helios-fhirpath 0.2.0

//! # FHIRPath Expression Parser
//!
//! This module provides a complete parser for the FHIRPath expression language,
//! implementing the official FHIRPath grammar specification. It transforms FHIRPath
//! text into an Abstract Syntax Tree (AST) that can be evaluated against FHIR resources.
//!
//! ## Overview
//!
//! The parser handles all FHIRPath syntax elements including:
//!
//! - **Literals**: Numbers, strings, booleans, dates, times, quantities
//! - **Path navigation**: Member access and nested paths (e.g., `Patient.name.given`)
//! - **Function calls**: Both built-in and user-defined functions
//! - **Operators**: Arithmetic, logical, comparison, and type operators
//! - **Collections**: Collection literals and operations
//! - **Comments**: Single-line (`//`) and multi-line (`/* */`) comments
//!
//! ## Key Types
//!
//! - [`Expression`]: The main AST node type representing any FHIRPath expression
//! - [`Literal`]: Represents literal values (strings, numbers, dates, etc.)
//! - [`Term`]: Basic terms (literals, invocations, variables)
//! - [`Invocation`]: Function calls and member access
//! - [`TypeSpecifier`]: Type names for `is` and `as` operations
//!
//! ## Parser Function
//!
//! The main entry point is [`parser()`], which returns a parser that can process
//! FHIRPath text and produce an [`Expression`] AST.
//!
//! ## Examples
//!
//! ```rust
//! use helios_fhirpath::parser::parser;
//! use chumsky::Parser;
//!
//! # fn main() -> Result<(), Box<dyn std::error::Error>> {
//! // Parse a simple path expression
//! let expr = parser().parse("Patient.name.given").unwrap();
//!
//! // Parse a function call with filter
//! let expr = parser().parse("Patient.name.where(use = 'official')").unwrap();
//!
//! // Parse arithmetic expression
//! let expr = parser().parse("(value + 10) * 2").unwrap();
//!
//! // Parse with literal values
//! let expr = parser().parse("birthDate >= @1990-01-01").unwrap();
//! # Ok(())
//! # }
//! ```
//!
//! ## Grammar Implementation
//!
//! The parser implements the FHIRPath grammar with proper operator precedence:
//!
//! 1. **Literals and Terms**: Numbers, strings, identifiers, parenthesized expressions
//! 2. **Postfix Operators**: Member access (`.`), indexing (`[]`)
//! 3. **Prefix Operators**: Unary plus/minus (`+`, `-`)
//! 4. **Multiplicative**: `*`, `/`, `div`, `mod`
//! 5. **Additive**: `+`, `-`, `&` (string concatenation)
//! 6. **Type Operations**: `is`, `as`
//! 7. **Union**: `|` (collection union)
//! 8. **Inequality**: `<`, `<=`, `>`, `>=`
//! 9. **Equality**: `=`, `~`, `!=`, `!~`
//! 10. **Membership**: `in`, `contains`
//! 11. **Logical AND**: `and`
//! 12. **Logical OR/XOR**: `or`, `xor`
//! 13. **Implies**: `implies`
//!
//! ## Literal Syntax
//!
//! The parser supports rich literal syntax:
//!
//! - **Strings**: `'text with \'escapes\''`
//! - **Numbers**: `42` (integer), `3.14` (decimal)
//! - **Booleans**: `true`, `false`
//! - **Dates**: `@2024-01-15`, `@2024-01`, `@2024`
//! - **DateTimes**: `@2024-01-15T14:30:00Z`, `@2024-01-15T14:30:00-05:00`
//! - **Times**: `@T14:30:00`, `@T14:30:00.123`
//! - **Quantities**: `5 'mg'`, `10.5 'cm'`, `3 days`
//! - **Empty**: `{}`
//!
//! ## Comments
//!
//! The parser supports comments that are ignored during parsing:
//!
//! ```fhirpath
//! Patient.name.given  // Get first name
//! /* Multi-line comment
//!    spanning multiple lines */
//! ```
//!
//! ## Error Handling
//!
//! Parse errors include detailed information about the location and nature of
//! syntax errors, making it easy to identify and fix issues in FHIRPath expressions.

