Changed comments a bit

README.md

@@ -15,8 +15,8 @@ go get git.streifling.com/jason/atom@latest
 ## Usage
 
 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.
+and attributes of Atom feeds. It also provides checks for all constructs'
+adherence to RFC4287.
 
 ```go
 package main
@@ -29,30 +29,25 @@ import (
 )
 
 func main() {
- feed, err := atom.NewFeed("Example Feed")
- if err != nil {
+ feed := atom.NewFeed("Example Feed")
+ if err := feed.Check(); err != nil {
   log.Fatalln(err)
  }
 
  author := atom.NewPerson("John Doe")
  author.Email = "john.doe@example.com"
+ if err := author.Check(); err != nil {
+  log.Fatalln(err)
+ }
  feed.AddAuthor(author)
 
- entry, err := atom.NewEntry("First Entry")
- if err != nil {
+ entry := atom.NewEntry("First Entry")
+ entry.Content = atom.NewContent(atom.InlineText, "text", "This is the content of the first entry.")
+ if err := entry.Check(); err != nil {
   log.Fatalln(err)
  }
- content, err := atom.NewContent(atom.InlineText, "text", "This is the content of the first entry.")
- if err != nil {
-  log.Fatalln(err)
- }
- entry.Content = content
  feed.AddEntry(entry)
 
- if err := feed.Check(); err != nil {
-  log.Fatalln(err)
- }
-
  feedString, err := feed.ToXML("utf-8")
  if err != nil {
   log.Fatalln(err)
@@ -62,7 +57,7 @@ func main() {
 ```
 
 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
@@ -115,7 +110,7 @@ func main() {
 }
 ```
 
-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"?>

atom.go

@@ -1,6 +1,7 @@
 package atom
 
 import (
+	"encoding/xml"
 	"fmt"
 	"html"
 	"mime"
@@ -11,6 +12,30 @@ import (
 	"golang.org/x/text/language"
 )
 
+type Countable interface {
+	*xml.Attr | *Person | *Category | *Link | *ExtensionElement | *Entry
+}
+
+// addToSlice adds a Countable to to a *[]Countable
+func addToSlice[C Countable](slice *[]C, countable C) {
+	if *slice == nil {
+		*slice = make([]C, 0)
+	}
+	*slice = append(*slice, countable)
+}
+
+// deleteFromSlice deletes the Countable with the index from the *[]Countable.
+// It return an error.
+func deleteFromSlice[C Countable](slice *[]C, index int) error {
+	length := len(*slice)
+	if index > length {
+		return fmt.Errorf("id %v out of range %v", index, length)
+	}
+
+	*slice = append((*slice)[:index], (*slice)[index+1:]...)
+	return nil
+}
+
 // isValidIRI checks whether an IRI is valid or not. It returns a bool.
 // https://www.w3.org/2011/04/XMLSchema/TypeLibrary-IRI-RFC3987.xsd
 func isValidIRI(iri string) bool {

commonAttributes.go

@@ -17,14 +17,18 @@ func newCommonAttributes() *CommonAttributes {
 	return new(CommonAttributes)
 }
 
-// AddAttribute adds the Attribute to the CommonAttributes.
+// AddAttribute adds the attribute to the CommonAttributes.
 func (c *CommonAttributes) AddAttribute(name, value string) {
-	if c.UndefinedAttributes == nil {
-		c.UndefinedAttributes = make([]*xml.Attr, 1)
-		c.UndefinedAttributes[0] = &xml.Attr{Name: xml.Name{Local: name}, Value: value}
-	} else {
-		c.UndefinedAttributes = append(c.UndefinedAttributes, &xml.Attr{Name: xml.Name{Local: name}, Value: value})
+	addToSlice(&c.UndefinedAttributes, &xml.Attr{Name: xml.Name{Local: name}, Value: value})
+}
+
+// DeleteAttribute deletes the attribute at index from the CommonAttributes. It
+// return an error.
