create element builder and refactor api

src/element.rs

@@ -1,25 +1,33 @@
 // elements resemble a final tree, including inherited namespace information
 
+#![feature(drain_filter)]
+
 use std::{
-    collections::{HashMap, HashSet},
+    collections::{HashMap, HashSet, VecDeque},
     convert::Infallible,
     str::FromStr,
 };
 
 use crate::{
-    error::Error,
+    error::{DeserializeError, Error},
     xml::{self, parsers_complete::Parser, Attribute},
     Result,
 };
 
+pub type DeserializeResult<T> = std::result::Result<T, DeserializeError>;
+
 pub trait FromElement: Sized {
-    fn from_element(element: Element) -> Result<Self>;
+    fn from_element(element: Element) -> DeserializeResult<Self>;
 }
 
 pub trait IntoElement {
-    fn into_element(&self) -> Element;
+    fn builder(&self) -> ElementBuilder;
 
-    fn get_content(&self) -> Vec<Content> {
+    fn into_element(&self) -> Element {
+        self.builder().build().unwrap()
+    }
+
+    fn get_content(&self) -> VecDeque<Content> {
         let element = self.into_element();
         element.content
     }
@@ -58,7 +66,8 @@ pub struct Element {
     // namespace: String,
     // hashmap of explicit namespace declarations on the element itself only
     // possibly not needed as can be calculated at write time depending on context and qualified namespace, and for reading, element validity and namespaces are kept track of by the reader.
-    pub namespace_declarations: HashSet<NamespaceDeclaration>,
+    // change this to custom namespace declarations only, so you can override the definition of namespaces if you wish
+    pub namespace_declaration_overrides: HashSet<NamespaceDeclaration>,
     // attributes can be in a different namespace than the element. how to make sure they are valid?
     // maybe include the namespace instead of or with the prefix
     // you can calculate the prefix from the namespaced name and the current writer context
@@ -66,7 +75,524 @@ pub struct Element {
     // this results in readers and writers being able to return qualification errors as they aren't able to create elements until every part is qualified.
     pub attributes: HashMap<Name, String>,
     // TODO: make a hashmap maybe? to be able to address parts of the content individually
-    pub content: Vec<Content>,
+    pub content: VecDeque<Content>,
+}
+
+impl Element {
+    pub fn identify(&self) -> (Option<&str>, &str) {
+        (self.name.namespace.as_deref(), &self.name.local_name)
+    }
+
+    pub fn check_name(&self, name: &str) -> DeserializeResult<()> {
+        if self.name.local_name == name {
+            Ok(())
+        } else {
+            return Err(DeserializeError::IncorrectName(
+                self.name.local_name.clone(),
+            ));
+        }
+    }
+
+    pub fn check_namespace(&self, namespace: &str) -> DeserializeResult<()> {
+        if self.name.namespace.as_deref() == Some(namespace) {
+            return Ok(());
+        } else {
+            if let Some(namespace) = &self.name.namespace {
+                return Err(DeserializeError::IncorrectNamespace(namespace.clone()));
+            } else {
+                return Err(DeserializeError::Unqualified);
+            }
+        }
+    }
+
+    pub fn attribute_opt<V: FromStr>(&mut self, att_name: &str) -> DeserializeResult<Option<V>> {
+        if let Some(att_value) = self.attributes.remove(&Name {
+            namespace: None,
+            local_name: att_name.to_string(),
+        }) {
+            let value = <V as FromStr>::from_str(&att_value)
+                .map_err(|_| DeserializeError::FromStr(att_value))?;
+            return Ok(Some(value));
+        } else {
+            return Ok(None);
+        }
+    }
+
+    pub fn attribute_opt_namespaced<V: FromStr>(
+        &mut self,
+        att_name: &str,
+        att_namespace: &str,
+    ) -> DeserializeResult<Option<V>> {
+        if let Some(att_value) = self.attributes.remove(&Name {
