servo/components/script/xpath/eval.rs

/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at https://mozilla.org/MPL/2.0/. */

use std::fmt;

use html5ever::{LocalName, Namespace, Prefix, QualName, local_name, namespace_prefix, ns};
use script_bindings::script_runtime::CanGc;

use super::parser::{
    AdditiveOp, Axis, EqualityOp, Expr, FilterExpr, KindTest, Literal, MultiplicativeOp, NodeTest,
    NumericLiteral, PathExpr, PredicateExpr, PredicateListExpr, PrimaryExpr,
    QName as ParserQualName, RelationalOp, StepExpr, UnaryOp,
};
use super::{EvaluationCtx, Value};
use crate::dom::attr::Attr;
use crate::dom::bindings::codegen::Bindings::NodeBinding::NodeMethods;
use crate::dom::bindings::domname::namespace_from_domstring;
use crate::dom::bindings::error::Error as JsError;
use crate::dom::bindings::inheritance::{Castable, CharacterDataTypeId, NodeTypeId};
use crate::dom::bindings::root::DomRoot;
use crate::dom::bindings::str::DOMString;
use crate::dom::bindings::xmlname;
use crate::dom::element::Element;
use crate::dom::node::{Node, ShadowIncluding};
use crate::dom::processinginstruction::ProcessingInstruction;
use crate::xpath::context::PredicateCtx;

#[derive(Clone, Debug)]
pub(crate) enum Error {
    NotANodeset,
    InvalidPath,
    UnknownFunction {
        name: QualName,
    },
    /// It is not clear where variables used in XPath expression should come from.
    /// Firefox throws "NS_ERROR_ILLEGAL_VALUE" when using them, chrome seems to return
    /// an empty result. We also error out.
    ///
    /// See <https://github.com/whatwg/dom/issues/67>
    CannotUseVariables,
    UnknownNamespace {
        prefix: String,
    },
    InvalidQName {
        qname: ParserQualName,
    },
    FunctionEvaluation {
        fname: String,
    },
    Internal {
        msg: String,
    },
    /// A JS exception that needs to be propagated to the caller.
    JsException(JsError),
}

impl std::fmt::Display for Error {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        match self {
            Error::NotANodeset => write!(f, "expression did not evaluate to a nodeset"),
            Error::InvalidPath => write!(f, "invalid path expression"),
            Error::UnknownFunction { name } => write!(f, "unknown function {:?}", name),
            Error::CannotUseVariables => write!(f, "cannot use variables"),
            Error::UnknownNamespace { prefix } => {
                write!(f, "unknown namespace prefix {:?}", prefix)
            },
            Error::InvalidQName { qname } => {
                write!(f, "invalid QName {:?}", qname)
            },
            Error::FunctionEvaluation { fname } => {
                write!(f, "error while evaluating function: {}", fname)
            },
            Error::Internal { msg } => {
                write!(f, "internal error: {}", msg)
            },
            Error::JsException(exception) => {
                write!(f, "JS exception: {:?}", exception)
            },
        }
    }
}

impl std::error::Error for Error {}

pub(crate) fn try_extract_nodeset(v: Value) -> Result<Vec<DomRoot<Node>>, Error> {
    match v {
        Value::Nodeset(ns) => Ok(ns),
        _ => Err(Error::NotANodeset),
    }
}

pub(crate) trait Evaluatable: fmt::Debug {
    fn evaluate(&self, context: &EvaluationCtx) -> Result<Value, Error>;
    /// Returns true if this expression evaluates to a primitive value, without needing to touch the DOM
    fn is_primitive(&self) -> bool;
}

impl<T: ?Sized> Evaluatable for Box<T>
where
    T: Evaluatable,
{
    fn evaluate(&self, context: &EvaluationCtx) -> Result<Value, Error> {
        (**self).evaluate(context)
    }

