Added more error handling and necessary functions

README.md

@@ -1,23 +1,24 @@
 # atom
 
-An extensible Atom feed generator library that aims to be very close to
-RFC4287. It diligently checks for compliance with the standard and provides
-functions for easy creation and extension of elements.
+An extensible Atom feed generator library that aims to be very close to RFC4287.
+It diligently checks for compliance with the standard and provides functions for
+easy creation and extension of elements.
 
 ## Installation
 
 To install the latest version of the module, use the following command:
 
-```
+```sh
 go get git.streifling.com/jason/atom@latest
 ```
 
 ## Usage
 
-This library provides easy to use functions to create and extend elements of an
-Atom feed. The intended way of using it entails using these functions.
+This library provides convenient functions to safely create and extend elements
+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.
 
-```
+```go
 package main
 
 import (
@@ -60,10 +61,10 @@ func main() {
 }
 ```
 
-It is also possible to use this library in a way similar to what other
-libraries would provide. This, of course, making it easier to make mistakes.
+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.
 
-```
+```go
 package main
 
 import (
@@ -114,9 +115,9 @@ func main() {
 }
 ```
 
-The output of both ways of using it being an RFC4287 compliant Atom feed:
+The output of both ways of using it is an RFC4287 compliant Atom feed:
 
-```
+```xml
 <?xml version="1.0" encoding="utf-8"?>
 <feed xmlns="http://www.w3.org/2005/Atom">
   <author>

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 {
-	mediaType, _, err := mime.ParseMediaType(mediaType)
+func isCompositeMediaType(m string) bool {
+	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 {
-	mediaType, _, err := mime.ParseMediaType(mediaType)
+func isXMLMediaType(m string) bool {
+	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 {
-	mediaType, _, err := mime.ParseMediaType(mediaType)
+func isValidMediaType(m string) bool {
+	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 {
-	_, err := language.Parse(string(tag))
+func isValidLanguageTag(languageTag string) bool {
+	_, 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 {
-	return IRI(fmt.Sprint("urn:uuid:", uuid.New()))
+func NewURN() string {
+	return fmt.Sprint("urn:uuid:", uuid.New())
 }

category.go

@@ -11,35 +11,64 @@ 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.
-func NewCategory(term string) *Category {
-	return &Category{Term: term}
+// NewCategory creates a new Category. It returns a *Category and an error.
+func NewCategory(term string) (*Category, error) {
+	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.
-func (c *Category) SetLabel(label string) {
+// 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
 // error.
 func (c *Category) Check() error {
 	if c.Term == "" {
-		return errors.New("term attribute of category empty")
+		return fmt.Errorf("term attribute of category %v empty", c)
 	}
 
 	if c.Scheme != "" {
 		if !isValidIRI(c.Scheme) {
-			return fmt.Errorf("scheme attribute %v of category not correctly formatted", c.Scheme)
+			return fmt.Errorf("scheme attribute of category %v not correctly formatted", c)
 		}
 	}
 
 	if !isCorrectlyEscaped(c.Label) {
-		return fmt.Errorf("label attribute %v of category not correctly escaped", c.Label)
+		return fmt.Errorf("label attribute of category %v not correctly escaped", c)
 	}
 
 	return nil

commonAttributes.go

@@ -2,11 +2,12 @@ package atom
 
 import (
 	"encoding/xml"
+	"errors"
 )
 
 type CommonAttributes struct {
-	Base                IRI         `xml:"base,attr,omitempty"`
-	Lang                LanguageTag `xml:"lang,attr,omitempty"`
+	Base                string      `xml:"base,attr,omitempty"` // IRI
+	Lang                string      `xml:"lang,attr,omitempty"` // LanguageTag
 	UndefinedAttributes []*xml.Attr `xml:",attr,omitempty"`
 }
 
@@ -16,12 +17,21 @@ func NewCommonAttributes() *CommonAttributes {
 	return new(CommonAttributes)
 }
 
-// AddExtensionAttribute adds the ExtensionAttribute to the CommonAttributes.
-func (c *CommonAttributes) AddExtensionAttribute(name, value string) {
+// AddAttribute adds the Attribute to the CommonAttributes. It returns an error.
