Added more error handling and necessary functions

README.md

@@ -14,9 +14,9 @@ go get git.streifling.com/jason/atom@latest
 
 ## Usage
 
-This library provides easy to use functions to safely create and extend elements
+This library provides convenient functions to safely create and extend elements
-and attributes of an Atom feed. The intended way of using it entails using these
+and attributes of an Atom feed. This is because it can be hard to know all
-functions.
+pitfalls of RFC4287. The intended way of using atom is with these functions.
 
 ```go
 package main

atom.go

@@ -10,18 +10,11 @@ import (
 	"golang.org/x/text/language"
 )
 
-type (
-	EmailAddress string
-	LanguageTag  string
-	MediaType    string
-	IRI          string
-)
-
 // 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 +33,8 @@ func isCorrectlyEscaped(text string) bool {
 
 // isCompositeMediaType checks whether a string is a composite media type. It
 // returns a bool.
-func isCompositeMediaType(mediaType string) bool {
+func isCompositeMediaType(m string) bool {
-	mediaType, _, err := mime.ParseMediaType(mediaType)
+	mediaType, _, err := mime.ParseMediaType(m)
 	if err != nil {
 		return false
 	}
@@ -51,8 +44,8 @@ func isCompositeMediaType(mediaType string) bool {
 
 // isXMLMediaType checks whether a string is an xml media type. It returns a
 // bool.
-func isXMLMediaType(mediaType string) bool {
+func isXMLMediaType(m string) bool {
-	mediaType, _, err := mime.ParseMediaType(mediaType)
+	mediaType, _, err := mime.ParseMediaType(m)
 	if err != nil {
 		return false
 	}
@@ -62,8 +55,8 @@ func isXMLMediaType(mediaType string) bool {
 
 // isValidMediaType checks whether a string is a valid media type. It returns a
 // bool.
-func isValidMediaType(mediaType string) bool {
+func isValidMediaType(m string) bool {
-	mediaType, _, err := mime.ParseMediaType(mediaType)
+	mediaType, _, err := mime.ParseMediaType(m)
 	if err != nil {
 		return false
 	}
@@ -77,8 +70,8 @@ func isValidMediaType(mediaType string) bool {
 }
 
 // isValidLanguageTag checks whether a LanguageTag is valid. It returns a bool.
-func isValidLanguageTag(tag LanguageTag) bool {
+func isValidLanguageTag(languageTag string) bool {
-	_, err := language.Parse(string(tag))
+	_, err := language.Parse(languageTag)
 	return err == nil
 }
 
@@ -89,6 +82,6 @@ func isValidAttribute(attribute string) bool {
 }
 
 // NewURN generates an new valid IRI based on a UUID. It returns an IRI.
-func NewURN() IRI {
+func NewURN() string {
-	return IRI(fmt.Sprint("urn:uuid:", uuid.New()))
+	return fmt.Sprint("urn:uuid:", uuid.New())
 }

category.go

@@ -2,6 +2,7 @@ package atom
 
 import (
 	"encoding/xml"
+	"errors"
 	"fmt"
 	"html"
 )
@@ -10,18 +11,47 @@ type Category struct {
 	XMLName xml.Name `xml:"category"`
 	*CommonAttributes
 	Term   string `xml:"term,attr"`
-	Scheme IRI    `xml:"scheme,attr,omitempty"`
+	Scheme string `xml:"scheme,attr,omitempty"` // IRI
 	Label  string `xml:"label,attr,omitempty"`
 }
 
-// NewCategory creates a new Category. It returns a *Category.
+// NewCategory creates a new Category. It returns a *Category and an error.
-func NewCategory(term string) *Category {
+func NewCategory(term string) (*Category, error) {
-	return &Category{Term: term}
+	if term == "" {
+		return nil, errors.New("error creating new category: term string empty")
+	}
+
+	return &Category{Term: term}, nil
 }
 
-// SetLabel sets the label of the Category.
+// SetTerm sets the Term attribute of the Category. It returns an error.