+            namespace: Some(att_namespace.to_string()),
+            local_name: att_name.to_string(),
+        }) {
+            let value = <V as FromStr>::from_str(&att_value)
+                .map_err(|_| DeserializeError::FromStr(att_value))?;
+            return Ok(Some(value));
+        } else {
+            return Ok(None);
+        }
+    }
+
+    pub fn attribute<V: FromStr>(&mut self, att_name: &str) -> DeserializeResult<V> {
+        let name = Name {
+            namespace: None,
+            local_name: att_name.to_string(),
+        };
+        if let Some(att_value) = self.attributes.remove(&name) {
+            let value = <V as FromStr>::from_str(&att_value)
+                .map_err(|_| DeserializeError::FromStr(att_value))?;
+            return Ok(value);
+        } else {
+            return Err(DeserializeError::MissingAttribute(name));
+        }
+    }
+
+    pub fn attribute_namespaced<V: FromStr>(
+        &mut self,
+        att_name: &str,
+        att_namespace: &str,
+    ) -> DeserializeResult<V> {
+        let name = Name {
+            namespace: Some(att_namespace.to_string()),
+            local_name: att_name.to_string(),
+        };
+        if let Some(att_value) = self.attributes.remove(&name) {
+            let value = <V as FromStr>::from_str(&att_value)
+                .map_err(|_| DeserializeError::FromStr(att_value))?;
+            return Ok(value);
+        } else {
+            return Err(DeserializeError::MissingAttribute(name));
+        }
+    }
+
+    pub fn no_more_attributes(self) -> DeserializeResult<Self> {
+        if self.attributes.is_empty() {
+            Ok(self)
+        } else {
+            Err(DeserializeError::UnexpectedAttributes(self.attributes))
+        }
+    }
+
+    // for xs:any
+
+    pub fn child_one<T: FromElement>(&mut self) -> DeserializeResult<T> {
+        if let Some(position) = self.content.iter().position(|content| match content {
+            Content::Element(element) => <T as FromElement>::from_element(element.clone()).is_ok(),
+            Content::Text(_) => false,
+            Content::PI => false,
+            Content::Comment(_) => false,
+        }) {
+            let element = self.content.remove(position).unwrap();
+            if let Content::Element(e) = element {
+                return <T as FromElement>::from_element(e);
+            } else {
+                return Err(DeserializeError::MissingChild);
+            }
+        } else {
+            return Err(DeserializeError::MissingChild);
+        }
+    }
+
+    pub fn child_opt<T: FromElement>(&mut self) -> DeserializeResult<Option<T>> {
+        if let Some(position) = self.content.iter().position(|content| match content {
+            Content::Element(element) => <T as FromElement>::from_element(element.clone()).is_ok(),
+            Content::Text(_) => false,
+            Content::PI => false,
+            Content::Comment(_) => false,
+        }) {
+            let element = self.content.remove(position).unwrap();
+            if let Content::Element(e) = element {
+                return Ok(Some(<T as FromElement>::from_element(e)?));
+            } else {
+                return Err(DeserializeError::MissingChild);
+            }
+        } else {
+            return Ok(None);
+        }
+    }
+
+    pub fn children<T: FromElement>(&mut self) -> DeserializeResult<Vec<T>> {
+        let (children, rest): (VecDeque<_>, VecDeque<_>) = self
+            .content
+            .clone()
+            .into_iter()
+            .partition(|content| match content {
+                Content::Element(element) => {
+                    <T as FromElement>::from_element(element.clone()).is_ok()
+                }
+                Content::Text(_) => false,
+                Content::PI => false,
+                Content::Comment(_) => false,
+            });
+        self.content = rest;
+        let children: Vec<T> = children