+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

@@ -1,8 +1,6 @@
 package atom
 
-import (
-	"fmt"
-)
+import "fmt"
 
 const (
 	InlineText = iota
@@ -30,6 +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

@@ -40,16 +40,16 @@ func (e *Entry) checkAuthors(authorInFeed bool) error {
 	if e.Authors == nil {
 		if !authorInFeed {
 			if e.Source == nil {
-				return errors.New("no authors set in entry")
+				return fmt.Errorf("no authors set in entry %v", e)
 			}
 			if e.Source.Authors == nil {
-				return errors.New("no authors set in entry")
+				return fmt.Errorf("no authors set in entry %v", e)
 			}
 		}
 	} else {
 		for i, a := range e.Authors {
 			if err := a.Check(); err != nil {
-				return fmt.Errorf("author element %v of entry: %v", i, err)
+				return fmt.Errorf("author element %v of entry %v: %v", i, e, err)
 			}
 		}
 	}
@@ -76,8 +76,12 @@ func NewEntry(title string) (*Entry, error) {
 	}, nil
 }
 
-// AddAuthor adds the Person as an author to the Entry.
-func (e *Entry) AddAuthor(p *Person) {
+// AddAuthor adds the Person as an author to the Entry. It returns an error.
+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
@@ -86,10 +90,15 @@ func (e *Entry) AddAuthor(p *Person) {
 	}
 
 	e.Updated.DateTime = DateTime(time.Now())
+	return nil
 }
 
-// AddCategory adds the Category to the Entry.
-func (e *Entry) AddCategory(c *Category) {
+// AddCategory adds the Category to the Entry. It returns an error.
+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
@@ -98,10 +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.
-func (e *Entry) AddContributor(c *Person) {
+// AddContributor adds the Person as a contributor to the Entry. It returns an
+// 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
@@ -110,10 +125,15 @@ func (e *Entry) AddContributor(c *Person) {
 	}
 
 	e.Updated.DateTime = DateTime(time.Now())
+	return nil
 }
 
-// AddLink adds the Link to the Entry.
-func (e *Entry) AddLink(l *Link) {
+// AddLink adds the Link to the Entry. It returns an error.
+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
@@ -122,10 +142,15 @@ func (e *Entry) AddLink(l *Link) {
 	}
 
 	e.Updated.DateTime = DateTime(time.Now())
+	return nil
 }
 
-// AddExtension adds the ExtensionElement to the Entry.
-func (e *Entry) AddExtension(x *ExtensionElement) {
+// AddExtension adds the ExtensionElement to the Entry. It returns an error.
+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
@@ -134,78 +159,79 @@ 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
 // error.
 func (e *Entry) Check() error {
 	if e.ID == nil {
-		return errors.New("no id element of entry")
+		return fmt.Errorf("no id element of entry %v", e)
 	} else {
 		if err := e.ID.Check(); err != nil {
-			return fmt.Errorf("id element of entry: %v", err)
+			return fmt.Errorf("id element of entry %v: %v", e, err)
 		}
 	}
 
 	if err := e.checkAuthors(true); err != nil {
-		return fmt.Errorf("entry %v: %v", e.ID.URI, err)
+		return fmt.Errorf("entry %v: %v", e, 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.ID.URI, err)
+			return fmt.Errorf("category element %v of entry %v: %v", i, e, err)
 		}
 	}
 
 	if e.Content != nil {
 		if err := e.Content.Check(); err != nil {
-			return fmt.Errorf("content element of entry %v: %v", e.ID.URI, err)
+			return fmt.Errorf("content element of entry %v: %v", e, 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 errors.New("no content element of entry %v and no link element with rel \"alternate\"")
+			return fmt.Errorf("no content element of entry %v and no link element with rel \"alternate\"", e)
 		}
 	}
 
 	for i, c := range e.Contributors {
 		if err := c.Check(); err != nil {
-			return fmt.Errorf("contributor element %v of entry %v: %v", i, e.ID.URI, err)
+			return fmt.Errorf("contributor element %v of entry %v: %v", i, e, 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.ID.URI, err)
+			return fmt.Errorf("link element %v of entry %v: %v", i, e, err)
 		}
 	}
 	if hasAlternateDuplicateLinks(e.Links) {
-		return fmt.Errorf("links with with a rel attribute value of \"alternate\" and duplicate type and hreflang attribute values found in entry %v", e.ID.URI)
+		return fmt.Errorf("links with a rel attribute value of \"alternate\" and duplicate type and hreflang attribute values found in entry %v", e)
 	}
 
 	if e.Published != nil {
 		if err := e.Published.Check(); err != nil {