    fn is_primitive(&self) -> bool {
        (**self).is_primitive()
    }
}

impl<T> Evaluatable for Option<T>
where
    T: Evaluatable,
{
    fn evaluate(&self, context: &EvaluationCtx) -> Result<Value, Error> {
        match self {
            Some(expr) => expr.evaluate(context),
            None => Ok(Value::Nodeset(vec![])),
        }
    }

    fn is_primitive(&self) -> bool {
        self.as_ref().is_some_and(|t| T::is_primitive(t))
    }
}

impl Evaluatable for Expr {
    fn evaluate(&self, context: &EvaluationCtx) -> Result<Value, Error> {
        match self {
            Expr::And(left, right) => {
                let left_bool = left.evaluate(context)?.boolean();
                let v = left_bool && right.evaluate(context)?.boolean();
                Ok(Value::Boolean(v))
            },
            Expr::Or(left, right) => {
                let left_bool = left.evaluate(context)?.boolean();
                let v = left_bool || right.evaluate(context)?.boolean();
                Ok(Value::Boolean(v))
            },
            Expr::Equality(left, equality_op, right) => {
                let left_val = left.evaluate(context)?;
                let right_val = right.evaluate(context)?;

                let v = match equality_op {
                    EqualityOp::Eq => left_val == right_val,
                    EqualityOp::NotEq => left_val != right_val,
                };

                Ok(Value::Boolean(v))
            },
            Expr::Relational(left, relational_op, right) => {
                let left_val = left.evaluate(context)?.number();
                let right_val = right.evaluate(context)?.number();

                let v = match relational_op {
                    RelationalOp::Lt => left_val < right_val,
                    RelationalOp::Gt => left_val > right_val,
                    RelationalOp::LtEq => left_val <= right_val,
                    RelationalOp::GtEq => left_val >= right_val,
                };
                Ok(Value::Boolean(v))
            },
            Expr::Additive(left, additive_op, right) => {
                let left_val = left.evaluate(context)?.number();
                let right_val = right.evaluate(context)?.number();

                let v = match additive_op {
                    AdditiveOp::Add => left_val + right_val,
                    AdditiveOp::Sub => left_val - right_val,
                };
                Ok(Value::Number(v))
            },
            Expr::Multiplicative(left, multiplicative_op, right) => {
                let left_val = left.evaluate(context)?.number();
                let right_val = right.evaluate(context)?.number();

                let v = match multiplicative_op {
                    MultiplicativeOp::Mul => left_val * right_val,
                    MultiplicativeOp::Div => left_val / right_val,
                    MultiplicativeOp::Mod => left_val % right_val,
                };
                Ok(Value::Number(v))
            },
            Expr::Unary(unary_op, expr) => {
                let v = expr.evaluate(context)?.number();

                match unary_op {
                    UnaryOp::Minus => Ok(Value::Number(-v)),
                }
            },
            Expr::Union(left, right) => {
                let as_nodes = |e: &Expr| e.evaluate(context).and_then(try_extract_nodeset);

                let mut left_nodes = as_nodes(left)?;
                let right_nodes = as_nodes(right)?;

                left_nodes.extend(right_nodes);
                Ok(Value::Nodeset(left_nodes))
            },
            Expr::Path(path_expr) => path_expr.evaluate(context),
        }
    }

    fn is_primitive(&self) -> bool {
        match self {
            Expr::Or(left, right) => left.is_primitive() && right.is_primitive(),
            Expr::And(left, right) => left.is_primitive() && right.is_primitive(),
            Expr::Equality(left, _, right) => left.is_primitive() && right.is_primitive(),
            Expr::Relational(left, _, right) => left.is_primitive() && right.is_primitive(),
            Expr::Additive(left, _, right) => left.is_primitive() && right.is_primitive(),
            Expr::Multiplicative(left, _, right) => left.is_primitive() && right.is_primitive(),
            Expr::Unary(_, expr) => expr.is_primitive(),
            Expr::Union(_, _) => false,
            Expr::Path(path_expr) => path_expr.is_primitive(),
        }
    }
}

impl Evaluatable for PathExpr {
    fn evaluate(&self, context: &EvaluationCtx) -> Result<Value, Error> {
        // Use starting_node for absolute/descendant paths, context_node otherwise
        let mut current_nodes = if self.is_absolute || self.is_descendant {
            vec![context.starting_node.clone()]
        } else {
            vec![context.context_node.clone()]
        };