-func (c *Category) SetLabel(label string) {
+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

@@ -1,10 +1,13 @@
 package atom
 
-import "encoding/xml"
+import (
+	"encoding/xml"
+	"errors"
+)
 
 type CommonAttributes struct {
-	Base                IRI         `xml:"base,attr,omitempty"`
+	Base                string      `xml:"base,attr,omitempty"` // IRI
-	Lang                LanguageTag `xml:"lang,attr,omitempty"`
+	Lang                string      `xml:"lang,attr,omitempty"` // LanguageTag
 	UndefinedAttributes []*xml.Attr `xml:",attr,omitempty"`
 }
 
@@ -14,12 +17,21 @@ func NewCommonAttributes() *CommonAttributes {
 	return new(CommonAttributes)
 }
 
-// AddExtensionAttribute adds the ExtensionAttribute to the CommonAttributes.
+// AddAttribute adds the Attribute to the CommonAttributes. It returns an error.
-func (c *CommonAttributes) AddExtensionAttribute(name, value string) {
+func (c *CommonAttributes) AddAttribute(name, value string) error {
+	if name == "" {
+		return errors.New("error adding attribute: name string empty")
+	}
+	if value == "" {
+		return errors.New("error adding attribute: value string empty")
+	}
+
 	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})
 	}
+
+	return nil
 }

content.go

@@ -28,6 +28,6 @@ func NewContent(contentType int, mediaType string, content any) (Content, error)
 	case 3:
 		return newOutOfLineContent(mediaType, content)
 	default:
-		return nil, fmt.Errorf("%v is not a valid text type", contentType)
+		return nil, fmt.Errorf("error creating new content: %v is not a valid text type", contentType)
 	}
 }

date.go

@@ -13,7 +13,7 @@ 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.

entry.go

@@ -2,6 +2,7 @@ package atom
 
 import (
 	"encoding/xml"
+	"errors"
 	"fmt"
 	"strings"
 	"time"
@@ -75,8 +76,12 @@ func NewEntry(title string) (*Entry, error) {
 	}, nil
 }
 
-// AddAuthor adds the Person as an author to the Entry.
+// AddAuthor adds the Person as an author to the Entry. It returns an error.
-func (e *Entry) AddAuthor(p *Person) {
+func (e *Entry) AddAuthor(p *Person) error {
+	if p == nil {
+		return errors.New("error adding author element to entry: *Person is nil")
+	}
+
 	if e.Authors == nil {
 		e.Authors = make([]*Person, 1)
 		e.Authors[0] = p
@@ -85,10 +90,15 @@ func (e *Entry) AddAuthor(p *Person) {
 	}
 
 	e.Updated.DateTime = DateTime(time.Now())
+	return nil
 }
 
-// AddCategory adds the Category to the Entry.
+// AddCategory adds the Category to the Entry. It returns an error.
-func (e *Entry) AddCategory(c *Category) {
+func (e *Entry) AddCategory(c *Category) error {
+	if c == nil {
+		return errors.New("error adding category element to entry: *Category is nil")
+	}
+
 	if e.Categories == nil {
 		e.Categories = make([]*Category, 1)
 		e.Categories[0] = c
@@ -97,10 +107,16 @@ func (e *Entry) AddCategory(c *Category) {
 	}
 
 	e.Updated.DateTime = DateTime(time.Now())
+	return nil
 }
 
-// AddContributor adds the Person as a contributor to the Entry.
+// AddContributor adds the Person as a contributor to the Entry. It returns an
-func (e *Entry) AddContributor(c *Person) {
+// error.
+func (e *Entry) AddContributor(c *Person) error {
+	if c == nil {
+		return errors.New("error adding contributor element to entry: *Person is nil")
+	}
+
 	if e.Contributors == nil {
 		e.Contributors = make([]*Person, 1)
 		e.Contributors[0] = c
@@ -109,10 +125,15 @@ func (e *Entry) AddContributor(c *Person) {
 	}
 
 	e.Updated.DateTime = DateTime(time.Now())
+	return nil
 }
 
-// AddLink adds the Link to the Entry.
+// AddLink adds the Link to the Entry. It returns an error.
-func (e *Entry) AddLink(l *Link) {
+func (e *Entry) AddLink(l *Link) error {
+	if l == nil {
+		return errors.New("error adding link element to entry: *Link is nil")
+	}
+
 	if e.Links == nil {
 		e.Links = make([]*Link, 1)
 		e.Links[0] = l
@@ -121,10 +142,15 @@ func (e *Entry) AddLink(l *Link) {
 	}
 
 	e.Updated.DateTime = DateTime(time.Now())
+	return nil
 }
 
-// AddExtension adds the ExtensionElement to the Entry.