-			return fmt.Errorf("published element of entry %v: %v", e.ID.URI, err)
+			return fmt.Errorf("published element of entry %v: %v", e, err)
 		}
 	}
 
 	if e.Rights != nil {
 		if err := e.Rights.Check(); err != nil {
-			return fmt.Errorf("rights element of entry %v: %v", e.ID.URI, err)
+			return fmt.Errorf("rights element of entry %v: %v", e, err)
 		}
 	}
 
 	if e.Source != nil {
 		if err := e.Source.Check(); err != nil {
-			return fmt.Errorf("source element of entry %v: %v", e.ID.URI, err)
+			return fmt.Errorf("source element of entry %v: %v", e, err)
 		}
 	}
 
 	if e.Summary != nil {
 		if err := e.Summary.Check(); err != nil {
-			return fmt.Errorf("summary element of entry %v: %v", e.ID.URI, err)
+			return fmt.Errorf("summary element of entry %v: %v", e, err)
 		}
 	} else {
 		// atom:entry elements MUST contain an atom:summary element in either
@@ -213,7 +239,7 @@ 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.ID.URI)
+			return fmt.Errorf("no summary element of entry %v but content of type out of line content", e)
 		}
 		// 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
@@ -221,29 +247,29 @@ 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.ID.URI)
+			return fmt.Errorf("no summary element of entry %v but media type not xml", e)
 		}
 	}
 
 	if e.Title == nil {
-		return fmt.Errorf("no title element of entry %v", e.ID.URI)
+		return fmt.Errorf("no title element of entry %v", e)
 	} else {
 		if err := e.Title.Check(); err != nil {
-			return fmt.Errorf("title element of entry %v: %v", e.ID.URI, err)
+			return fmt.Errorf("title element of entry %v: %v", e, err)
 		}
 	}
 
 	if e.Updated == nil {
-		return fmt.Errorf("no updated element of entry %v", e.ID.URI)
+		return fmt.Errorf("no updated element of entry %v", e)
 	} else {
 		if err := e.Updated.Check(); err != nil {
-			return fmt.Errorf("updated element of entry %v: %v", e.ID.URI, err)
+			return fmt.Errorf("updated element of entry %v: %v", e, 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.ID.URI, err)
+			return fmt.Errorf("extension element %v of entry %v: %v", i, e, err)
 		}
 	}
 

extensionElement.go

@@ -11,16 +11,27 @@ type ExtensionElement struct {
 }
 
 // NewExtensionElement creates a new ExtensionElement. It returns a
-// *ExtensionElement.
-func NewExtensionElement(name string, value any) *ExtensionElement {
-	return &ExtensionElement{XMLName: xml.Name{Local: name}, Value: value}
+// *ExtensionElement and an error.
+func NewExtensionElement(name string, value any) (*ExtensionElement, error) {
+	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 element empty")
+		return errors.New("value element of extension empty")
 	}
 
 	return nil

feed.go

@@ -45,8 +45,12 @@ func NewFeed(title string) (*Feed, error) {
 	}, nil
 }
 
-// AddAuthor adds the Person as an author to the Feed.
-func (f *Feed) AddAuthor(p *Person) {
+// AddAuthor adds the Person as an author to the Feed. It returns an error.
+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
@@ -55,10 +59,15 @@ func (f *Feed) AddAuthor(p *Person) {
 	}
 
 	f.Updated.DateTime = DateTime(time.Now())
+	return nil
 }
 
-// AddCategory adds the Category to the Feed.
-func (f *Feed) AddCategory(c *Category) {
+// AddCategory adds the Category to the Feed. It returns an error.
+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
@@ -67,10 +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.
-func (f *Feed) AddContributor(c *Person) {
+// AddContributor adds the Person as a contributor to the Feed. It returns an
+// 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
@@ -79,10 +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".
-func (f *Feed) AddLink(l *Link) {
+// AddLink adds the Link to the Feed. It returns an error. There should be one
+// 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
@@ -91,10 +112,15 @@ func (f *Feed) AddLink(l *Link) {
 	}
 
 	f.Updated.DateTime = DateTime(time.Now())
+	return nil
 }
 
-// AddExtension adds the Extension to the Feed.
-func (f *Feed) AddExtension(e *ExtensionElement) {
+// AddExtension adds the Extension to the Feed. It returns an error.
+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
@@ -103,10 +129,15 @@ func (f *Feed) AddExtension(e *ExtensionElement) {
 	}
 
 	f.Updated.DateTime = DateTime(time.Now())
+	return nil
 }
 
-// AddEntry adds the Entry to the Feed.
-func (f *Feed) AddEntry(e *Entry) {
+// AddEntry adds the Entry to the Feed. It returns an error.