+func (c *CommonAttributes) DeleteAttribute(index int) error {
+	if err := deleteFromSlice(&c.UndefinedAttributes, index); err != nil {
+		return fmt.Errorf("error deleting undefined attribute %v from common attributes %v: %v", index, c, err)
 	}
+	return nil
 }
 
 // Check checks the CommonAttributes for incompatibilities with RFC4287. It

entry.go

@@ -39,16 +39,16 @@ func (e *Entry) checkAuthors(authorInFeed bool) error {
 	if e.Authors == nil {
 		if !authorInFeed {
 			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)
 			}
 		}
 	}
@@ -68,62 +68,85 @@ func NewEntry(title string) *Entry {
 
 // AddAuthor adds the Person as an author to the Entry.
 func (e *Entry) AddAuthor(p *Person) {
-	if e.Authors == nil {
-		e.Authors = make([]*Person, 1)
-		e.Authors[0] = p
-	} else {
-		e.Authors = append(e.Authors, p)
+	addToSlice(&e.Authors, p)
+	e.Updated = NewDate(time.Now())
+}
+
+// DeleteAuthor deletes the Person at index from the Entry. It return an error.
+func (e *Entry) DeleteAuthor(index int) error {
+	if err := deleteFromSlice(&e.Authors, index); err != nil {
+		return fmt.Errorf("error deleting author %v from entry %v: %v", index, e.ID.URI, err)
 	}
 
-	e.Updated.DateTime = DateTime(time.Now())
+	e.Updated = NewDate(time.Now())
+	return nil
 }
 
 // AddCategory adds the Category to the Entry.
 func (e *Entry) AddCategory(c *Category) {
-	if e.Categories == nil {
-		e.Categories = make([]*Category, 1)
-		e.Categories[0] = c
-	} else {
-		e.Categories = append(e.Categories, c)
+	addToSlice(&e.Categories, c)
+	e.Updated = NewDate(time.Now())
+}
+
+// DeleteCategory deletes the Category at index from the Entry. It return an
+// error.
+func (e *Entry) DeleteCategory(index int) error {
+	if err := deleteFromSlice(&e.Categories, index); err != nil {
+		return fmt.Errorf("error deleting category %v from entry %v: %v", index, e.ID.URI, err)
 	}
 
-	e.Updated.DateTime = DateTime(time.Now())
+	e.Updated = NewDate(time.Now())
+	return nil
 }
 
 // AddContributor adds the Person as a contributor to the Entry.
 func (e *Entry) AddContributor(c *Person) {
-	if e.Contributors == nil {
-		e.Contributors = make([]*Person, 1)
-		e.Contributors[0] = c
-	} else {
-		e.Contributors = append(e.Contributors, c)
+	addToSlice(&e.Contributors, c)
+	e.Updated = NewDate(time.Now())
+}
+
+// DeleteContributor deletes the Person at index from the Entry. It return an
+// error.
+func (e *Entry) DeleteContributor(index int) error {
+	if err := deleteFromSlice(&e.Contributors, index); err != nil {
+		return fmt.Errorf("error deleting contributor %v from entry %v: %v", index, e.ID.URI, err)
 	}
 
-	e.Updated.DateTime = DateTime(time.Now())
+	e.Updated = NewDate(time.Now())
+	return nil
 }
 
 // AddLink adds the Link to the Entry.
 func (e *Entry) AddLink(l *Link) {
-	if e.Links == nil {
-		e.Links = make([]*Link, 1)
-		e.Links[0] = l
-	} else {
-		e.Links = append(e.Links, l)
+	addToSlice(&e.Links, l)
+	e.Updated = NewDate(time.Now())
+}
+
+// DeleteLink deletes the Link at index from the Entry. It return an error.
