update()
-		f.Links = make([]*Link, 1)
-		f.Links[0] = l
-	} else {
-		f.Links = append(f.Links, l)
-	}
-
-	f.Updated.DateTime = DateTime(time.Now())
 	return nil
 }
 
-// AddExtension adds the Extension to the Feed. It returns an error.
+// AddLink adds the Link l to the Feed. It returns the index as an int.
-func (f *Feed) AddExtension(e *ExtensionElement) error {
+//
-	if e == nil {
+// There should be one Link with Rel "self".
-		return errors.New("error adding extension element to feed: *ExtensionElement is nil")
+func (f *Feed) AddLink(l *Link) int {
+	f.update()
+	return addToSlice(&f.Links, l)
+}
+
+// DeleteLink deletes the Link at index from the Feed. It returns 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.Extensions == nil {
+	f.update()
-		f.Extensions = make([]*ExtensionElement, 1)
-		f.Extensions[0] = e
-	} else {
-		f.Extensions = append(f.Extensions, e)
-	}
-
-	f.Updated.DateTime = DateTime(time.Now())
 	return nil
 }
 
-// AddEntry adds the Entry to the Feed. It returns an error.
+// AddExtension adds the Extension e to the Feed. It returns the index as an
-func (f *Feed) AddEntry(e *Entry) error {
+// int.
-	if e == nil {
+func (f *Feed) AddExtension(e *ExtensionElement) int {
-		return errors.New("error adding entry element to feed: *Entry is nil")
+	f.update()
+	return addToSlice(&f.Extensions, e)
+}
+
+// DeleteExtension deletes the Extension at index from the Feed. It returns 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.Entries == nil {
+	f.update()
-		f.Entries = make([]*Entry, 1)
+	return nil
-		f.Entries[0] = e
+}
-	} else {
+
-		f.Entries = append(f.Entries, e)
+// AddEntry adds the Entry e to the Feed. It returns the index as an int.
+func (f *Feed) AddEntry(e *Entry) int {
+	f.update()
+	return addToSlice(&f.Entries, e)
+}
+
+// DeleteEntry deletes the Entry at index from the Feed. It returns 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)
 	}
 
-	f.Updated.DateTime = DateTime(time.Now())
+	f.update()
+	return nil
+}
+
+// DeleteEntryByURI deletes the Entry from the Feed. It takes in a string uri
+// and returns 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:]...)
+	f.update()
+
 	return nil
 }
 
@@ -166,93 +189,93 @@ func (f *Feed) Check() error {
 	if f.Authors == nil {
 		for _, e := range f.Entries {
 			if err := e.checkAuthors(false); err != nil {
-				return fmt.Errorf("no authors set in feed %v: %v", f, err)
+				return fmt.Errorf("no authors set in feed %v: %v", f.ID.URI, err)
 			}
 		}
 	} else {
 		for i, a := range f.Authors {
 			if err := a.Check(); err != nil {
-				return fmt.Errorf("author element %v of feed %v: %v", i, f, err)
+				return fmt.Errorf("author element %v of feed %v: %v", i, f.ID.URI, err)
 			}
 		}
 	}
 
 	for i, c := range f.Categories {
 		if err := c.Check(); err != nil {
-			return fmt.Errorf("category element %v of feed %v: %v", i, f, err)
+			return fmt.Errorf("category element %v of feed %v: %v", i, f.ID.URI, err)
 		}
 	}
 
 	for i, c := range f.Contributors {
 		if err := c.Check(); err != nil {
-			return fmt.Errorf("contributor element %v of feed %v: %v", i, f, err)
+			return fmt.Errorf("contributor element %v of feed %v: %v", i, f.ID.URI, err)
 		}
 	}
 
 	if f.Generator != nil {
 		if err := f.Generator.Check(); err != nil {
-			return fmt.Errorf("generator element of feed %v: %v", f, err)
+			return fmt.Errorf("generator element of feed %v: %v", f.ID.URI, err)
 		}
 	}
 
 	if f.Icon != nil {
 		if err := f.Icon.Check(); err != nil {
-			return fmt.Errorf("icon element of feed %v: %v", f, err)
+			return fmt.Errorf("icon element of feed %v: %v", f.ID.URI, err)
 		}
 	}
 