use chumsky::Parser;
use chumsky::error::Rich;
use chumsky::prelude::*;
use rust_decimal::Decimal;
use std::fmt;
use std::str::FromStr;

/// Represents a literal value in FHIRPath
///
/// This enum represents all the different types of literal values that can appear
/// in a FHIRPath expression, including:
/// - Empty value (`{}`)
/// - Boolean values (true/false)
/// - String literals (e.g., 'text')
/// - Numeric values (integers and decimals)
/// - Date/time literals (date, datetime, time)
/// - Quantity values (numeric values with units)
///
/// These literals are used in the abstract syntax tree (AST) produced by the parser
/// and are later evaluated into concrete values during expression evaluation.
#[derive(Debug, Clone, PartialEq)]
pub enum Literal {
    /// The empty value, represented as `{}` in FHIRPath
    Null,
    /// Boolean true/false values
    Boolean(bool),
    /// String literals enclosed in single quotes
    String(String),
    /// Decimal numbers (with a decimal point)
    Number(Decimal),
    /// Integer numbers (without a decimal point)
    Integer(i64),
    /// Date literals, starting with @, such as @2022-01-01
    Date(helios_fhir::PrecisionDate),
    /// DateTime literals with optional time and timezone parts
    DateTime(helios_fhir::PrecisionDateTime),
    /// Time literals, starting with @T, such as @T12:00:00
    Time(helios_fhir::PrecisionTime),
    /// Quantity values with a numeric value and a unit, such as 5 'mg'
    Quantity(Decimal, String),
}

/// Represents a FHIRPath expression
///
/// This enum represents the different kinds of expressions that can appear
/// in a FHIRPath expression tree. It forms the core of the abstract syntax tree (AST)
/// produced by the parser. Each variant corresponds to a different type of expression
/// in the FHIRPath language, including basic terms, operators, and function invocations.
///
/// The Expression tree is built during parsing and later evaluated by the evaluator
/// to produce a result value. The structure preserves operator precedence and
/// expression nesting as specified in the FHIRPath grammar.
#[derive(Debug, Clone, PartialEq)]
pub enum Expression {
    /// A basic term (literal, invocation, etc.)
    Term(Term),

    /// A method or function invocation on an expression
    /// (e.g., `Patient.name.given.first()`)
    Invocation(Box<Expression>, Invocation),

    /// An indexer expression (e.g., `Patient.name[0]`)
    Indexer(Box<Expression>, Box<Expression>),

    /// A unary polarity expression (+ or -)
    /// (e.g., `-5` or `+value`)
    Polarity(char, Box<Expression>),

    /// A multiplicative expression (*, /, div, mod)
    /// (e.g., `value * 2` or `amount div 10`)
    Multiplicative(Box<Expression>, String, Box<Expression>),

    /// An additive expression (+ or -)
    /// (e.g., `value + 5` or `total - tax`)
    Additive(Box<Expression>, String, Box<Expression>),

    /// A type operation (is, as)
    /// (e.g., `value is Integer` or `patient as Patient`)
    Type(Box<Expression>, String, TypeSpecifier),

    /// A union operation (|)
    /// (e.g., `Patient.name | Patient.address`)
    Union(Box<Expression>, Box<Expression>),

    /// An inequality comparison (<, <=, >, >=)
    /// (e.g., `value > 5` or `date <= today()`)
    Inequality(Box<Expression>, String, Box<Expression>),

    /// An equality comparison (=, !=, ~, !~)
    /// (e.g., `name = 'John'` or `birthDate ~ @2020`)
    Equality(Box<Expression>, String, Box<Expression>),

    /// A membership test (in, contains)
    /// (e.g., `'John' in Patient.name.given` or `Patient.name contains 'John'`)
    Membership(Box<Expression>, String, Box<Expression>),

    /// A logical AND operation
    /// (e.g., `value > 5 and value < 10`)
    And(Box<Expression>, Box<Expression>),

    /// A logical OR or XOR operation
    /// (e.g., `status = 'active' or status = 'pending'`)
    Or(Box<Expression>, String, Box<Expression>),

    /// A logical IMPLIES operation
    /// (e.g., `exists() implies value > 0`)
    Implies(Box<Expression>, Box<Expression>),

    /// A lambda expression with optional identifier
    /// (e.g., `item => item.value > 10`)
    Lambda(Option<String>, Box<Expression>),