+            .into_iter()
+            .map(|content| {
+                let child = match content {
+                    Content::Element(element) => <T as FromElement>::from_element(element).ok(),
+                    Content::Text(_) => None,
+                    Content::PI => None,
+                    Content::Comment(_) => None,
+                }
+                .unwrap();
+                child
+            })
+            .collect();
+        Ok(children)
+    }
+
+    pub fn value<V: FromStr>(&mut self) -> DeserializeResult<V> {
+        if let Some(position) = self.content.iter().position(|content| match content {
+            Content::Element(_) => false,
+            Content::Text(s) => <V as FromStr>::from_str(s).is_ok(),
+            Content::PI => false,
+            Content::Comment(_) => false,
+        }) {
+            let element = self.content.remove(position).unwrap();
+            if let Content::Text(v) = element {
+                return Ok(<V as FromStr>::from_str(&v).ok().unwrap());
+            } else {
+                panic!("infallible")
+            }
+        } else {
+            return Err(DeserializeError::MissingValue);
+        }
+    }
+
+    pub fn value_opt<V: FromStr>(&mut self) -> DeserializeResult<Option<V>> {
+        if let Some(position) = self.content.iter().position(|content| match content {
+            Content::Element(_) => false,
+            Content::Text(s) => <V as FromStr>::from_str(s).is_ok(),
+            Content::PI => false,
+            Content::Comment(_) => false,
+        }) {
+            let element = self.content.remove(position).unwrap();
+            if let Content::Text(v) = element {
+                return Ok(<V as FromStr>::from_str(&v).ok());
+            } else {
+                panic!("infallible")
+            }
+        } else {
+            return Ok(None);
+        }
+    }
+
+    // for xs:sequence
+
+    pub fn pop_child_one<T: FromElement>(&mut self) -> DeserializeResult<T> {
+        loop {
+            let child = self
+                .content
+                .pop_front()
+                .ok_or(DeserializeError::MissingChild)?;
+            match child {
+                Content::Element(element) => return Ok(<T as FromElement>::from_element(element)?),
+                Content::Text(_) => {
+                    return Err(DeserializeError::UnexpectedContent(self.content.clone()))
+                }
+                Content::PI => {}
+                Content::Comment(_) => {}
+            }
+        }
+    }
+
+    pub fn pop_child_opt<T: FromElement>(&mut self) -> DeserializeResult<Option<T>> {
+        loop {
+            let child = self.content.pop_front();
+            if let Some(child) = child {
+                match child {
+                    Content::Element(element) => {
+                        return Ok(Some(<T as FromElement>::from_element(element)?))
+                    }
+                    Content::Text(_) => {
+                        return Err(DeserializeError::UnexpectedContent(self.content.clone()))
+                    }
+                    Content::PI => {}
+                    Content::Comment(_) => {}
+                }
+            } else {
+                return Ok(None);
+            }
+        }
+    }
+
+    pub fn pop_children<T: FromElement>(&mut self) -> DeserializeResult<Vec<T>> {
+        let mut children = Vec::new();
+        loop {
+            let child = self.content.front();
+            if let Some(child) = child {
+                match child {
+                    Content::Element(element) => {
+                        if let Ok(child) = <T as FromElement>::from_element(element.clone()) {
+                            children.push(child);
+                            self.content.pop_front();
+                        }
+                    }
+                    Content::Text(_) => return Ok(children),
+                    Content::PI => {}