+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
@@ -115,16 +146,17 @@ 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
 // error.
 func (f *Feed) Check() error {
 	if f.ID == nil {
-		return errors.New("no id element of feed")
+		return fmt.Errorf("no id element of feed %v", f)
 	} else {
 		if err := f.ID.Check(); err != nil {
-			return fmt.Errorf("id element of feed: %v", err)
+			return fmt.Errorf("id element of feed %v: %v", f, err)
 		}
 	}
 
@@ -134,93 +166,93 @@ 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.ID.URI, err)
+				return fmt.Errorf("no authors set in feed %v: %v", f, 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.ID.URI, err)
+				return fmt.Errorf("author element %v of feed %v: %v", i, f, 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.ID.URI, err)
+			return fmt.Errorf("category element %v of feed %v: %v", i, f, 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.ID.URI, err)
+			return fmt.Errorf("contributor element %v of feed %v: %v", i, f, err)
 		}
 	}
 
 	if f.Generator != nil {
 		if err := f.Generator.Check(); err != nil {
-			return fmt.Errorf("generator element of feed %v: %v", f.ID.URI, err)
+			return fmt.Errorf("generator element of feed %v: %v", f, err)
 		}
 	}
 
 	if f.Icon != nil {
 		if err := f.Icon.Check(); err != nil {
-			return fmt.Errorf("icon element of feed %v: %v", f.ID.URI, err)
+			return fmt.Errorf("icon element of feed %v: %v", f, 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.ID.URI, err)
+			return fmt.Errorf("link element %v of feed %v: %v", i, f, err)
 		}
 	}
 	if hasAlternateDuplicateLinks(f.Links) {
-		return fmt.Errorf("links with with a rel attribute value of \"alternate\" and duplicate type and hreflang attribute values found in feed %v", f.ID.URI)
+		return fmt.Errorf("links with a rel attribute value of \"alternate\" and duplicate type and hreflang attribute values found in feed %v", f)
 	}
 
 	if f.Logo != nil {
 		if err := f.Logo.Check(); err != nil {
-			return fmt.Errorf("logo element of feed %v: %v", f.ID.URI, err)
+			return fmt.Errorf("logo element of feed %v: %v", f, err)
 		}
 	}
 
 	if f.Rights != nil {
 		if err := f.Rights.Check(); err != nil {
-			return fmt.Errorf("rights element of feed %v: %v", f.ID.URI, err)
+			return fmt.Errorf("rights element of feed %v: %v", f, err)
 		}
 	}
 
 	if f.Subtitle != nil {
 		if err := f.Subtitle.Check(); err != nil {
-			return fmt.Errorf("subtitle element of feed %v: %v", f.ID.URI, err)
+			return fmt.Errorf("subtitle element of feed %v: %v", f, err)
 		}
 	}
 
 	if f.Title == nil {
-		return fmt.Errorf("no title element of feed %v", f.ID.URI)
+		return fmt.Errorf("no title element of feed %v", f)
 	} else {
 		if err := f.Title.Check(); err != nil {
-			return fmt.Errorf("title element of feed %v: %v", f.ID.URI, err)
+			return fmt.Errorf("title element of feed %v: %v", f, err)
 		}
 	}
 
 	if f.Updated == nil {
-		return fmt.Errorf("no updated element of feed %v", f.ID)
+		return fmt.Errorf("no updated element of feed %v", f)
 	} else {
 		if err := f.Updated.Check(); err != nil {
-			return fmt.Errorf("updated element of feed %v: %v", f.ID.URI, err)
+			return fmt.Errorf("updated element of feed %v: %v", f, 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.ID.URI, err)
+			return fmt.Errorf("extension element %v of feed %v: %v", i, f, 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.ID.URI, err)
+			return fmt.Errorf("entry element %v of feed %v: %v", i, f, err)
 		}
 	}
 

generator.go

@@ -10,14 +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.
-func NewGenerator(text string) *Generator {
-	return &Generator{Text: html.UnescapeString(text)}
+// NewGenerator creates a new Generator. It returns a *Generator and an error.
+func NewGenerator(text string) (*Generator, error) {
+	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
@@ -25,16 +38,16 @@ func NewGenerator(text string) *Generator {
 func (g *Generator) Check() error {
 	if g.URI != "" {
 		if !isValidIRI(g.URI) {
-			return fmt.Errorf("uri attribute %v of generator not correctly formatted", g.URI)
+			return fmt.Errorf("uri attribute of generator %v not correctly formatted", g)
 		}
 	}
 
 	if g.Text == "" {
-		return errors.New("text element of generator empty")
+		return fmt.Errorf("text element of generator %v empty", g)
 	}
 
 	if !isCorrectlyEscaped(g.Text) {
-		return fmt.Errorf("text element %v of generator not correctly escaped", g.Text)
+		return fmt.Errorf("text element of generator %v not correctly escaped", g)
 	}
 
 	return nil

icon.go

@@ -9,11 +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)
 	}
@@ -28,7 +28,7 @@ func (i *Icon) Check() error {
 		return errors.New("uri element of icon empty")
 	} else {
 		if !isValidIRI(i.URI) {
-			return fmt.Errorf("uri attribute %v of icon not correctly formatted", i.URI)
+			return fmt.Errorf("uri attribute of icon %v not correctly formatted", i)
 		}
 	}
 

id.go

@@ -9,16 +9,16 @@ 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.