+// AddExtension adds the ExtensionElement to the Entry. It returns an error.
-func (e *Entry) AddExtension(x *ExtensionElement) {
+func (e *Entry) AddExtension(x *ExtensionElement) error {
+	if x == nil {
+		return errors.New("error adding extension element to entry: *ExtensionElement is nil")
+	}
+
 	if e.Extensions == nil {
 		e.Extensions = make([]*ExtensionElement, 1)
 		e.Extensions[0] = x
@@ -133,6 +159,7 @@ func (e *Entry) AddExtension(x *ExtensionElement) {
 	}
 
 	e.Updated.DateTime = DateTime(time.Now())
+	return nil
 }
 
 // Check checks the Entry for incompatibilities with RFC4287. It returns an

extensionElement.go

@@ -11,14 +11,25 @@ type ExtensionElement struct {
 }
 
 // NewExtensionElement creates a new ExtensionElement. It returns a
-// *ExtensionElement.
+// *ExtensionElement and an error.
-func NewExtensionElement(name string, value any) *ExtensionElement {
+func NewExtensionElement(name string, value any) (*ExtensionElement, error) {
-	return &ExtensionElement{XMLName: xml.Name{Local: name}, Value: value}
+	if name == "" {
+		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")
+	}
+
 	if e.Value == nil {
 		return errors.New("value element of extension empty")
 	}

feed.go

@@ -2,6 +2,7 @@ package atom
 
 import (
 	"encoding/xml"
+	"errors"
 	"fmt"
 	"time"
 )
@@ -44,8 +45,12 @@ func NewFeed(title string) (*Feed, error) {
 	}, nil
 }
 
-// AddAuthor adds the Person as an author to the Feed.
+// AddAuthor adds the Person as an author to the Feed. It returns an error.
-func (f *Feed) AddAuthor(p *Person) {
+func (f *Feed) AddAuthor(p *Person) error {
+	if p == nil {
+		return errors.New("error adding author element to feed: *Person is nil")
+	}
+
 	if f.Authors == nil {
 		f.Authors = make([]*Person, 1)
 		f.Authors[0] = p
@@ -54,10 +59,15 @@ func (f *Feed) AddAuthor(p *Person) {
 	}
 
 	f.Updated.DateTime = DateTime(time.Now())
+	return nil
 }
 
-// AddCategory adds the Category to the Feed.
+// AddCategory adds the Category to the Feed. It returns an error.
-func (f *Feed) AddCategory(c *Category) {
+func (f *Feed) AddCategory(c *Category) error {
+	if c == nil {
+		return errors.New("error adding category element to feed: *Category is nil")
+	}
+
 	if f.Categories == nil {
 		f.Categories = make([]*Category, 1)
 		f.Categories[0] = c
@@ -66,10 +76,16 @@ func (f *Feed) AddCategory(c *Category) {
 	}
 
 	f.Updated.DateTime = DateTime(time.Now())
+	return nil
 }
 
-// AddContributor adds the Person as a contributor to the Feed.
+// AddContributor adds the Person as a contributor to the Feed. It returns an
-func (f *Feed) AddContributor(c *Person) {
+// error.
+func (f *Feed) AddContributor(c *Person) error {
+	if c == nil {
+		return errors.New("error adding contributor element to feed: *Person is nil")
+	}
+
 	if f.Contributors == nil {
 		f.Contributors = make([]*Person, 1)
 		f.Contributors[0] = c
@@ -78,10 +94,16 @@ func (f *Feed) AddContributor(c *Person) {
 	}
 
 	f.Updated.DateTime = DateTime(time.Now())
+	return nil
 }
 
-// AddLink adds the Link to the Feed. There should be one Link with Rel "self".
+// AddLink adds the Link to the Feed. It returns an error. There should be one
-func (f *Feed) AddLink(l *Link) {
+// Link with Rel "self".
+func (f *Feed) AddLink(l *Link) error {
+	if l == nil {
+		return errors.New("error adding link element to feed: *Link is nil")
+	}
+
 	if f.Links == nil {
 		f.Links = make([]*Link, 1)
 		f.Links[0] = l
@@ -90,10 +112,15 @@ func (f *Feed) AddLink(l *Link) {
 	}
 
 	f.Updated.DateTime = DateTime(time.Now())
+	return nil
 }
 
-// AddExtension adds the Extension to the Feed.