    /// An instance selector expression
    /// (e.g., `Quantity { value: 5, unit: 'mg' }`)
    /// Contains the type name and a list of field name/expression pairs.
    InstanceSelector(String, Vec<(String, Box<Expression>)>),
}

/// Represents a type specifier in FHIRPath
///
/// This enum is used to represent types in type operations like 'is' and 'as'.
/// It supports both simple types and namespace-qualified types as defined in the
/// FHIRPath specification.
///
/// Type specifiers are used in expressions like:
/// - `value is Integer`
/// - `patient is FHIR.Patient`
/// - `value as System.Decimal`
///
/// The parser determines whether an identifier is a simple type name or a
/// namespace-qualified type name based on the presence of a dot separator.
#[derive(Debug, Clone, PartialEq)]
pub enum TypeSpecifier {
    /// A qualified identifier representing a type, possibly with a namespace
    ///
    /// The first String is either:
    /// - The namespace (when `Option<String>` is Some), or
    /// - The type name (when `Option<String>` is None)
    ///
    /// The `Option<String>` is:
    /// - Some(type_name) when a namespace is provided, or
    /// - None when it's a simple type without a namespace
    ///
    /// Examples:
    /// - FHIR.Patient -> QualifiedIdentifier("FHIR", Some("Patient"))
    /// - Boolean -> QualifiedIdentifier("Boolean", None)
    /// - System.Boolean -> QualifiedIdentifier("System", Some("Boolean"))
    QualifiedIdentifier(String, Option<String>),
}

/// Represents a basic term in a FHIRPath expression
///
/// A term is the most fundamental unit in a FHIRPath expression.
/// It can be a literal value, an invocation, a variable reference,
/// or a parenthesized expression. Terms are the leaves of the expression
/// tree in the abstract syntax tree (AST).
///
/// Terms can appear alone or as part of more complex expressions,
/// and they are the starting point for expression evaluation.
#[derive(Debug, Clone, PartialEq)]
pub enum Term {
    /// An invocation, such as a member access, function call, or special identifier
    /// (e.g., `name`, `first()`, `$this`)
    Invocation(Invocation),

    /// A literal value like a number, string, boolean, or date
    /// (e.g., `42`, `'text'`, `true`, `@2022-01-01`)
    Literal(Literal),

    /// An external constant or environment variable reference
    /// (e.g., `%context`, `%ucum`, `%terminologies`)
    ExternalConstant(String),

    /// A parenthesized expression
    /// (e.g., `(1 + 2)`, `(Patient.name)`)
    Parenthesized(Box<Expression>),
}

/// Represents an invocation in a FHIRPath expression
///
/// An invocation represents different ways to reference or call something in FHIRPath.
/// This includes member access, function calls, and special contextual identifiers
/// like $this, $index, and $total.
///
/// Invocations are fundamental building blocks in FHIRPath expressions and
/// are used for navigation, function application, and context references.
#[derive(Debug, Clone, PartialEq)]
pub enum Invocation {
    /// A member access, referencing a property by name
    /// (e.g., `Patient.name`, `Observation.value`)
    Member(String),

    /// A function call with optional arguments
    /// (e.g., `first()`, `where(value > 5)`, `substring(2, 5)`)
    Function(String, Vec<Expression>),

    /// A reference to the current focus item ($this)
    /// Used in expressions like `$this.name` or in lambda expressions
    This,

    /// A reference to the current index ($index)
    /// Used in expressions like `$index > 5` in filtering operations
    Index,

    /// A reference to the current aggregate total ($total)
    /// Used in the aggregate() function to access the running total
    Total,
}

// Removed Unit, DateTimePrecision, PluralDateTimePrecision enums

/// Span information: (start_position, length) in the source text
#[derive(Debug, Clone, PartialEq)]
pub struct ExprSpan {
    pub position: usize,
    pub length: usize,
}

/// An Expression node with span information attached
#[derive(Debug, Clone)]
pub struct SpannedExpression {
    pub kind: SpannedExprKind,
    pub span: ExprSpan,
}