        // If path starts with '//', add an implicit descendant-or-self::node() step
        if self.is_descendant {
            current_nodes = current_nodes
                .iter()
                .flat_map(|n| n.traverse_preorder(ShadowIncluding::No))
                .collect();
        }

        trace!("[PathExpr] Evaluating path expr: {:?}", self);

        let have_multiple_steps = self.steps.len() > 1;

        for step in &self.steps {
            let mut next_nodes = Vec::new();
            for node in current_nodes {
                let step_context = context.subcontext_for_node(&node);
                let step_result = step.evaluate(&step_context)?;
                match (have_multiple_steps, step_result) {
                    (_, Value::Nodeset(mut nodes)) => {
                        // as long as we evaluate to nodesets, keep going
                        next_nodes.append(&mut nodes);
                    },
                    (false, value) => {
                        trace!("[PathExpr] Got single primitive value: {:?}", value);
                        return Ok(value);
                    },
                    (true, value) => {
                        error!(
                            "Expected nodeset from step evaluation, got: {:?} node: {:?}, step: {:?}",
                            value, node, step
                        );
                        return Ok(value);
                    },
                }
            }
            current_nodes = next_nodes;
        }

        trace!("[PathExpr] Got nodes: {:?}", current_nodes);

        Ok(Value::Nodeset(current_nodes))
    }

    fn is_primitive(&self) -> bool {
        !self.is_absolute &&
            !self.is_descendant &&
            self.steps.len() == 1 &&
            self.steps[0].is_primitive()
    }
}

/// Error types for validate and extract a qualified name following
/// the XML naming rules.
#[derive(Debug)]
enum ValidationError {
    InvalidCharacter,
    Namespace,
}

/// Validate a qualified name following the XML naming rules.
///
/// On success, this returns a tuple `(prefix, local name)`.
fn validate_and_extract_qualified_name(
    qualified_name: &str,
) -> Result<(Option<&str>, &str), ValidationError> {
    if qualified_name.is_empty() {
        // Qualified names must not be empty
        return Err(ValidationError::InvalidCharacter);
    }
    let mut colon_offset = None;
    let mut at_start_of_name = true;

    for (byte_position, c) in qualified_name.char_indices() {
        if c == ':' {
            if colon_offset.is_some() {
                // Qualified names must not contain more than one colon
                return Err(ValidationError::InvalidCharacter);
            }
            colon_offset = Some(byte_position);
            at_start_of_name = true;
            continue;
        }

        if at_start_of_name {
            if !xmlname::is_valid_start(c) {
                // Name segments must begin with a valid start character
                return Err(ValidationError::InvalidCharacter);
            }
            at_start_of_name = false;
        } else if !xmlname::is_valid_continuation(c) {
            // Name segments must consist of valid characters
            return Err(ValidationError::InvalidCharacter);
        }
    }

    let Some(colon_offset) = colon_offset else {
        // Simple case: there is no prefix
        return Ok((None, qualified_name));
    };

    let (prefix, local_name) = qualified_name.split_at(colon_offset);
    let local_name = &local_name[1..]; // Remove the colon

    if prefix.is_empty() || local_name.is_empty() {
        // Neither prefix nor local name can be empty
        return Err(ValidationError::InvalidCharacter);
    }

    Ok((Some(prefix), local_name))
}

/// Validate a namespace and qualified name following the XML naming rules
/// and extract their parts.
fn validate_and_extract(
    namespace: Option<DOMString>,
    qualified_name: &str,
) -> Result<(Namespace, Option<Prefix>, LocalName), ValidationError> {
    // Step 1. If namespace is the empty string, then set it to null.
    let namespace = namespace_from_domstring(namespace);