+                    Content::Comment(_) => {}
+                }
+            } else {
+                return Ok(children);
+            }
+        }
+    }
+
+    pub fn pop_value<V: FromStr>(&mut self) -> DeserializeResult<V> {
+        loop {
+            let child = self
+                .content
+                .pop_front()
+                .ok_or(DeserializeError::MissingChild)?;
+            match child {
+                Content::Element(_) => {
+                    return Err(DeserializeError::UnexpectedContent(self.content.clone()))
+                }
+                Content::Text(t) => {
+                    return Ok(
+                        <V as FromStr>::from_str(&t).map_err(|_| DeserializeError::FromStr(t))?
+                    )
+                }
+                Content::PI => {}
+                Content::Comment(_) => {}
+            }
+        }
+    }
+
+    pub fn pop_value_opt<V: FromStr>(&mut self) -> DeserializeResult<Option<V>> {
+        loop {
+            let child = self.content.pop_front();
+            if let Some(child) = child {
+                match child {
+                    Content::Element(_) => {
+                        return Err(DeserializeError::UnexpectedContent(self.content.clone()))
+                    }
+                    Content::Text(t) => {
+                        return Ok(Some(
+                            <V as FromStr>::from_str(&t)
+                                .map_err(|_| DeserializeError::FromStr(t))?,
+                        ))
+                    }
+                    Content::PI => {}
+                    Content::Comment(_) => {}
+                }
+            } else {
+                return Ok(None);
+            }
+        }
+    }
+
+    pub fn no_more_content(self) -> DeserializeResult<Self> {
+        if self
+            .content
+            .iter()
+            .filter(|content| match content {
+                Content::Element(_) => true,
+                Content::Text(_) => true,
+                Content::PI => false,
+                Content::Comment(_) => false,
+            })
+            .collect::<Vec<_>>()
+            .is_empty()
+        {
+            Ok(self)
+        } else {
+            Err(DeserializeError::UnexpectedContent(self.content))
+        }
+    }
+
+    pub fn builder(name: impl ToString, namespace: Option<impl ToString>) -> ElementBuilder {
+        ElementBuilder::new(name, namespace)
+    }
+}
+
+pub struct ElementBuilder {
+    name: Name,
+    namespace_declaration_overrides: Vec<NamespaceDeclaration>,
+    attributes: Vec<(Name, String)>,
+    content: Vec<ContentBuilder>,
+}
+
+impl ElementBuilder {
+    pub fn new(name: impl ToString, namespace: Option<impl ToString>) -> Self {
+        Self {
+            name: Name {
+                namespace: namespace.map(|namespace| namespace.to_string()),
+                local_name: name.to_string(),
+            },
+            namespace_declaration_overrides: Vec::new(),
+            attributes: Vec::new(),
+            content: Vec::new(),
+        }
+    }
+
+    pub fn push_namespace_declaration_override(
+        mut self,
+        prefix: Option<impl ToString>,
+        namespace: impl ToString,
+    ) -> Self {
+        self.namespace_declaration_overrides
+            .push(NamespaceDeclaration {
+                prefix: prefix.map(|prefix| prefix.to_string()),
+                namespace: namespace.to_string(),
+            });
+        self
+    }
+
+    pub fn push_attribute<N: ToString, V: ToString>(mut self, name: N, value: V) -> Self {
+        self.attributes.push((
+            // TODO: make sure name is a valid name, same for prefixes
+            Name {
+                namespace: None,
+                local_name: name.to_string(),
+            },
+            value.to_string(),
+        ));
+        self
+    }
+
+    pub fn push_attribute_namespaced(
+        mut self,