@@ -27,7 +27,7 @@ func (i *ID) Check() error {
 		return errors.New("uri element of id empty")
 	} else {
 		if !isValidIRI(i.URI) {
-			return fmt.Errorf("uri element %v of id not correctly formatted", i.URI)
+			return fmt.Errorf("uri element of id %v not correctly formatted", i)
 		}
 	}
 

inlineOtherContent.go

@@ -2,7 +2,6 @@ package atom
 
 import (
 	"encoding/xml"
-	"errors"
 	"fmt"
 	"mime"
 )
@@ -11,17 +10,18 @@ type InlineOtherContent struct {
 	XMLName xml.Name `xml:"content"`
 	*CommonAttributes
 	AnyElement any    `xml:",chardata"`
-	Type       MediaType `xml:"type,attr,omitempty"`
+	Type       string `xml:"type,attr,omitempty"` // MediaType
 }
 
 // newInlineOtherContent creates a new InlineOtherContent. It returns a
 // *InlineOtherContent and an error.
 func newInlineOtherContent(mediaType string, content any) (*InlineOtherContent, error) {
-	if mediaType, _, err := mime.ParseMediaType(mediaType); err != nil {
-		return nil, fmt.Errorf("media type %v incompatible with inline other content", mediaType)
+	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(mediaType), AnyElement: content}, nil
+	return &InlineOtherContent{Type: mediaType, AnyElement: content}, nil
 }
 
 // isContent checks whether the InlineOtherContent is a Content. It returns a
@@ -33,19 +33,19 @@ func (i *InlineOtherContent) isContent() bool { return true }
 func (i *InlineOtherContent) hasSRC() bool { return false }
 
 // getType returns the Type of the InlineOtherContent as a string.
-func (i *InlineOtherContent) getType() string { return string(i.Type) }
+func (i *InlineOtherContent) getType() string { return i.Type }
 
 // Check checks the InlineOtherContent for incompatibilities with RFC4287. It
 // returns an error.
 func (i *InlineOtherContent) Check() error {
 	mediaType := i.getType()
 
-	if mediaType, _, err := mime.ParseMediaType(mediaType); err != nil {
-		return fmt.Errorf("type attribute %v incompatible with inline other content", mediaType)
+	if !isValidMediaType(mediaType) {
+		return fmt.Errorf("type attribute of inline other content %v invalid media type", i)
 	}
 
 	if isCompositeMediaType(mediaType) {
-		return errors.New("type attribute of inline other content must not be a composite type")
+		return fmt.Errorf("type attribute of inline other content %v must not be a composite type", i)
 	}
 
 	return nil

inlineTextContent.go

@@ -2,7 +2,6 @@ package atom
 
 import (
 	"encoding/xml"
-	"errors"
 	"fmt"
 )
 
@@ -43,7 +42,7 @@ func (i *InlineTextContent) getType() string { return i.Type }
 // returns an error.
 func (i *InlineTextContent) Check() error {
 	if i.Type != "" && i.Type != "text" && i.Type != "html" {
-		return errors.New("type attribute of inline text content must be text or html if not omitted")
+		return fmt.Errorf("type attribute of inline text content %v must be text or html if not omitted", i)
 	}
 
 	return nil

inlineXHTMLContent.go

@@ -2,7 +2,6 @@ package atom
 
 import (
 	"encoding/xml"
-	"errors"
 	"fmt"
 )
 
@@ -43,11 +42,11 @@ func (i *InlineXHTMLContent) getType() string { return i.Type }
 // returns an error.
 func (i *InlineXHTMLContent) Check() error {
 	if i.Type != "xhtml" {
-		return errors.New("type attribute of inline xhtml content must be xhtml")
+		return fmt.Errorf("type attribute of inline xhtml content %v must be xhtml", i)
 	}
 
 	if err := i.XHTMLDiv.Check(); err != nil {
-		return fmt.Errorf("xhtml div element %v of inline xhtml content %v: %v", i.XHTMLDiv, i, err)
+		return fmt.Errorf("xhtml div element of inline xhtml content %v: %v", i, err)
 	}
 
 	return nil

link.go

@@ -2,7 +2,6 @@ package atom
 
 import (
 	"encoding/xml"
-	"errors"
 	"fmt"
 	"strings"
 )
@@ -11,44 +10,66 @@ type Link struct {
 	XMLName xml.Name `xml:"link"`
 	*CommonAttributes
 	Title    string `xml:"title,attr,omitempty"`
-	Href     IRI         `xml:"href,attr"`
+	Href     string `xml:"href,attr"` // IRI
 	Rel      string `xml:"rel,attr,omitempty"`
-	Type     MediaType   `xml:"type,attr,omitempty"`
-	HrefLang LanguageTag `xml:"hreflang,attr,omitempty"`
+	Type     string `xml:"type,attr,omitempty"`     // MediaType
+	HrefLang string `xml:"hreflang,attr,omitempty"` // LanguageTag
 	Length   uint   `xml:"length,attr,omitempty"`
 }
 
-// NewLink creates a new Link. It returns a *Link.