    // Step 2. Validate qualifiedName.
    // Step 3. Let prefix be null.
    // Step 4. Let localName be qualifiedName.
    // Step 5. If qualifiedName contains a U+003A (:):
    // NOTE: validate_and_extract_qualified_name does all of these things for us, because
    // it's easier to do them together
    let (prefix, local_name) = validate_and_extract_qualified_name(qualified_name)?;
    debug_assert!(!local_name.contains(':'));

    match (namespace, prefix) {
        (ns!(), Some(_)) => {
            // Step 6. If prefix is non-null and namespace is null, then throw a "NamespaceError" DOMException.
            Err(ValidationError::Namespace)
        },
        (ref ns, Some("xml")) if ns != &ns!(xml) => {
            // Step 7. If prefix is "xml" and namespace is not the XML namespace,
            // then throw a "NamespaceError" DOMException.
            Err(ValidationError::Namespace)
        },
        (ref ns, p) if ns != &ns!(xmlns) && (qualified_name == "xmlns" || p == Some("xmlns")) => {
            // Step 8. If either qualifiedName or prefix is "xmlns" and namespace is not the XMLNS namespace,
            // then throw a "NamespaceError" DOMException.
            Err(ValidationError::Namespace)
        },
        (ns!(xmlns), p) if qualified_name != "xmlns" && p != Some("xmlns") => {
            // Step 9. If namespace is the XMLNS namespace and neither qualifiedName nor prefix is "xmlns",
            // then throw a "NamespaceError" DOMException.
            Err(ValidationError::Namespace)
        },
        (ns, p) => {
            // Step 10. Return namespace, prefix, and localName.
            Ok((ns, p.map(Prefix::from), LocalName::from(local_name)))
        },
    }
}

pub(crate) fn convert_parsed_qname_to_qualified_name(
    qname: &ParserQualName,
    context: &EvaluationCtx,
    can_gc: CanGc,
) -> Result<QualName, Error> {
    let qname_as_str = qname.to_string();
    let namespace = context
        .resolve_namespace(qname.prefix.as_deref(), can_gc)
        .map_err(Error::JsException)?;

    if let Ok((ns, prefix, local)) = validate_and_extract(namespace, &qname_as_str) {
        Ok(QualName { prefix, ns, local })
    } else {
        Err(Error::InvalidQName {
            qname: qname.clone(),
        })
    }
}

#[derive(Debug)]
pub(crate) enum NameTestComparisonMode {
    /// Namespaces must match exactly
    XHtml,
    /// Missing namespace information is treated as the HTML namespace
    Html,
}

pub(crate) fn element_name_test(
    expected_name: QualName,
    element_qualname: QualName,
    comparison_mode: NameTestComparisonMode,
) -> bool {
    let is_wildcard = expected_name.local == local_name!("*");

    let test_prefix = expected_name
        .prefix
        .clone()
        .unwrap_or(namespace_prefix!(""));
    let test_ns_uri = match test_prefix {
        namespace_prefix!("*") => ns!(*),
        namespace_prefix!("html") => ns!(html),
        namespace_prefix!("xml") => ns!(xml),
        namespace_prefix!("xlink") => ns!(xlink),
        namespace_prefix!("svg") => ns!(svg),
        namespace_prefix!("mathml") => ns!(mathml),
        namespace_prefix!("") => {
            if matches!(comparison_mode, NameTestComparisonMode::XHtml) {
                ns!()
            } else {
                ns!(html)
            }
        },
        _ => {
            // We don't support custom namespaces, use fallback or panic depending on strictness
            if matches!(comparison_mode, NameTestComparisonMode::XHtml) {
                panic!("Unrecognized namespace prefix: {}", test_prefix)
            } else {
                ns!(html)
            }
        },
    };

    if is_wildcard {
        test_ns_uri == element_qualname.ns
    } else {
        test_ns_uri == element_qualname.ns && expected_name.local == element_qualname.local
    }
}