+func (e *Entry) DeleteLink(index int) error {
+	if err := deleteFromSlice(&e.Links, index); err != nil {
+		return fmt.Errorf("error deleting link %v from entry %v: %v", index, e.ID.URI, err)
 	}
 
-	e.Updated.DateTime = DateTime(time.Now())
+	e.Updated = NewDate(time.Now())
+	return nil
 }
 
 // AddExtension adds the ExtensionElement to the Entry.
 func (e *Entry) AddExtension(x *ExtensionElement) {
-	if e.Extensions == nil {
-		e.Extensions = make([]*ExtensionElement, 1)
-		e.Extensions[0] = x
-	} else {
-		e.Extensions = append(e.Extensions, x)
+	addToSlice(&e.Extensions, x)
+	e.Updated = NewDate(time.Now())
+}
+
+// DeleteExtension deletes the Extension at index from the Entry. It return an
+// error.
+func (e *Entry) DeleteExtension(index int) error {
+	if err := deleteFromSlice(&e.Extensions, index); err != nil {
+		return fmt.Errorf("error deleting extension %v from entry %v: %v", index, e.ID.URI, err)
 	}
 
-	e.Updated.DateTime = DateTime(time.Now())
+	e.Updated = NewDate(time.Now())
+	return nil
 }
 
 // Check checks the Entry for incompatibilities with RFC4287. It returns an
@@ -138,64 +161,64 @@ 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
@@ -203,7 +226,7 @@ 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
@@ -211,29 +234,29 @@ 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)
+			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)
 		}
 	}
 
@@ -244,7 +267,7 @@ func (e *Entry) Check() error {
 func (e *Entry) ToXML(encoding string) (string, error) {
 	xml, err := xml.MarshalIndent(e, "", "  ")
 	if err != nil {
-		return "", fmt.Errorf("error xml encoding entry: %v", err)
+		return "", fmt.Errorf("error xml encoding entry %v: %v", e.ID.URI, err)
 	}
 
 	return fmt.Sprintln(`<?xml version="1.0" encoding="`+encoding+`"?>`) + string(xml), nil

feed.go

@@ -37,74 +37,123 @@ func NewFeed(title string) *Feed {
 
 // AddAuthor adds the Person as an author to the Feed.
 func (f *Feed) AddAuthor(p *Person) {
-	if f.Authors == nil {
-		f.Authors = make([]*Person, 1)
-		f.Authors[0] = p
-	} else {
-		f.Authors = append(f.Authors, p)
+	addToSlice(&f.Authors, p)
+	f.Updated = NewDate(time.Now())
+}
+
+// DeleteAuthor deletes the Person at index from the Feed. It return an error.
+func (f *Feed) DeleteAuthor(index int) error {
+	if err := deleteFromSlice(&f.Authors, index); err != nil {
+		return fmt.Errorf("error deleting author %v from entry %v: %v", index, f.ID.URI, err)
 	}
 
-	f.Updated.DateTime = DateTime(time.Now())
+	f.Updated = NewDate(time.Now())
+	return nil
 }
 
 // AddCategory adds the Category to the Feed.
 func (f *Feed) AddCategory(c *Category) {
-	if f.Categories == nil {
-		f.Categories = make([]*Category, 1)
-		f.Categories[0] = c
-	} else {
-		f.Categories = append(f.Categories, c)
+	addToSlice(&f.Categories, c)
+	f.Updated = NewDate(time.Now())
+}
+
+// DeleteCategory deletes the Category at index from the Feed. It return an
+// error.
+func (f *Feed) DeleteCategory(index int) error {
+	if err := deleteFromSlice(&f.Categories, index); err != nil {
+		return fmt.Errorf("error deleting category %v from entry %v: %v", index, f.ID.URI, err)
 	}
 
-	f.Updated.DateTime = DateTime(time.Now())
+	f.Updated = NewDate(time.Now())
+	return nil
 }
 
 // AddContributor adds the Person as a contributor to the Feed.
 func (f *Feed) AddContributor(c *Person) {
-	if f.Contributors == nil {
-		f.Contributors = make([]*Person, 1)
-		f.Contributors[0] = c
-	} else {
-		f.Contributors = append(f.Contributors, c)
+	addToSlice(&f.Contributors, c)
+	f.Updated = NewDate(time.Now())
+}
+
+// DeleteContributor deletes the Person at index from the Feed. It return an
+// error.