 	for i, l := range f.Links {
 		if err := l.Check(); err != nil {
-			return fmt.Errorf("link element %v of feed %v: %v", i, f, err)
+			return fmt.Errorf("link element %v of feed %v: %v", i, f.ID.URI, err)
 		}
 	}
 	if hasAlternateDuplicateLinks(f.Links) {
-		return fmt.Errorf("links with a rel attribute value of \"alternate\" and duplicate type and hreflang attribute values found in feed %v", f)
+		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 {
 		if err := f.Logo.Check(); err != nil {
-			return fmt.Errorf("logo element of feed %v: %v", f, err)
+			return fmt.Errorf("logo element of feed %v: %v", f.ID.URI, err)
 		}
 	}
 
 	if f.Rights != nil {
 		if err := f.Rights.Check(); err != nil {
-			return fmt.Errorf("rights element of feed %v: %v", f, err)
+			return fmt.Errorf("rights element of feed %v: %v", f.ID.URI, err)
 		}
 	}
 
 	if f.Subtitle != nil {
 		if err := f.Subtitle.Check(); err != nil {
-			return fmt.Errorf("subtitle element of feed %v: %v", f, err)
+			return fmt.Errorf("subtitle element of feed %v: %v", f.ID.URI, err)
 		}
 	}
 
 	if f.Title == nil {
-		return fmt.Errorf("no title element of feed %v", f)
+		return fmt.Errorf("no title element of feed %v", f.ID.URI)
 	} else {
 		if err := f.Title.Check(); err != nil {
-			return fmt.Errorf("title element of feed %v: %v", f, err)
+			return fmt.Errorf("title element of feed %v: %v", f.ID.URI, err)
 		}
 	}
 
 	if f.Updated == nil {
-		return fmt.Errorf("no updated element of feed %v", f)
+		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, err)
+			return fmt.Errorf("updated element of feed %v: %v", f.ID.URI, err)
 		}
 	}
 
 	for i, x := range f.Extensions {
 		if err := x.Check(); err != nil {
-			return fmt.Errorf("extension element %v of feed %v: %v", i, f, err)
+			return fmt.Errorf("extension element %v of feed %v: %v", i, f.ID.URI, err)
 		}
 	}
 
 	for i, n := range f.Entries {
 		if err := n.Check(); err != nil {
-			return fmt.Errorf("entry element %v of feed %v: %v", i, f, err)
+			return fmt.Errorf("entry element %v of feed %v: %v", i, f.ID.URI, err)
 		}
 	}
 
@@ -263,7 +286,7 @@ 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"
 )
@@ -15,22 +14,13 @@ type Generator struct {
 	Text    string `xml:",chardata"`
 }
 
-// NewGenerator creates a new Generator. It returns a *Generator and an error.
+// NewGenerator creates a new Generator. It takes in a string text and returns a
-func NewGenerator(text string) (*Generator, error) {
+// *Generator.
-	if text == "" {
+func NewGenerator(text string) *Generator {
-		return nil, errors.New("error creating new generator: text string empty")
+	return &Generator{
+		CommonAttributes: NewCommonAttributes(),
+		Text:             html.UnescapeString(text),
 	}
-
-	return &Generator{Text: html.UnescapeString(text)}, nil
-}
-
-// SetURI sets the URI attribute of the Generator. It returns an error.
-func (g *Generator) SetURI(uri string) error {
-	if !isValidIRI(uri) {
-		return fmt.Errorf("uri %v not correctly formatted", g)
-	}
-
-	return nil
 }
 
 // Check checks the Generator for incompatibilities with RFC4287. It returns an

icon.go

@@ -12,13 +12,12 @@ type Icon struct {
 	URI string `xml:",chardata"` // IRI
 }
 
-// NewIcon creates a new Icon. It returns a *Icon and an error.
+// NewIcon creates a new Icon. It takes in a string uri and returns a *Icon.
-func NewIcon(uri string) (*Icon, error) {
+func NewIcon(uri string) *Icon {
-	if !isValidIRI(uri) {
+	return &Icon{
-		return nil, fmt.Errorf("uri %v not correctly formatted", uri)
+		CommonAttributes: NewCommonAttributes(),
+		URI:              uri,
 	}
-
-	return &Icon{URI: uri}, nil
 }
 
 // Check checks the Icon for incompatibilities with RFC4287. It returns an

id.go

@@ -12,13 +12,12 @@ type ID struct {
 	URI string `xml:",chardata"` // IRI
 }
 
-// NewID creates a new ID. It returns a *ID and an error.
+// NewID creates a new ID. It takes in a string uri and returns a *ID.
-func NewID(uri string) (*ID, error) {
+func NewID(uri string) *ID {
-	if !isValidIRI(uri) {
+	return &ID{
-		return nil, fmt.Errorf("uri %v not correctly formatted", uri)
+		CommonAttributes: NewCommonAttributes(),
+		URI:              uri,
 	}
-
-	return &ID{URI: uri}, nil
 }
 