+        namespace: impl ToString,
+        name: impl ToString,
+        value: impl ToString,
+    ) -> Self {
+        self.attributes.push((
+            Name {
+                namespace: Some(namespace.to_string()),
+                local_name: name.to_string(),
+            },
+            value.to_string(),
+        ));
+        self
+    }
+
+    pub fn push_child(mut self, child: ElementBuilder) -> Self {
+        self.content.push(ContentBuilder::Element(child));
+        self
+    }
+
+    pub fn push_text(mut self, text: impl ToString) -> Self {
+        self.content.push(ContentBuilder::Text(text.to_string()));
+        self
+    }
+
+    pub fn push_attribute_opt(self, name: impl ToString, value: Option<impl ToString>) -> Self {
+        if let Some(value) = value {
+            self.push_attribute(name, value)
+        } else {
+            self
+        }
+    }
+
+    pub fn push_attribute_opt_namespaced(
+        self,
+        namespace: impl ToString,
+        name: impl ToString,
+        value: Option<impl ToString>,
+    ) -> Self {
+        if let Some(value) = value {
+            self.push_attribute_namespaced(namespace, name, value)
+        } else {
+            self
+        }
+    }
+
+    pub fn push_child_opt(self, child: Option<ElementBuilder>) -> Self {
+        if let Some(child) = child {
+            self.push_child(child)
+        } else {
+            self
+        }
+    }
+
+    pub fn push_text_opt(self, text: Option<impl ToString>) -> Self {
+        if let Some(text) = text {
+            self.push_text(text)
+        } else {
+            self
+        }
+    }
+
+    pub fn push_content(mut self, content: ContentBuilder) -> Self {
+        self.content.push(content);
+        self
+    }
+
+    pub fn push_children(self, children: Vec<impl IntoContent>) -> Self {
+        let mut element_builder = self;
+        for child in children {
+            element_builder = element_builder.push_content(child.builder())
+        }
+        element_builder
+    }
+
+    pub fn build(&self) -> Result<Element> {
+        let mut namespace_declaration_overrides = HashSet::new();
+        for namespace_declaration in &self.namespace_declaration_overrides {
+            if !namespace_declaration_overrides.insert(namespace_declaration.clone()) {
+                return Err(Error::DuplicateNameSpaceDeclaration(
+                    namespace_declaration.clone(),
+                ));
+            }
+        }
+
+        let mut attributes = HashMap::new();
+        for (att_name, att_value) in &self.attributes {
+            if attributes
+                .insert(att_name.clone(), att_value.to_string())
+                .is_some()
+            {
+                // TODO: better error
+                return Err(Error::DuplicateAttribute(att_name.local_name.to_string()));
+            }
+        }
+
+        let content: Result<VecDeque<Content>> = self
+            .content
+            .iter()
+            .map(|content_builder| -> Result<Content> { Ok(content_builder.build()?) })
+            .collect();
+        let content = content?;
+
+        Ok(Element {
+            name: self.name.clone(),
+            namespace_declaration_overrides,
+            attributes,
+            content,
+        })
+    }
+}
+
+pub trait IntoContent {
+    fn builder(&self) -> ContentBuilder;
+}
+
+impl<T> IntoContent for T
+where
+    T: IntoElement,
+{
+    fn builder(&self) -> ContentBuilder {
+        ContentBuilder::Element(self.builder())
+    }
+}
+
+pub enum ContentBuilder {
+    Element(ElementBuilder),
+    Text(String),
+}
+
+impl ContentBuilder {
+    pub fn build(&self) -> Result<Content> {
+        match self {
+            ContentBuilder::Element(element_builder) => {
+                Ok(Content::Element(element_builder.build()?))
+            }
+            ContentBuilder::Text(text) => Ok(Content::Text(text.to_string())),
+        }
+    }
 }
 