+func (f *Feed) DeleteContributor(index int) error {
+	if err := deleteFromSlice(&f.Contributors, index); err != nil {
+		return fmt.Errorf("error deleting contributor %v from entry %v: %v", index, f.ID.URI, err)
 	}
 
-	f.Updated.DateTime = DateTime(time.Now())
+	f.Updated = NewDate(time.Now())
+	return nil
 }
 
 // AddLink adds the Link to the Feed. There should be one Link with Rel "self".
 func (f *Feed) AddLink(l *Link) {
-	if f.Links == nil {
-		f.Links = make([]*Link, 1)
-		f.Links[0] = l
-	} else {
-		f.Links = append(f.Links, l)
+	addToSlice(&f.Links, l)
+	f.Updated = NewDate(time.Now())
+}
+
+// DeleteLink deletes the Link at index from the Feed. It return an error.
+func (f *Feed) DeleteLink(index int) error {
+	if err := deleteFromSlice(&f.Links, index); err != nil {
+		return fmt.Errorf("error deleting link %v from entry %v: %v", index, f.ID.URI, err)
 	}
 
-	f.Updated.DateTime = DateTime(time.Now())
+	f.Updated = NewDate(time.Now())
+	return nil
 }
 
 // AddExtension adds the Extension to the Feed.
 func (f *Feed) AddExtension(e *ExtensionElement) {
-	if f.Extensions == nil {
-		f.Extensions = make([]*ExtensionElement, 1)
-		f.Extensions[0] = e
-	} else {
-		f.Extensions = append(f.Extensions, e)
+	addToSlice(&f.Extensions, e)
+	f.Updated = NewDate(time.Now())
+}
+
+// DeleteExtension deletes the Extension at index from the Feed. It return an
+// error.
+func (f *Feed) DeleteExtension(index int) error {
+	if err := deleteFromSlice(&f.Extensions, index); err != nil {
+		return fmt.Errorf("error deleting extension %v from entry %v: %v", index, f.ID.URI, err)
 	}
 
-	f.Updated.DateTime = DateTime(time.Now())
+	f.Updated = NewDate(time.Now())
+	return nil
 }
 
 // AddEntry adds the Entry to the Feed.
 func (f *Feed) AddEntry(e *Entry) {
-	if f.Entries == nil {
-		f.Entries = make([]*Entry, 1)
-		f.Entries[0] = e
-	} else {
-		f.Entries = append(f.Entries, e)
+	addToSlice(&f.Entries, e)
+	f.Updated = NewDate(time.Now())
+}
+
+// DeleteEntry deletes the Entry at index from the Feed. It return an error.
+func (f *Feed) DeleteEntry(index int) error {
+	if err := deleteFromSlice(&f.Entries, index); err != nil {
+		return fmt.Errorf("error deleting entry %v from entry %v: %v", index, f.ID.URI, err)
 	}
 
-	f.Updated.DateTime = DateTime(time.Now())
+	f.Updated = NewDate(time.Now())
+	return nil
+}
+
+// DeleteEntryByURI deletes the Entry from the Feed. It return an error.
+func (f *Feed) DeleteEntryByURI(uri string) error {
+	if !isValidIRI(uri) {
+		return fmt.Errorf("error deleting entry from feed %v: uri %v invalid", f.ID.URI, uri)
+	}
+
+	index := -1
+	for i, e := range f.Entries {
+		if e.ID.URI == uri {
+			index = i
+			break
+		}
+	}
+	if index < 0 {
+		return fmt.Errorf("error deleting entry from feed %v: id %v not found", f.ID.URI, uri)
+	}
+
+	f.Entries = append(f.Entries[:index], f.Entries[index+1:]...)