 // Check checks the ID for incompatibilities with RFC4287. It returns an error.

inlineOtherContent.go

@@ -13,15 +13,16 @@ type InlineOtherContent struct {
 	Type       string `xml:"type,attr,omitempty"` // MediaType
 }
 
-// newInlineOtherContent creates a new InlineOtherContent. It returns a
+// newInlineOtherContent creates a new InlineOtherContent. It takes in the string
-// *InlineOtherContent and an error.
+// mediaType and any content and returns a *InlineOtherContent and an error.
-func newInlineOtherContent(mediaType string, content any) (*InlineOtherContent, error) {
+func newInlineOtherContent(mediaType string, content any) *InlineOtherContent {
-	if !isValidMediaType(mediaType) {
-		return nil, fmt.Errorf("error creating new inline other content: media type %v invalid", mediaType)
-	}
 	mediaType, _, _ = mime.ParseMediaType(mediaType)
 
-	return &InlineOtherContent{Type: mediaType, AnyElement: content}, nil
+	return &InlineOtherContent{
+		CommonAttributes: NewCommonAttributes(),
+		Type:             mediaType,
+		AnyElement:       content,
+	}
 }
 
 // isContent checks whether the InlineOtherContent is a Content. It returns a

inlineTextContent.go

@@ -12,19 +12,14 @@ type InlineTextContent struct {
 	Text string `xml:",chardata"`
 }
 
-// newInlineTextContent creates a new InlineTextContent. It returns a
+// newInlineTextContent creates a new InlineTextContent. It takes in the strings
-// *InlineTextContent and an error.
+// mediaType and text and returns a *InlineTextContent.
-func newInlineTextContent(mediaType string, content any) (*InlineTextContent, error) {
+func newInlineTextContent(mediaType, text string) *InlineTextContent {
-	if mediaType != "text" && mediaType != "html" && mediaType != "" {
+	return &InlineTextContent{
-		return nil, fmt.Errorf("media type %v incompatible with inline text content", mediaType)
+		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

inlineXHTMLContent.go

@@ -12,19 +12,14 @@ type InlineXHTMLContent struct {
 	Type     string `xml:"type,attr"`
 }
 
-// newInlineXHTMLContent creates a new InlineXHTMLContent. It returns a
+// newInlineXHTMLContent creates a new InlineXHTMLContent. It takes in the
-// *InlineXHTMLContent and an error.
+// string mediaType and the XHTMLDiv div and returns a *InlineXHTMLContent.
-func newInlineXHTMLContent(mediaType string, content any) (*InlineXHTMLContent, error) {
+func newInlineXHTMLContent(mediaType string, div *XHTMLDiv) *InlineXHTMLContent {
-	if mediaType != "xhtml" {
+	return &InlineXHTMLContent{
-		return nil, fmt.Errorf("media type %v incompatible with inline xhtml content", mediaType)
+		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

link.go

@@ -17,31 +17,12 @@ type Link struct {
 	Length   uint   `xml:"length,attr,omitempty"`
 }
 
-// NewLink creates a new Link. It returns a *Link and an error.
+// NewLink creates a new Link. It takes in the string href and returns a *Link.
-func NewLink(href string) (*Link, error) {
+func NewLink(href string) *Link {
-	if !isValidIRI(href) {
+	return &Link{
-		return nil, fmt.Errorf("href %v not correctly formatted", href)
+		CommonAttributes: NewCommonAttributes(),
+		Href:             href,
 	}
-
-	return &Link{Href: href}, nil
-}
-
-// SetType sets the Type attribute of the Link. It returns an error.
-func (l *Link) SetType(t string) error {
-	if !isValidMediaType(t) {
-		return fmt.Errorf("type %v invalid media type", l)
-	}
-
-	return nil
-}
-
-// SetHrefLang sets the HrefLang attribute of the Link. It returns an error.
-func (l *Link) SetHrefLang(h string) error {
-	if !isValidLanguageTag(h) {
-		return fmt.Errorf("hreflang %v invalid language tag", l)
-	}
-
-	return nil
 }
 