+// AddExtension adds the Extension to the Feed. It returns an error.
-func (f *Feed) AddExtension(e *ExtensionElement) {
+func (f *Feed) AddExtension(e *ExtensionElement) error {
+	if e == nil {
+		return errors.New("error adding extension element to feed: *ExtensionElement is nil")
+	}
+
 	if f.Extensions == nil {
 		f.Extensions = make([]*ExtensionElement, 1)
 		f.Extensions[0] = e
@@ -102,10 +129,15 @@ func (f *Feed) AddExtension(e *ExtensionElement) {
 	}
 
 	f.Updated.DateTime = DateTime(time.Now())
+	return nil
 }
 
-// AddEntry adds the Entry to the Feed.
+// AddEntry adds the Entry to the Feed. It returns an error.
-func (f *Feed) AddEntry(e *Entry) {
+func (f *Feed) AddEntry(e *Entry) error {
+	if e == nil {
+		return errors.New("error adding entry element to feed: *Entry is nil")
+	}
+
 	if f.Entries == nil {
 		f.Entries = make([]*Entry, 1)
 		f.Entries[0] = e
@@ -114,6 +146,7 @@ func (f *Feed) AddEntry(e *Entry) {
 	}
 
 	f.Updated.DateTime = DateTime(time.Now())
+	return nil
 }
 
 // Check checks the Feed for incompatibilities with RFC4287. It returns an

generator.go

@@ -2,6 +2,7 @@ package atom
 
 import (
 	"encoding/xml"
+	"errors"
 	"fmt"
 	"html"
 )
@@ -9,14 +10,27 @@ 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.
+// NewGenerator creates a new Generator. It returns a *Generator and an error.
-func NewGenerator(text string) *Generator {
+func NewGenerator(text string) (*Generator, error) {
-	return &Generator{Text: html.UnescapeString(text)}
+	if text == "" {
+		return nil, errors.New("error creating new generator: text string empty")
+	}
+
+	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

@@ -9,11 +9,11 @@ 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) {
+func NewIcon(uri string) (*Icon, error) {
 	if !isValidIRI(uri) {
 		return nil, fmt.Errorf("uri %v not correctly formatted", uri)
 	}

id.go

@@ -9,16 +9,16 @@ 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) {
+func NewID(uri string) (*ID, error) {
 	if !isValidIRI(uri) {
 		return nil, fmt.Errorf("uri %v not correctly formatted", uri)
 	}
 
-	return &ID{URI: IRI(uri)}, nil
+	return &ID{URI: uri}, nil
 }
 
 // Check checks the ID for incompatibilities with RFC4287. It returns an error.

inlineOtherContent.go

@@ -9,18 +9,19 @@ import (
 type InlineOtherContent struct {
 	XMLName xml.Name `xml:"content"`
 	*CommonAttributes
-	AnyElement any       `xml:",chardata"`
+	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 {
+	if !isValidMediaType(mediaType) {
-		return nil, fmt.Errorf("media type %v incompatible with inline other content", mediaType)
+		return nil, fmt.Errorf("error creating new inline other content: media type %v invalid", mediaType)
 	}
+	mediaType, _, _ = mime.ParseMediaType(mediaType)
 
-	return &InlineOtherContent{Type: MediaType(mediaType), AnyElement: content}, nil
+	return &InlineOtherContent{Type: mediaType, AnyElement: content}, nil
 }
 
 // isContent checks whether the InlineOtherContent is a Content. It returns a
@@ -32,15 +33,15 @@ 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 {
+	if !isValidMediaType(mediaType) {
-		return fmt.Errorf("type attribute %v incompatible with inline other content", mediaType)
+		return fmt.Errorf("type attribute of inline other content %v invalid media type", i)
 	}
 
 	if isCompositeMediaType(mediaType) {

link.go

@@ -9,17 +9,39 @@ import (
 type Link struct {
 	XMLName xml.Name `xml:"link"`
 	*CommonAttributes
-	Title    string      `xml:"title,attr,omitempty"`
+	Title    string `xml:"title,attr,omitempty"`
-	Href     IRI         `xml:"href,attr"`
+	Href     string `xml:"href,attr"` // IRI
-	Rel      string      `xml:"rel,attr,omitempty"`
+	Rel      string `xml:"rel,attr,omitempty"`
-	Type     MediaType   `xml:"type,attr,omitempty"`
+	Type     string `xml:"type,attr,omitempty"`     // MediaType
-	HrefLang LanguageTag `xml:"hreflang,attr,omitempty"`
+	HrefLang string `xml:"hreflang,attr,omitempty"` // LanguageTag
-	Length   uint        `xml:"length,attr,omitempty"`
+	Length   uint   `xml:"length,attr,omitempty"`
 }
 
-// NewLink creates a new Link. It returns a *Link.