fn apply_node_test(
    context: &EvaluationCtx,
    test: &NodeTest,
    node: &Node,
    can_gc: CanGc,
) -> Result<bool, Error> {
    let result = match test {
        NodeTest::Name(qname) => {
            // Convert the unvalidated "parser QualName" into the proper QualName structure
            let wanted_name = convert_parsed_qname_to_qualified_name(qname, context, can_gc)?;
            match node.type_id() {
                NodeTypeId::Element(_) => {
                    let element = node.downcast::<Element>().unwrap();
                    let comparison_mode = if node.owner_doc().is_html_document() {
                        NameTestComparisonMode::Html
                    } else {
                        NameTestComparisonMode::XHtml
                    };
                    let element_qualname = QualName::new(
                        element.prefix().as_ref().cloned(),
                        element.namespace().clone(),
                        element.local_name().clone(),
                    );
                    element_name_test(wanted_name, element_qualname, comparison_mode)
                },
                NodeTypeId::Attr => {
                    let attr = node.downcast::<Attr>().unwrap();
                    let attr_qualname = QualName::new(
                        attr.prefix().cloned(),
                        attr.namespace().clone(),
                        attr.local_name().clone(),
                    );
                    // attributes are always compared with strict namespace matching
                    let comparison_mode = NameTestComparisonMode::XHtml;
                    element_name_test(wanted_name, attr_qualname, comparison_mode)
                },
                _ => false,
            }
        },
        NodeTest::Wildcard => matches!(node.type_id(), NodeTypeId::Element(_)),
        NodeTest::Kind(kind) => match kind {
            KindTest::PI(target) => {
                if NodeTypeId::CharacterData(CharacterDataTypeId::ProcessingInstruction) ==
                    node.type_id()
                {
                    let pi = node.downcast::<ProcessingInstruction>().unwrap();
                    match (target, pi.target()) {
                        (Some(target_name), node_target_name)
                            if target_name == &node_target_name.to_string() =>
                        {
                            true
                        },
                        (Some(_), _) => false,
                        (None, _) => true,
                    }
                } else {
                    false
                }
            },
            KindTest::Comment => matches!(
                node.type_id(),
                NodeTypeId::CharacterData(CharacterDataTypeId::Comment)
            ),
            KindTest::Text => matches!(
                node.type_id(),
                NodeTypeId::CharacterData(CharacterDataTypeId::Text(_))
            ),
            KindTest::Node => true,
        },
    };
    Ok(result)
}

impl Evaluatable for StepExpr {
    fn evaluate(&self, context: &EvaluationCtx) -> Result<Value, Error> {
        match self {
            StepExpr::Filter(filter_expr) => filter_expr.evaluate(context),
            StepExpr::Axis(axis_step) => {
                let nodes: Vec<DomRoot<Node>> = match axis_step.axis {
                    Axis::Child => context.context_node.children().collect(),
                    Axis::Descendant => context
                        .context_node
                        .traverse_preorder(ShadowIncluding::No)
                        .skip(1)
                        .collect(),
                    Axis::Parent => vec![context.context_node.GetParentNode()]
                        .into_iter()
                        .flatten()
                        .collect(),
                    Axis::Ancestor => context.context_node.ancestors().collect(),
                    Axis::Following => context
                        .context_node
                        .following_nodes(&context.context_node)
                        .skip(1)
                        .collect(),
                    Axis::Preceding => context
                        .context_node
                        .preceding_nodes(&context.context_node)
                        .skip(1)
                        .collect(),
                    Axis::FollowingSibling => context.context_node.following_siblings().collect(),
                    Axis::PrecedingSibling => context.context_node.preceding_siblings().collect(),
                    Axis::Attribute => {
                        if matches!(Node::type_id(&context.context_node), NodeTypeId::Element(_)) {
                            let element = context.context_node.downcast::<Element>().unwrap();
                            element
                                .attrs()
                                .iter()
                                .map(|attr| attr.upcast::<Node>())
                                .map(DomRoot::from_ref)
                                .collect()
                        } else {
                            vec![]
                        }
                    },
                    Axis::Self_ => vec![context.context_node.clone()],
                    Axis::DescendantOrSelf => context
                        .context_node
                        .traverse_preorder(ShadowIncluding::No)
                        .collect(),
                    Axis::AncestorOrSelf => context
                        .context_node
                        .inclusive_ancestors(ShadowIncluding::No)
                        .collect(),
                    Axis::Namespace => Vec::new(), // Namespace axis is not commonly implemented
                };