-func NewLink(href string) *Link {
-	return &Link{Href: IRI(href)}
+// NewLink creates a new Link. It returns a *Link and an error.
+func NewLink(href string) (*Link, error) {
+	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
 // error.
 func (l *Link) Check() error {
 	if l.Href == "" {
-		return errors.New("href attribute of link empty")
+		return fmt.Errorf("href attribute of link %v empty", l)
 	} else {
 		if !isValidIRI(l.Href) {
-			return fmt.Errorf("href attribute %v of link not correctly formatted", l.Href)
+			return fmt.Errorf("href attribute of link %v not correctly formatted", l)
 		}
 	}
 
 	if l.Rel != "" {
-		if strings.Contains(l.Rel, ":") && !isValidIRI(IRI(l.Rel)) {
-			return fmt.Errorf("rel attribute %v of link %v not correctly formatted", l.Rel, l.Href)
+		if strings.Contains(l.Rel, ":") && !isValidIRI(l.Rel) {
+			return fmt.Errorf("rel attribute of link %v not correctly formatted", l)
 		}
 	}
 
 	if l.Type != "" {
-		if !isValidMediaType(string(l.Type)) {
-			return fmt.Errorf("type attribute %v of link %v invalid media type", l.Type, l.Href)
+		if !isValidMediaType(l.Type) {
+			return fmt.Errorf("type attribute of link %v invalid media type", l)
 		}
 	}
 
 	if l.HrefLang != "" {
 		if !isValidLanguageTag(l.HrefLang) {
-			return fmt.Errorf("hreflang attribute %v of link %v invalid language tag", l.Type, l.HrefLang)
+			return fmt.Errorf("hreflang attribute of link %v invalid language tag", l)
 		}
 	}
 

logo.go

@@ -2,18 +2,17 @@ package atom
 
 import (
 	"encoding/xml"
-	"errors"
 	"fmt"
 )
 
 type Logo struct {
 	XMLName xml.Name `xml:"logo"`
 	*CommonAttributes
-	URI IRI `xml:",chardata"`
+	URI string `xml:",chardata"` // IRI
 }
 
-// NewLogo creates a new Logo. It returns a *Logo.
-func NewLogo(uri IRI) (*Logo, error) {
+// NewLogo creates a new Logo. It returns a *Logo and an error.
+func NewLogo(uri string) (*Logo, error) {
 	if !isValidIRI(uri) {
 		return nil, fmt.Errorf("uri %v not correctly formatted", uri)
 	}
@@ -25,10 +24,10 @@ func NewLogo(uri IRI) (*Logo, error) {
 // error.
 func (l *Logo) Check() error {
 	if l.URI == "" {
-		return errors.New("uri element of logo empty")
+		return fmt.Errorf("uri element of logo %v empty", l)
 	} else {
 		if !isValidIRI(l.URI) {
-			return fmt.Errorf("uri element %v of logo not correctly formatted", l.URI)
+			return fmt.Errorf("uri element of logo %v not correctly formatted", l)
 		}
 	}
 

outOfLineContent.go

@@ -2,7 +2,6 @@ package atom
 
 import (
 	"encoding/xml"
-	"errors"
 	"fmt"
 	"mime"
 )
@@ -10,27 +9,28 @@ import (
 type OutOfLineContent struct {
 	XMLName xml.Name `xml:"content"`
 	*CommonAttributes
-	Type MediaType `xml:"type,attr,omitempty"`
-	SRC  IRI       `xml:"src,attr"`
+	Type string `xml:"type,attr,omitempty"` // MediaType
+	SRC  string `xml:"src,attr"`            // IRI
 }
 
 // newOutOfLineContent creates a new OutOfLineContent. It returns a
 // *OutOfLineContent and an error.
 func newOutOfLineContent(mediaType string, content any) (*OutOfLineContent, error) {
-	if mediaType, _, err := mime.ParseMediaType(mediaType); err != nil {
-		return nil, fmt.Errorf("media type %v incompatible with out of line content", mediaType)
+	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.(IRI)
+	iri, ok := content.(string)
 	if !ok {
 		return nil, fmt.Errorf("content type %T incompatible with out of line content", content)
 	}
 
 	if !isValidIRI(iri) {
-		return nil, errors.New("content not a valid uri")
+		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
@@ -42,23 +42,23 @@ func (o *OutOfLineContent) isContent() bool { return true }
 func (o *OutOfLineContent) hasSRC() bool { return true }
 
 // getType returns the Type of the OutOfLineContent as a string.