 pub fn escape_str(s: &str) -> String {

src/error.rs

@@ -1,6 +1,26 @@
-use std::{num::ParseIntError, str::Utf8Error};
+use std::{
+    collections::{HashMap, VecDeque},
+    num::ParseIntError,
+    str::{FromStr, Utf8Error},
+};
 
-use crate::element::{Content, Name, NamespaceDeclaration};
+use crate::{
+    element::{Content, Name, NamespaceDeclaration},
+    Element,
+};
+
+#[derive(Debug)]
+pub enum DeserializeError {
+    FromStr(String),
+    UnexpectedAttributes(HashMap<Name, String>),
+    UnexpectedContent(VecDeque<Content>),
+    MissingAttribute(Name),
+    IncorrectName(String),
+    IncorrectNamespace(String),
+    Unqualified,
+    MissingChild,
+    MissingValue,
+}
 
 #[derive(Debug)]
 pub enum Error {
@@ -23,6 +43,13 @@ pub enum Error {
     UnexpectedNumberOfContents(usize),
     UnexpectedContent(Content),
     UnexpectedElement(Name),
+    Deserialize(DeserializeError),
+}
+
+impl From<DeserializeError> for Error {
+    fn from(e: DeserializeError) -> Self {
+        Self::Deserialize(e)
+    }
 }
 
 impl From<std::io::Error> for Error {

src/lib.rs

@@ -11,6 +11,7 @@ pub const XML_NS: &str = "http://www.w3.org/XML/1998/namespace";
 pub const XMLNS_NS: &str = "http://www.w3.org/2000/xmlns/";
 
 pub use element::Element;
+pub use error::DeserializeError;
 pub use error::Error;
 pub use reader::Reader;
 pub use writer::Writer;

src/reader.rs

@@ -2,7 +2,7 @@ use circular::Buffer;
 use futures::{FutureExt, Stream};
 use nom::Err;
 use std::{
-    collections::{hash_set, BTreeMap, HashMap, HashSet},
+    collections::{hash_set, BTreeMap, HashMap, HashSet, VecDeque},
     future::Future,
     path::Prefix,
     pin::{pin, Pin},
@@ -102,12 +102,12 @@ where
 
     pub async fn read_start<'s, T: FromElement>(&'s mut self) -> Result<T> {
         let element = self.read_start_tag().await?;
-        FromElement::from_element(element)
+        Ok(FromElement::from_element(element)?)
     }
 
     pub async fn read<'s, T: FromElement>(&'s mut self) -> Result<T> {
         let element = self.read_element().await?;
-        FromElement::from_element(element)
+        Ok(FromElement::from_element(element)?)
     }
 
     pub async fn read_start_tag<'s>(&'s mut self) -> Result<Element> {
@@ -436,9 +436,9 @@ impl<R> Reader<R> {
 
         return Ok(Element {
             name: element_name,
-            namespace_declarations: element_namespace_declarations,
+            namespace_declaration_overrides: element_namespace_declarations,
             attributes,
-            content: Vec::new(),
+            content: VecDeque::new(),
         });
     }
 
@@ -652,12 +652,12 @@ impl<R> Reader<R> {
 
             namespace_declarations.pop();
         } else {
-            content = Vec::new();
+            content = VecDeque::new();
         }
 
         return Ok(Element {
             name: element_name,
-            namespace_declarations: element_namespace_declarations,
+            namespace_declaration_overrides: element_namespace_declarations,
             attributes,
             content,
         });
@@ -666,18 +666,18 @@ impl<R> Reader<R> {
     fn content_from_xml(
         namespaces: &mut Vec<HashSet<NamespaceDeclaration>>,
         xml_content: xml::Content,
-    ) -> Result<Vec<Content>> {
-        let mut content = Vec::new();
+    ) -> Result<VecDeque<Content>> {
+        let mut content = VecDeque::new();
         let mut text = xml_content.char_data.map(|str| String::from(*str));
         for (content_item, char_data) in xml_content.content {
             match content_item {
                 xml::ContentItem::Element(element) => {
                     text.map(|text| {
                         if !text.is_empty() {
-                            content.push(Content::Text(text))
+                            content.push_back(Content::Text(text))
                         }
                     });
-                    content.push(Content::Element(Self::element_from_xml(
+                    content.push_back(Content::Element(Self::element_from_xml(
                         namespaces, element,
                     )?));
                     text = char_data.map(|str| String::from(*str));
@@ -711,7 +711,7 @@ impl<R> Reader<R> {
         }
         text.map(|text| {
             if !text.is_empty() {
-                content.push(Content::Text(text))
+                content.push_back(Content::Text(text))
             }
         });
         Ok(content)

src/writer.rs

@@ -93,20 +93,38 @@ impl<W: AsyncWrite + Unpin + Send> Writer<W> {
     }
 
     pub async fn write_empty(&mut self, element: &Element) -> Result<()> {
-        let namespace_declarations_stack: Vec<_> = self
+        let mut namespace_declarations_stack: Vec<_> = self
             .namespace_declarations
             .iter()
             .flatten()
-            .chain(&element.namespace_declarations)
+            .chain(&element.namespace_declaration_overrides)
             .collect();
 