                trace!("[StepExpr] Axis {:?} got nodes {:?}", axis_step.axis, nodes);

                // Filter nodes according to the step's node_test. Will error out if any NodeTest
                // application errors out.
                let filtered_nodes: Vec<DomRoot<Node>> = nodes
                    .into_iter()
                    .map(|node| {
                        // FIXME: propagate this can_gc up further. This likely requires removing the "Evaluate"
                        // trait or changing the signature of "evaluate". The trait is not really necessary anyways.
                        apply_node_test(context, &axis_step.node_test, &node, CanGc::note())
                            .map(|matches| matches.then_some(node))
                    })
                    .collect::<Result<Vec<_>, _>>()?
                    .into_iter()
                    .flatten()
                    .collect();

                trace!("[StepExpr] Filtering got nodes {:?}", filtered_nodes);

                if axis_step.predicates.predicates.is_empty() {
                    trace!(
                        "[StepExpr] No predicates, returning nodes {:?}",
                        filtered_nodes
                    );
                    Ok(Value::Nodeset(filtered_nodes))
                } else {
                    // Apply predicates
                    let predicate_list_subcontext = context
                        .update_predicate_nodes(filtered_nodes.iter().map(|n| &**n).collect());
                    axis_step.predicates.evaluate(&predicate_list_subcontext)
                }
            },
        }
    }

    fn is_primitive(&self) -> bool {
        match self {
            StepExpr::Filter(filter_expr) => filter_expr.is_primitive(),
            StepExpr::Axis(_) => false,
        }
    }
}

impl Evaluatable for PredicateListExpr {
    fn evaluate(&self, context: &EvaluationCtx) -> Result<Value, Error> {
        if let Some(ref predicate_nodes) = context.predicate_nodes {
            let mut matched_nodes: Vec<DomRoot<Node>> = predicate_nodes.clone();

            for predicate_expr in &self.predicates {
                let size = matched_nodes.len();
                let mut new_matched = Vec::new();

                for (i, node) in matched_nodes.iter().enumerate() {
                    // 1-based position, per XPath spec
                    let predicate_ctx = EvaluationCtx {
                        starting_node: context.starting_node.clone(),
                        context_node: node.clone(),
                        predicate_nodes: context.predicate_nodes.clone(),
                        predicate_ctx: Some(PredicateCtx { index: i + 1, size }),
                        resolver: context.resolver.clone(),
                    };

                    let eval_result = predicate_expr.expr.evaluate(&predicate_ctx);

                    let keep = match eval_result {
                        Ok(Value::Number(n)) => (i + 1) as f64 == n,
                        Ok(Value::Boolean(b)) => b,
                        Ok(v) => v.boolean(),
                        Err(_) => false,
                    };

                    if keep {
                        new_matched.push(node.clone());
                    }
                }

                matched_nodes = new_matched;
                trace!(
                    "[PredicateListExpr] Predicate {:?} matched nodes {:?}",
                    predicate_expr, matched_nodes
                );
            }
            Ok(Value::Nodeset(matched_nodes))
        } else {
            Err(Error::Internal {
                msg: "[PredicateListExpr] No nodes on stack for predicate to operate on"
                    .to_string(),
            })
        }
    }

    fn is_primitive(&self) -> bool {
        self.predicates.len() == 1 && self.predicates[0].is_primitive()
    }
}