/// Mirrors Expression variants but children are Box<SpannedExpression>
#[derive(Debug, Clone)]
pub enum SpannedExprKind {
    Term(SpannedTerm),
    Invocation(Box<SpannedExpression>, SpannedInvocation),
    Indexer(Box<SpannedExpression>, Box<SpannedExpression>),
    Polarity(char, Box<SpannedExpression>),
    Multiplicative(Box<SpannedExpression>, String, Box<SpannedExpression>),
    Additive(Box<SpannedExpression>, String, Box<SpannedExpression>),
    Type(Box<SpannedExpression>, String, TypeSpecifier),
    Union(Box<SpannedExpression>, Box<SpannedExpression>),
    Inequality(Box<SpannedExpression>, String, Box<SpannedExpression>),
    Equality(Box<SpannedExpression>, String, Box<SpannedExpression>),
    Membership(Box<SpannedExpression>, String, Box<SpannedExpression>),
    And(Box<SpannedExpression>, Box<SpannedExpression>),
    Or(Box<SpannedExpression>, String, Box<SpannedExpression>),
    Implies(Box<SpannedExpression>, Box<SpannedExpression>),
    Lambda(Option<String>, Box<SpannedExpression>),
    InstanceSelector(String, Vec<(String, Box<SpannedExpression>)>),
}

#[derive(Debug, Clone)]
pub enum SpannedTerm {
    Literal(Literal),
    Invocation(SpannedInvocation),
    ExternalConstant(String),
    Parenthesized(Box<SpannedExpression>),
}

#[derive(Debug, Clone)]
pub enum SpannedInvocation {
    Member(String),
    Function(String, Vec<SpannedExpression>),
    This,
    Index,
    Total,
}

impl SpannedExpression {
    pub fn to_expression(&self) -> Expression {
        match &self.kind {
            SpannedExprKind::Term(t) => Expression::Term(t.to_term()),
            SpannedExprKind::Invocation(base, inv) => {
                Expression::Invocation(Box::new(base.to_expression()), inv.to_invocation())
            }
            SpannedExprKind::Indexer(expr, idx) => Expression::Indexer(
                Box::new(expr.to_expression()),
                Box::new(idx.to_expression()),
            ),
            SpannedExprKind::Polarity(op, expr) => {
                Expression::Polarity(*op, Box::new(expr.to_expression()))
            }
            SpannedExprKind::Multiplicative(l, op, r) => Expression::Multiplicative(
                Box::new(l.to_expression()),
                op.clone(),
                Box::new(r.to_expression()),
            ),
            SpannedExprKind::Additive(l, op, r) => Expression::Additive(
                Box::new(l.to_expression()),
                op.clone(),
                Box::new(r.to_expression()),
            ),
            SpannedExprKind::Type(expr, op, ts) => {
                Expression::Type(Box::new(expr.to_expression()), op.clone(), ts.clone())
            }
            SpannedExprKind::Union(l, r) => {
                Expression::Union(Box::new(l.to_expression()), Box::new(r.to_expression()))
            }
            SpannedExprKind::Inequality(l, op, r) => Expression::Inequality(
                Box::new(l.to_expression()),
                op.clone(),
                Box::new(r.to_expression()),
            ),
            SpannedExprKind::Equality(l, op, r) => Expression::Equality(
                Box::new(l.to_expression()),
                op.clone(),
                Box::new(r.to_expression()),
            ),
            SpannedExprKind::Membership(l, op, r) => Expression::Membership(
                Box::new(l.to_expression()),
                op.clone(),
                Box::new(r.to_expression()),
            ),
            SpannedExprKind::And(l, r) => {
                Expression::And(Box::new(l.to_expression()), Box::new(r.to_expression()))
            }
            SpannedExprKind::Or(l, op, r) => Expression::Or(
                Box::new(l.to_expression()),
                op.clone(),
                Box::new(r.to_expression()),
            ),
            SpannedExprKind::Implies(l, r) => {
                Expression::Implies(Box::new(l.to_expression()), Box::new(r.to_expression()))
            }
            SpannedExprKind::Lambda(param, expr) => {
                Expression::Lambda(param.clone(), Box::new(expr.to_expression()))
            }
            SpannedExprKind::InstanceSelector(type_name, fields) => Expression::InstanceSelector(
                type_name.clone(),
                fields
                    .iter()
                    .map(|(name, expr)| (name.clone(), Box::new(expr.to_expression())))
                    .collect(),
            ),
        }
    }
}