 // Check checks the Link for incompatibilities with RFC4287. It returns an

logo.go

@@ -11,13 +11,12 @@ type Logo struct {
 	URI string `xml:",chardata"` // IRI
 }
 
-// NewLogo creates a new Logo. It returns a *Logo and an error.
+// NewLogo creates a new Logo. It takes in a string uri and returns a *Logo.
-func NewLogo(uri string) (*Logo, error) {
+func NewLogo(uri string) *Logo {
-	if !isValidIRI(uri) {
+	return &Logo{
-		return nil, fmt.Errorf("uri %v not correctly formatted", uri)
+		CommonAttributes: NewCommonAttributes(),
+		URI:              uri,
 	}
-
-	return &Logo{URI: uri}, nil
 }
 
 // Check checks the Logo for incompatibilities with RFC4287. It returns an

outOfLineContent.go

@@ -13,24 +13,16 @@ type OutOfLineContent struct {
 	SRC  string `xml:"src,attr"`            // IRI
 }
 
-// newOutOfLineContent creates a new OutOfLineContent. It returns a
+// newOutOfLineContent creates a new OutOfLineContent. It takes in the strings
-// *OutOfLineContent and an error.
+// mediaType and src and returns a *OutOfLineContent.
-func newOutOfLineContent(mediaType string, content any) (*OutOfLineContent, error) {
+func newOutOfLineContent(mediaType, src string) *OutOfLineContent {
-	if !isValidMediaType(mediaType) {
-		return nil, fmt.Errorf("error creating new out of line content: media type %v invalid", mediaType)
-	}
 	mediaType, _, _ = mime.ParseMediaType(mediaType)
 
-	iri, ok := content.(string)
+	return &OutOfLineContent{
-	if !ok {
+		CommonAttributes: NewCommonAttributes(),
-		return nil, fmt.Errorf("content type %T incompatible with out of line content", content)
+		Type:             mediaType,
+		SRC:              src,
 	}
-
-	if !isValidIRI(iri) {
-		return nil, fmt.Errorf("content %v not a valid uri", iri)
-	}
-
-	return &OutOfLineContent{Type: mediaType, SRC: iri}, nil
 }
 
 // isContent checks whether the OutOfLineContent is a Content. It returns a

person.go

@@ -1,7 +1,6 @@
 package atom
 
 import (
-	"errors"
 	"fmt"
 	"net/mail"
 )
@@ -14,37 +13,27 @@ type Person struct {
 	Extensions []*ExtensionElement `xml:",any,omitempty"`
 }
 
-// NewPerson creates a new Person. It returns a *Person and an error.
+// NewPerson creates a new Person. It takes in a string name and returns a
-func NewPerson(name string) (*Person, error) {
+// *Person.
-	if name == "" {
+func NewPerson(name string) *Person {
-		return nil, errors.New("error creating new person: name string empty")
+	return &Person{
+		CommonAttributes: NewCommonAttributes(),
+		Name:             name,
 	}
-
-	return &Person{Name: name}, nil
 }
 
-// SetURI sets the URI element of the Person. It returns an error.
+// AddExtension adds the Extension e to the Person. It returns the index as an
-func (l *Link) SetURI(uri string) error {
+// int.
-	if !isValidIRI(uri) {
+func (p *Person) AddExtension(e *ExtensionElement) int {
-		return fmt.Errorf("uri %v not correctly formatted", uri)
+	return addToSlice(&p.Extensions, e)
-	}
-
-	return nil
 }
 
-// AddExtension adds the Extension to the Person. It returns an error.
+// DeleteExtension deletes the Extension at index from the Person. It returns an
-func (p *Person) AddExtension(e *ExtensionElement) error {
+// error.
-	if e == nil {
+func (p *Person) DeleteExtension(index int) error {
-		return errors.New("error adding extension element to person: *ExtensionElement is nil")
+	if err := deleteFromSlice(&p.Extensions, index); err != nil {
+		return fmt.Errorf("error deleting extension %v from person %v: %v", index, p, err)
 	}
-
-	if p.Extensions == nil {
-		p.Extensions = make([]*ExtensionElement, 1)
-		p.Extensions[0] = e
-	} else {
-		p.Extensions = append(p.Extensions, e)
-	}
-
 	return nil
 }
 
@@ -67,11 +56,9 @@ func (p *Person) Check() error {
 		}
 	}
 
-	if p.Extensions != nil {
+	for i, e := range p.Extensions {
-		for i, e := range p.Extensions {
+		if err := e.Check(); err != nil {
-			if err := e.Check(); err != nil {
+			return fmt.Errorf("extension element %v of person %v: %v", i, p, err)
-				return fmt.Errorf("extension element %v of person %v: %v", i, p, err)
-			}
 		}
 	}
 

plainText.go

@@ -1,7 +1,6 @@
 package atom
 
 import (
-	"errors"
 	"fmt"
 )
 