+        let mut namespace_declarations = element.namespace_declaration_overrides.clone();
+
+        let default_namespace_declaration;
         let prefix;
         if let Some(namespace) = &element.name.namespace {
-            let name_namespace_declaration = namespace_declarations_stack
+            if let Some(name_namespace_declaration) = namespace_declarations_stack
                 .iter()
                 .rfind(|namespace_declaration| namespace_declaration.namespace == *namespace)
-                .ok_or(Error::UndeclaredNamespace(namespace.clone()))?;
-            prefix = name_namespace_declaration.prefix.as_ref();
+            {
+                prefix = name_namespace_declaration.prefix.as_ref();
+            } else {
+                default_namespace_declaration = NamespaceDeclaration {
+                    prefix: None,
+                    namespace: namespace.clone(),
+                };
+                if namespace_declarations.insert(default_namespace_declaration.clone()) {
+                    namespace_declarations_stack.push(&default_namespace_declaration);
+                    prefix = None
+                } else {
+                    return Err(Error::DuplicateNameSpaceDeclaration(NamespaceDeclaration {
+                        prefix: None,
+                        namespace: namespace.clone(),
+                    }));
+                }
+            }
         } else {
             prefix = None
         }
@@ -125,7 +143,7 @@ impl<W: AsyncWrite + Unpin + Send> Writer<W> {
 
         let mut attributes = Vec::new();
 
-        for namespace_declaration in &element.namespace_declarations {
+        for namespace_declaration in namespace_declarations.iter() {
             let ns_name = namespace_declaration
                 .prefix
                 .as_ref()
@@ -180,20 +198,38 @@ impl<W: AsyncWrite + Unpin + Send> Writer<W> {
     }
 
     pub async fn write_element_start(&mut self, element: &Element) -> Result<()> {
-        let namespace_declarations_stack: Vec<_> = self
+        let mut namespace_declarations_stack: Vec<_> = self
             .namespace_declarations
             .iter()
             .flatten()
-            .chain(&element.namespace_declarations)
+            .chain(&element.namespace_declaration_overrides)
             .collect();
 
+        let mut namespace_declarations = element.namespace_declaration_overrides.clone();
+
+        let default_namespace_declaration;
         let prefix;
         if let Some(namespace) = &element.name.namespace {
-            let name_namespace_declaration = namespace_declarations_stack
+            if let Some(name_namespace_declaration) = namespace_declarations_stack
                 .iter()
                 .rfind(|namespace_declaration| namespace_declaration.namespace == *namespace)
-                .ok_or(Error::UndeclaredNamespace(namespace.clone()))?;
-            prefix = name_namespace_declaration.prefix.as_ref();
+            {
+                prefix = name_namespace_declaration.prefix.as_ref();
+            } else {
+                default_namespace_declaration = NamespaceDeclaration {
+                    prefix: None,
+                    namespace: namespace.clone(),
+                };
+                if namespace_declarations.insert(default_namespace_declaration.clone()) {
+                    namespace_declarations_stack.push(&default_namespace_declaration);
+                    prefix = None
+                } else {
+                    return Err(Error::DuplicateNameSpaceDeclaration(NamespaceDeclaration {
+                        prefix: None,
+                        namespace: namespace.clone(),
+                    }));
+                }
+            }
         } else {
             prefix = None
         }
@@ -212,7 +248,7 @@ impl<W: AsyncWrite + Unpin + Send> Writer<W> {
 
         let mut attributes = Vec::new();
 
-        for namespace_declaration in &element.namespace_declarations {
+        for namespace_declaration in namespace_declarations.iter() {
             let ns_name = namespace_declaration
                 .prefix
                 .as_ref()
@@ -265,7 +301,7 @@ impl<W: AsyncWrite + Unpin + Send> Writer<W> {
 
         self.depth.push(element.name.clone());
         self.namespace_declarations
-            .push(element.namespace_declarations.clone());
+            .push(namespace_declarations.clone());
         Ok(())
     }