impl SpannedTerm {
    pub fn to_term(&self) -> Term {
        match self {
            SpannedTerm::Literal(l) => Term::Literal(l.clone()),
            SpannedTerm::Invocation(i) => Term::Invocation(i.to_invocation()),
            SpannedTerm::ExternalConstant(s) => Term::ExternalConstant(s.clone()),
            SpannedTerm::Parenthesized(e) => Term::Parenthesized(Box::new(e.to_expression())),
        }
    }
}

impl SpannedInvocation {
    pub fn to_invocation(&self) -> Invocation {
        match self {
            SpannedInvocation::Member(s) => Invocation::Member(s.clone()),
            SpannedInvocation::Function(name, args) => Invocation::Function(
                name.clone(),
                args.iter().map(|a| a.to_expression()).collect(),
            ),
            SpannedInvocation::This => Invocation::This,
            SpannedInvocation::Index => Invocation::Index,
            SpannedInvocation::Total => Invocation::Total,
        }
    }
}

impl fmt::Display for Literal {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match self {
            Literal::Null => write!(f, "{{}}"),
            Literal::Boolean(b) => write!(f, "{}", b),
            Literal::String(s) => write!(f, "'{}'", s),
            Literal::Number(d) => write!(f, "{}", d), // Use Decimal's Display
            Literal::Integer(n) => write!(f, "{}", n),
            Literal::Date(d) => write!(f, "@{}", d.original_string()),
            Literal::DateTime(dt) => write!(f, "@{}", dt.original_string()),
            Literal::Time(t) => write!(f, "@T{}", t.original_string()),
            Literal::Quantity(d, u) => write!(f, "{} '{}'", d, u), // Use Decimal's Display and unit string
        }
    }
}

/// Helper function to remove backticks from identifiers if present
fn clean_backtick_identifier(id: &str) -> String {
    if id.starts_with('`') && id.ends_with('`') && id.len() >= 3 {
        id[1..id.len() - 1].to_string()
    } else {
        id.to_string()
    }
}

/// Combines a sequence of code units (from literal scalars and `\uXXXX` escapes)
/// into a FHIRPath string. Adjacent high/low UTF-16 surrogate escapes are joined
/// into a single Unicode scalar value; unpaired surrogates are rejected. This
/// matches the FHIRPath string-literal rules (FHIR-53554).
fn combine_string_code_units(codes: Vec<u32>) -> Result<String, &'static str> {
    let mut out = String::with_capacity(codes.len());
    let mut i = 0;
    while i < codes.len() {
        let c = codes[i];
        if (0xD800..=0xDBFF).contains(&c) {
            if let Some(&lo) = codes.get(i + 1) {
                if (0xDC00..=0xDFFF).contains(&lo) {
                    let scalar = 0x10000 + ((c - 0xD800) << 10) + (lo - 0xDC00);
                    match char::from_u32(scalar) {
                        Some(ch) => {
                            out.push(ch);
                            i += 2;
                            continue;
                        }
                        None => return Err("Invalid surrogate pair"),
                    }
                }
            }
            return Err("Unpaired high surrogate in \\uXXXX escape");
        }
        if (0xDC00..=0xDFFF).contains(&c) {
            return Err("Unpaired low surrogate in \\uXXXX escape");
        }
        match char::from_u32(c) {
            Some(ch) => out.push(ch),
            None => return Err("Invalid Unicode code point"),
        }
        i += 1;
    }
    Ok(out)
}