+	f.Updated = NewDate(time.Now())
+	return nil
 }
 
 // Check checks the Feed for incompatibilities with RFC4287. It returns an
@@ -124,93 +173,93 @@ 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)
 		}
 	}
 
@@ -221,7 +270,7 @@ 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

person.go

@@ -23,12 +23,16 @@ func NewPerson(name string) *Person {
 
 // AddExtension adds the Extension to the Person.
 func (p *Person) AddExtension(e *ExtensionElement) {
-	if p.Extensions == nil {
-		p.Extensions = make([]*ExtensionElement, 1)
-		p.Extensions[0] = e
-	} else {
-		p.Extensions = append(p.Extensions, e)
+	addToSlice(&p.Extensions, e)
+}
+
+// DeleteExtension deletes the Extension at index from the Person. It return an
+// error.
+func (p *Person) DeleteExtension(index int) error {
+	if err := deleteFromSlice(&p.Extensions, index); err != nil {
+		return fmt.Errorf("error deleting extension %v from person %v: %v", index, p, err)
 	}
+	return nil
 }
 
 // Check checks the Person for incompatibilities with RFC4287. It returns an
@@ -50,11 +54,9 @@ func (p *Person) Check() error {
 		}
 	}
 
-	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, err)
-			}
+	for i, e := range p.Extensions {
+		if err := e.Check(); err != nil {
+			return fmt.Errorf("extension element %v of person %v: %v", i, p, err)
 		}
 	}
 

source.go

@@ -28,6 +28,74 @@ func NewSource() *Source {
 	return &Source{CommonAttributes: newCommonAttributes()}
 }
 
+// AddAuthor adds the Person as an author to the Source.
+func (s *Source) AddAuthor(p *Person) {
+	addToSlice(&s.Authors, p)
+}
+
+// DeleteAuthor deletes the Person at index from the Source. It return an error.
+func (s *Source) DeleteAuthor(index int) error {
+	if err := deleteFromSlice(&s.Authors, index); err != nil {
+		return fmt.Errorf("error deleting author %v from source %v: %v", index, s, err)
+	}
+	return nil
+}
+
+// AddCategory adds the Category to the Source.
+func (s *Source) AddCategory(c *Category) {
+	addToSlice(&s.Categories, c)
+}
+
+// DeleteCategory deletes the Category at index from the Source. It return an
+// error.
+func (s *Source) DeleteCategory(index int) error {
+	if err := deleteFromSlice(&s.Categories, index); err != nil {
+		return fmt.Errorf("error deleting category %v from source %v: %v", index, s, err)
+	}
+	return nil
+}
+
+// AddContributor adds the Person as a contributor to the Source.
+func (s *Source) AddContributor(c *Person) {
+	addToSlice(&s.Contributors, c)
+}
+
+// DeleteContributor deletes the Person at index from the Source. It return an
+// error.
+func (s *Source) DeleteContributor(index int) error {
+	if err := deleteFromSlice(&s.Contributors, index); err != nil {
+		return fmt.Errorf("error deleting contributor %v from source %v: %v", index, s, err)
+	}
+	return nil
+}
+
+// AddLink adds the Link to the Source.
+func (s *Source) AddLink(l *Link) {
+	addToSlice(&s.Links, l)
+}
+
+// DeleteLink deletes the Link at index from the Source. It return an error.
+func (s *Source) DeleteLink(index int) error {
+	if err := deleteFromSlice(&s.Links, index); err != nil {
+		return fmt.Errorf("error deleting link %v from source %v: %v", index, s, err)
+	}
+	return nil
+}
+
+// AddExtension adds the ExtensionElement to the Source.
+func (s *Source) AddExtension(e *ExtensionElement) {
+	addToSlice(&s.Extensions, e)
+}
+
+// DeleteExtension deletes the Extension at index from the Source. It return an
+// error.
+func (s *Source) DeleteExtension(index int) error {
+	if err := deleteFromSlice(&s.Extensions, index); err != nil {
+		return fmt.Errorf("error deleting extension %v from source %v: %v", index, s, err)
+	}
+	return nil
+}
+
 // Check checks the Source for incompatibilities with RFC4287. It returns an
 // error.
 func (s *Source) Check() error {