+// NewLink creates a new Link. It returns a *Link and an error.
-func NewLink(href string) *Link {
+func NewLink(href string) (*Link, error) {
-	return &Link{Href: IRI(href)}
+	if !isValidIRI(href) {
+		return nil, fmt.Errorf("href %v not correctly formatted", 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
@@ -34,13 +56,13 @@ func (l *Link) Check() error {
 	}
 
 	if l.Rel != "" {
-		if strings.Contains(l.Rel, ":") && !isValidIRI(IRI(l.Rel)) {
+		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)) {
+		if !isValidMediaType(l.Type) {
 			return fmt.Errorf("type attribute of link %v invalid media type", l)
 		}
 	}

logo.go

@@ -8,11 +8,11 @@ import (
 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.
+// NewLogo creates a new Logo. It returns a *Logo and an error.
-func NewLogo(uri IRI) (*Logo, error) {
+func NewLogo(uri string) (*Logo, error) {
 	if !isValidIRI(uri) {
 		return nil, fmt.Errorf("uri %v not correctly formatted", uri)
 	}

outOfLineContent.go

@@ -9,18 +9,19 @@ import (
 type OutOfLineContent struct {
 	XMLName xml.Name `xml:"content"`
 	*CommonAttributes
-	Type MediaType `xml:"type,attr,omitempty"`
+	Type string `xml:"type,attr,omitempty"` // MediaType
-	SRC  IRI       `xml:"src,attr"`
+	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 {
+	if !isValidMediaType(mediaType) {
-		return nil, fmt.Errorf("media type %v incompatible with out of line content", mediaType)
+		return nil, fmt.Errorf("error creating new out of line content: media type %v invalid", mediaType)
 	}
+	mediaType, _, _ = mime.ParseMediaType(mediaType)
 
-	iri, ok := content.(IRI)
+	iri, ok := content.(string)
 	if !ok {
 		return nil, fmt.Errorf("content type %T incompatible with out of line content", content)
 	}
@@ -29,7 +30,7 @@ func newOutOfLineContent(mediaType string, content any) (*OutOfLineContent, erro
 		return nil, fmt.Errorf("content %v not a valid uri", iri)
 	}
 
-	return &OutOfLineContent{Type: MediaType(mediaType), SRC: iri}, nil
+	return &OutOfLineContent{Type: mediaType, SRC: iri}, nil
 }
 
 // isContent checks whether the OutOfLineContent is a Content. It returns a
@@ -41,15 +42,15 @@ 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 {
+	if !isValidMediaType(mediaType) {
-		return fmt.Errorf("type attribute %v incompatible with out of line content", mediaType)
+		return fmt.Errorf("type attribute of out of line content %v invalid media type", o)
 	}
 
 	if isCompositeMediaType(mediaType) {

person.go

@@ -1,6 +1,7 @@
 package atom
 
 import (
+	"errors"
 	"fmt"
 	"net/mail"
 )
@@ -8,24 +9,43 @@ import (
 type Person struct {
 	*CommonAttributes
 	Name       string              `xml:"name"`
-	URI        IRI                 `xml:"uri,omitempty"`
+	URI        string              `xml:"uri,omitempty"`   // IRI
-	Email      EmailAddress        `xml:"email,omitempty"`
+	Email      string              `xml:"email,omitempty"` // EmailAddress
 	Extensions []*ExtensionElement `xml:",any,omitempty"`
 }
 
-// NewPerson creates a new Person. It returns a *Person.
+// NewPerson creates a new Person. It returns a *Person and an error.
-func NewPerson(name string) *Person {
+func NewPerson(name string) (*Person, error) {
-	return &Person{Name: name}
+	if name == "" {
+		return nil, errors.New("error creating new person: name string empty")
+	}
+
+	return &Person{Name: name}, nil
 }
 
-// AddExtension adds the Extension to the Person.