impl Evaluatable for PredicateExpr {
    fn evaluate(&self, context: &EvaluationCtx) -> Result<Value, Error> {
        let narrowed_nodes: Result<Vec<DomRoot<Node>>, Error> = context
            .subcontext_iter_for_nodes()
            .filter_map(|ctx| {
                if let Some(predicate_ctx) = ctx.predicate_ctx {
                    let eval_result = self.expr.evaluate(&ctx);

                    let v = match eval_result {
                        Ok(Value::Number(v)) => Ok(predicate_ctx.index == v as usize),
                        Ok(Value::Boolean(v)) => Ok(v),
                        Ok(v) => Ok(v.boolean()),
                        Err(e) => Err(e),
                    };

                    match v {
                        Ok(true) => Some(Ok(ctx.context_node)),
                        Ok(false) => None,
                        Err(e) => Some(Err(e)),
                    }
                } else {
                    Some(Err(Error::Internal {
                        msg: "[PredicateExpr] No predicate context set".to_string(),
                    }))
                }
            })
            .collect();

        Ok(Value::Nodeset(narrowed_nodes?))
    }

    fn is_primitive(&self) -> bool {
        self.expr.is_primitive()
    }
}

impl Evaluatable for FilterExpr {
    fn evaluate(&self, context: &EvaluationCtx) -> Result<Value, Error> {
        let primary_result = self.primary.evaluate(context)?;
        let have_predicates = !self.predicates.predicates.is_empty();

        match (have_predicates, &primary_result) {
            (false, _) => {
                trace!(
                    "[FilterExpr] No predicates, returning primary result: {:?}",
                    primary_result
                );
                Ok(primary_result)
            },
            (true, Value::Nodeset(vec)) => {
                let predicate_list_subcontext =
                    context.update_predicate_nodes(vec.iter().map(|n| &**n).collect());
                let result_filtered_by_predicates =
                    self.predicates.evaluate(&predicate_list_subcontext);
                trace!(
                    "[FilterExpr] Result filtered by predicates: {:?}",
                    result_filtered_by_predicates
                );
                result_filtered_by_predicates
            },
            // You can't use filtering expressions `[]` on other than node-sets
            (true, _) => Err(Error::NotANodeset),
        }
    }

    fn is_primitive(&self) -> bool {
        self.predicates.predicates.is_empty() && self.primary.is_primitive()
    }
}

impl Evaluatable for PrimaryExpr {
    fn evaluate(&self, context: &EvaluationCtx) -> Result<Value, Error> {
        match self {
            PrimaryExpr::Literal(literal) => literal.evaluate(context),
            PrimaryExpr::Variable(_qname) => Err(Error::CannotUseVariables),
            PrimaryExpr::Parenthesized(expr) => expr.evaluate(context),
            PrimaryExpr::ContextItem => Ok(Value::Nodeset(vec![context.context_node.clone()])),
            PrimaryExpr::Function(core_function) => core_function.evaluate(context),
        }
    }

    fn is_primitive(&self) -> bool {
        match self {
            PrimaryExpr::Literal(_) => true,
            PrimaryExpr::Variable(_qname) => false,
            PrimaryExpr::Parenthesized(expr) => expr.is_primitive(),
            PrimaryExpr::ContextItem => false,
            PrimaryExpr::Function(_) => false,
        }
    }
}

impl Evaluatable for Literal {
    fn evaluate(&self, _context: &EvaluationCtx) -> Result<Value, Error> {
        match self {
            Literal::Numeric(numeric_literal) => match numeric_literal {
                // We currently make no difference between ints and floats
                NumericLiteral::Integer(v) => Ok(Value::Number(*v as f64)),
                NumericLiteral::Decimal(v) => Ok(Value::Number(*v)),
            },
            Literal::String(s) => Ok(Value::String(s.into())),
        }
    }

    fn is_primitive(&self) -> bool {
        true
    }
}