-func (o *OutOfLineContent) getType() string { return string(o.Type) }
+func (o *OutOfLineContent) getType() string { return o.Type }
 
 // Check checks the OutOfLineContent for incompatibilities with RFC4287. It
 // returns an error.
 func (o *OutOfLineContent) Check() error {
 	mediaType := o.getType()
 
-	if mediaType, _, err := mime.ParseMediaType(mediaType); err != nil {
-		return fmt.Errorf("type attribute %v incompatible with out of line content", mediaType)
+	if !isValidMediaType(mediaType) {
+		return fmt.Errorf("type attribute of out of line content %v invalid media type", o)
 	}
 
 	if isCompositeMediaType(mediaType) {
-		return errors.New("type attribute of out of line content must not be a composite type")
+		return fmt.Errorf("type attribute of out of line content %v must not be a composite type", o)
 	}
 
 	if o.SRC == "" {
-		return errors.New("src attribute of out of line content empty")
+		return fmt.Errorf("src attribute of out of line content %v empty", o)
 	}
 
 	return nil

person.go

@@ -9,49 +9,68 @@ import (
 type Person struct {
 	*CommonAttributes
 	Name       string              `xml:"name"`
-	URI        IRI                 `xml:"uri,omitempty"`
-	Email      EmailAddress        `xml:"email,omitempty"`
+	URI        string              `xml:"uri,omitempty"`   // IRI
+	Email      string              `xml:"email,omitempty"` // EmailAddress
 	Extensions []*ExtensionElement `xml:",any,omitempty"`
 }
 
-// NewPerson creates a new Person. It returns a *Person.
-func NewPerson(name string) *Person {
-	return &Person{Name: name}
+// NewPerson creates a new Person. It returns a *Person and an error.
+func NewPerson(name string) (*Person, error) {
+	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.
-func (p *Person) AddExtension(e *ExtensionElement) {
+// SetURI sets the URI element of the Person. It returns an error.
+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
 // error.
 func (p *Person) Check() error {
 	if p.Name == "" {
-		return errors.New("name element of person element empty")
+		return fmt.Errorf("name element of person %v empty", p)
 	}
 
 	if p.URI != "" {
 		if !isValidIRI(p.URI) {
-			return fmt.Errorf("uri element of person %v not correctly formatted", p.Name)
+			return fmt.Errorf("uri element of person %v not correctly formatted", p)
 		}
 	}
 
 	if p.Email != "" {
-		if _, err := mail.ParseAddress(string(p.Email)); err != nil {
-			return fmt.Errorf("email element of person %v not correctly formatted", p.Name)
+		if _, err := mail.ParseAddress(p.Email); err != nil {
+			return fmt.Errorf("email element of person %v not correctly formatted", p)
 		}
 	}
 
 	if p.Extensions != nil {
 		for i, e := range p.Extensions {
 			if err := e.Check(); err != nil {
-				return fmt.Errorf("extension element %v of person %v: %v", i, p.Name, err)
+				return fmt.Errorf("extension element %v of person %v: %v", i, p, err)
 			}
 		}
 	}

plainText.go

@@ -1,6 +1,9 @@
 package atom
 
-import "errors"
+import (
+	"errors"
+	"fmt"
+)
 
 type PlainText struct {
 	*CommonAttributes
@@ -11,19 +14,28 @@ type PlainText struct {
 // isText checks whether the PlainText is a Text. It returns a bool.