+// SetURI sets the URI element of the Person. It returns an error.
-func (p *Person) AddExtension(e *ExtensionElement) {
+func (l *Link) SetURI(uri string) error {
+	if !isValidIRI(uri) {
+		return fmt.Errorf("uri %v not correctly formatted", uri)
+	}
+
+	return nil
+}
+
+// AddExtension adds the Extension to the Person. It returns an error.
+func (p *Person) AddExtension(e *ExtensionElement) error {
+	if e == nil {
+		return errors.New("error adding extension element to person: *ExtensionElement is nil")
+	}
+
 	if p.Extensions == nil {
 		p.Extensions = make([]*ExtensionElement, 1)
 		p.Extensions[0] = e
 	} else {
 		p.Extensions = append(p.Extensions, e)
 	}
+
+	return nil
 }
 
 // Check checks the Person for incompatibilities with RFC4287. It returns an
@@ -42,7 +62,7 @@ func (p *Person) Check() error {
 	}
 
 	if p.Email != "" {
-		if _, err := mail.ParseAddress(string(p.Email)); err != nil {
+		if _, err := mail.ParseAddress(p.Email); err != nil {
 			return fmt.Errorf("email element of person %v not correctly formatted", p)
 		}
 	}

plainText.go

@@ -1,6 +1,9 @@
 package atom
 
-import "fmt"
+import (
+	"errors"
+	"fmt"
+)
 
 type PlainText struct {
 	*CommonAttributes
@@ -11,6 +14,15 @@ 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.
+func newPlainText(textType, content string) (*PlainText, error) {
+	if content == "" {
+		return nil, errors.New("error creating new plain text: content string empty")
+	}
+
+	return &PlainText{Type: textType, Text: content}, nil
+}
+
 // Check checks the PlainText for incompatibilities with RFC4287. It returns an
 // error.
 func (p *PlainText) Check() error {

source.go

@@ -23,6 +23,11 @@ type Source struct {
 	Extensions   []*ExtensionElement `xml:",any,omitempty"`
 }
 
+// NewSource creates a new Source. It returns a *Source.
+func NewSource() *Source {
+	return new(Source)
+}
+
 // Check checks the Source for incompatibilities with RFC4287. It returns an
 // error.
 func (s *Source) Check() error {

text.go

@@ -14,17 +14,11 @@ type Text interface {
 func NewText(textType, content string) (Text, error) {
 	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{
+		return newXHTMLText(textType, content)
-			Type: textType,
-			XHTMLDiv: &XHTMLDiv{
-				XMLNS:   "http://www.w3.org/1999/xhtml",
-				Content: content,
-			},
-		}, nil
 	default:
 		return nil, fmt.Errorf("%v is not a valid text type", textType)
 	}

xhtmlDiv.go

@@ -2,6 +2,7 @@ package atom
 
 import (
 	"encoding/xml"
+	"errors"
 	"fmt"
 )
 
@@ -11,12 +12,16 @@ type XHTMLDiv struct {
 	Content string   `xml:",innerxml"`
 }
 
-// NewXHTMLDiv creates a new XHTMLDiv. It returns a *XHTMLDiv.
+// NewXHTMLDiv creates a new XHTMLDiv. It returns a *XHTMLDiv and an error.
-func NewXHTMLDiv(content string) *XHTMLDiv {
+func NewXHTMLDiv(content string) (*XHTMLDiv, error) {
+	if content == "" {
+		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

@@ -1,6 +1,8 @@
 package atom
 
-import "fmt"
+import (
+	"fmt"
+)
 
 type XHTMLText struct {
 	*CommonAttributes
@@ -11,6 +13,19 @@ 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.
+func newXHTMLText(textType, content string) (*XHTMLText, error) {
+	xhtmlDiv, err := NewXHTMLDiv(content)
+	if err != nil {
+		return nil, fmt.Errorf("error creating new xhtml text: %v", err)
+	}
+
+	return &XHTMLText{
+		Type:     textType,
+		XHTMLDiv: xhtmlDiv,
+	}, nil
+}
+
 // Check checks the XHTMLText for incompatibilities with RFC4287. It returns an
 // error.
 func (x *XHTMLText) Check() error {