@@ -14,13 +13,14 @@ 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 and an error.
+// newPlainText creates a new PlainText. It takes in the strings textType and
-func newPlainText(textType, content string) (*PlainText, error) {
+// content and returns a *PlainText.
-	if content == "" {
+func newPlainText(textType, content string) *PlainText {
-		return nil, errors.New("error creating new plain text: content string empty")
+	return &PlainText{
+		CommonAttributes: NewCommonAttributes(),
+		Type:             textType,
+		Text:             content,
 	}
-
-	return &PlainText{Type: textType, Text: content}, nil
 }
 
 // Check checks the PlainText for incompatibilities with RFC4287. It returns an

source.go

@@ -25,7 +25,79 @@ type Source struct {
 
 // NewSource creates a new Source. It returns a *Source.
 func NewSource() *Source {
-	return new(Source)
+	return &Source{CommonAttributes: NewCommonAttributes()}
+}
+
+// AddAuthor adds the Person a as an author to the Source. It returns the index
+// as an int.
+func (s *Source) AddAuthor(a *Person) int {
+	return addToSlice(&s.Authors, a)
+}
+
+// DeleteAuthor deletes the Person at index from the Source. It returns 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 c to the Source. It returns the 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 returns 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 c as a contributor to the Source. It returns
+// the 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 returns 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 l to the Source. It returns the 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 returns 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 e to the Source. It returns the 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 returns 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

text.go

@@ -1,17 +1,17 @@
 package atom
 
-import (
+import "html"
-	"fmt"
-	"html"
-)
 
 type Text interface {
 	isText() bool
 	Check() error
 }
 
-// NewText creates a new Text. It returns a Text and an error.
+// NewText creates a new Text. It takes in the strings textType and content and
-func NewText(textType, content string) (Text, error) {
+// returns a Text.
+//
+// If textType is invalid it returns nil.
+func NewText(textType, content string) Text {
 	switch textType {
 	case "text", "":
 		return newPlainText(textType, content)
@@ -20,6 +20,6 @@ func NewText(textType, content string) (Text, error) {
 	case "xhtml":
 		return newXHTMLText(textType, content)
 	default:
-		return nil, fmt.Errorf("%v is not a valid text type", textType)
+		return nil
 	}
 }

xhtmlDiv.go

@@ -2,7 +2,6 @@ package atom
 
 import (
 	"encoding/xml"
-	"errors"
 	"fmt"
 )
 
@@ -12,16 +11,13 @@ type XHTMLDiv struct {
 	Content string   `xml:",innerxml"`
 }
 
-// NewXHTMLDiv creates a new XHTMLDiv. It returns a *XHTMLDiv and an error.
+// NewXHTMLDiv creates a new XHTMLDiv. It takes in a string content and returns
-func NewXHTMLDiv(content string) (*XHTMLDiv, error) {
+// a *XHTMLDiv.
-	if content == "" {
+func NewXHTMLDiv(content string) *XHTMLDiv {
-		return nil, errors.New("error creating new xhtml div: content string empty")
-	}
-
 	return &XHTMLDiv{
 		XMLNS:   "http://www.w3.org/1999/xhtml",
 		Content: content,
-	}, nil
+	}
 }
 
 // Check checks the XHTMLDiv for incompatibilities with RFC4287. It returns an

xhtmlText.go

@@ -13,17 +13,14 @@ 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 and an error.
+// newPlainText creates a new PlainText. It takes in the strings textType and
-func newXHTMLText(textType, content string) (*XHTMLText, error) {
+// content and returns a *PlainText.
-	xhtmlDiv, err := NewXHTMLDiv(content)
+func newXHTMLText(textType, content string) *XHTMLText {
-	if err != nil {
-		return nil, fmt.Errorf("error creating new xhtml text: %v", err)
-	}
-
 	return &XHTMLText{
-		Type:     textType,
+		CommonAttributes: NewCommonAttributes(),
-		XHTMLDiv: xhtmlDiv,
+		Type:             textType,
-	}, nil
+		XHTMLDiv:         NewXHTMLDiv(content),
+	}
 }
 
 // Check checks the XHTMLText for incompatibilities with RFC4287. It returns an