 func (p *PlainText) isText() bool { return true }
 
+// newPlainText creates a new PlainText. It returns a *PlainText 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 {
 	if p.Type != "" && p.Type != "text" && p.Type != "html" {
-		return errors.New("type attribute of plain text must be text or html if not omitted")
+		return fmt.Errorf("type attribute of plain text %v must be text or html if not omitted", p)
 	}
 
 	if p.Type == "html" && !isCorrectlyEscaped(p.Text) {
-		return errors.New("text element of plain text not correctly escaped")
+		return fmt.Errorf("text element of plain text %v not correctly escaped", p)
 	}
 
 	if p.Text == "" {
-		return errors.New("text element of plain text empty")
+		return fmt.Errorf("text element of plain text %v empty", p)
 	}
 
 	return nil

source.go

@@ -23,84 +23,89 @@ 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 {
 	for i, a := range s.Authors {
 		if err := a.Check(); err != nil {
-			return fmt.Errorf("author element %v of source %v: %v", i, s.ID.URI, err)
+			return fmt.Errorf("author element %v of source %v: %v", i, s, err)
 		}
 	}
 
 	for i, c := range s.Categories {
 		if err := c.Check(); err != nil {
-			return fmt.Errorf("category element %v of source %v: %v", i, s.ID.URI, err)
+			return fmt.Errorf("category element %v of source %v: %v", i, s, err)
 		}
 	}
 
 	for i, c := range s.Contributors {
 		if err := c.Check(); err != nil {
-			return fmt.Errorf("contributor element %v of source %v: %v", i, s.ID.URI, err)
+			return fmt.Errorf("contributor element %v of source %v: %v", i, s, err)
 		}
 	}
 
 	if s.Generator != nil {
 		if err := s.Generator.Check(); err != nil {
-			return fmt.Errorf("generator element of source %v: %v", s.ID.URI, err)
+			return fmt.Errorf("generator element of source %v: %v", s, err)
 		}
 	}
 
 	if s.Icon != nil {
 		if err := s.Icon.Check(); err != nil {
-			return fmt.Errorf("icon element of source %v: %v", s.ID.URI, err)
+			return fmt.Errorf("icon element of source %v: %v", s, err)
 		}
 	}
 
 	if s.ID != nil {
 		if err := s.ID.Check(); err != nil {
-			return fmt.Errorf("id element of source %v: %v", s.ID.URI, err)
+			return fmt.Errorf("id element of source %v: %v", s, err)
 		}
 	}
 
 	for i, l := range s.Links {
 		if err := l.Check(); err != nil {
-			return fmt.Errorf("link element %v of source %v: %v", i, s.ID.URI, err)
+			return fmt.Errorf("link element %v of source %v: %v", i, s, err)
 		}
 	}
 
 	if s.Logo != nil {
 		if err := s.Logo.Check(); err != nil {
-			return fmt.Errorf("logo element of source %v: %v", s.ID.URI, err)
+			return fmt.Errorf("logo element of source %v: %v", s, err)
 		}
 	}
 
 	if s.Rights != nil {
 		if err := s.Rights.Check(); err != nil {
-			return fmt.Errorf("rights element of source %v: %v", s.ID.URI, err)
+			return fmt.Errorf("rights element of source %v: %v", s, err)
 		}
 	}
 
 	if s.Subtitle != nil {
 		if err := s.Subtitle.Check(); err != nil {
-			return fmt.Errorf("subtitle element of source %v: %v", s.ID.URI, err)
+			return fmt.Errorf("subtitle element of source %v: %v", s, err)
 		}
 	}
 
 	if s.Title != nil {
 		if err := s.Title.Check(); err != nil {
-			return fmt.Errorf("title element of source %v: %v", s.ID.URI, err)
+			return fmt.Errorf("title element of source %v: %v", s, err)
 		}
 	}
 
 	if s.Updated != nil {
 		if err := s.Updated.Check(); err != nil {
-			return fmt.Errorf("updated element of source %v: %v", s.ID.URI, err)
+			return fmt.Errorf("updated element of source %v: %v", s, err)
 		}
 	}
 
 	for i, e := range s.Extensions {
 		if err := e.Check(); err != nil {
-			return fmt.Errorf("extension element %v of source %v: %v", i, s.ID.URI, err)
+			return fmt.Errorf("extension element %v of source %v: %v", i, s, err)
 		}
 	}
 

text.go

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

xhtmlDiv.go

@@ -3,6 +3,7 @@ package atom
 import (
 	"encoding/xml"
 	"errors"
+	"fmt"
 )
 
 type XHTMLDiv struct {
@@ -11,19 +12,23 @@ type XHTMLDiv struct {
 	Content string   `xml:",innerxml"`
 }
 
-// NewXHTMLDiv creates a new XHTMLDiv. It returns a *XHTMLDiv.
-func NewXHTMLDiv(content string) *XHTMLDiv {
+// NewXHTMLDiv creates a new XHTMLDiv. It returns a *XHTMLDiv and an error.
+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
 // error.
 func (x *XHTMLDiv) Check() error {
 	if x.XMLNS != "http://www.w3.org/1999/xhtml" {
-		return errors.New("xmlns attribute of xhtml text must be http://www.w3.org/1999/xhtml")
+		return fmt.Errorf("xmlns attribute of xhtml text %v must be http://www.w3.org/1999/xhtml", x)
 	}
 
 	return nil

xhtmlText.go

@@ -1,7 +1,6 @@
 package atom
 
 import (
-	"errors"
 	"fmt"
 )
 
@@ -14,11 +13,24 @@ 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 {
 	if x.Type != "xhtml" {
-		return errors.New("type attribute of xhtml text must be xhtml")
+		return fmt.Errorf("type attribute of xhtml text %v must be xhtml", x)
 	}
 
 	if err := x.XHTMLDiv.Check(); err != nil {