/// Creates a parser for FHIRPath expressions
///
/// This function creates and returns a parser that can parse FHIRPath expressions
/// according to the official FHIRPath grammar specification. The parser uses the
/// chumsky parsing library to implement a recursive descent parser with proper
/// handling of operator precedence and associativity.
///
/// The parser handles all FHIRPath syntax elements including:
/// - Literals (numbers, strings, dates, times, etc.)
/// - Path navigation and member access
/// - Function invocation
/// - Mathematical operations
/// - Logical operations
/// - Comparison and equality tests
/// - Collection operators
/// - Type testing operations
///
/// # Returns
///
/// A parser that can consume a string of characters and produce an Expression
/// representing the abstract syntax tree (AST) of the parsed FHIRPath expression.
///
/// # Errors
///
/// The parser returns detailed error information when it encounters syntax errors
/// in the input, including the location and nature of the error.
/// Parser that matches a custom whitespace including comments  
fn custom_padded<'src, T, P>(
    parser: P,
) -> impl Parser<'src, &'src str, T, extra::Err<Rich<'src, char>>> + Clone
where
    P: Parser<'src, &'src str, T, extra::Err<Rich<'src, char>>> + Clone,
    T: Clone,
{
    // First consume any leading whitespace/comments
    let ws_or_comment = choice((
        // Regular whitespace
        text::whitespace().at_least(1).ignored(),
        // Single-line comment: // ... newline or EOF
        just("//")
            .then(any().and_is(text::newline().or(end()).not()).repeated())
            .ignored(),
        // Multi-line comment: /* ... */
        just("/*")
            .then(any().and_is(just("*/").not()).repeated())
            .then(just("*/"))
            .ignored(),
    ))
    .repeated()
    .ignored();

    ws_or_comment
        .then(parser)
        .map(|(_, result)| result)
        .then_ignore(ws_or_comment)
}

pub fn parser<'src>()
-> impl Parser<'src, &'src str, Expression, extra::Err<Rich<'src, char>>> + Clone + 'src {
    // Parser for escape sequences within string literals. Produces a raw
    // code unit (u32) so that `\uXXXX` UTF-16 surrogate escapes can be paired
    // into a single scalar value by `combine_string_code_units`.
    let esc = just('\\').ignore_then(choice((
        just('`').to('`' as u32),
        just('\'').to('\'' as u32),
        just('\\').to('\\' as u32),
        just('/').to('/' as u32),
        just('f').to(0x000C_u32),
        just('n').to('\n' as u32),
        just('r').to('\r' as u32),
        just('t').to('\t' as u32),
        just('"').to('"' as u32),
        // Unicode escape sequence: \uXXXX. May be a surrogate half that will
        // be paired with a following escape in `combine_string_code_units`.
        just('u').ignore_then(
            any()
                .filter(|c: &char| c.is_ascii_hexdigit())
                .repeated()
                .exactly(4)
                .collect::<String>()
                .try_map(
                    |digits: String, span| match u32::from_str_radix(&digits, 16) {
                        Ok(code) => Ok(code),
                        Err(_) => Err(Rich::custom(span, "Invalid hex digits")),
                    },
                ),
        ),
    )));

    // Helper macro to make a parser skip whitespace and comments
    macro_rules! padded {
        ($p:expr) => {
            custom_padded($p)
        };
    }

    // LITERAL PARSERS

    // Parser for null/empty literals: {}
    // In FHIRPath, the empty collection is represented as {}
    let null = just('{').then(just('}')).to(Literal::Null);

    // Parser for boolean literals: true, false
    // Note: These need to be parsed before identifiers to avoid ambiguity
    let boolean = choice((
        text::keyword("true").to(Literal::Boolean(true)),
        text::keyword("false").to(Literal::Boolean(false)),
    ))
    .boxed();

    // Parser for string literals: 'text'
    // Handles escape sequences and allows any characters between single quotes
    let string = just('\'')
        .ignore_then(
            none_of("\\\'")
                .map(|c: char| c as u32)
                .or(esc)
                .repeated()
                .collect::<Vec<u32>>(),
        )
        .then_ignore(just('\''))
        .try_map(|codes, span| combine_string_code_units(codes).map_err(|e| Rich::custom(span, e)))
        .map(Literal::String)
        .boxed();

    // Parser for integer literals
    //
    // Parses sequences of digits without a decimal point into an i64 value.
    // The FHIRPath specification defines integers as 64-bit signed values.
    // This parser validates that the integer is within the valid range.
    let integer = any()
        .filter(|c: &char| c.is_ascii_digit())
        .repeated()
        .at_least(1) // Require at least one digit
        .collect::<String>()
        .try_map(|digits: String, span| match i64::from_str(&digits) {
            Ok(n) => Ok(Literal::Integer(n)),
            Err(_) => Err(Rich::custom(span, format!("Invalid integer: {}", digits))),
        });
    let integer = padded!(integer); // Allow whitespace around integers

    // Parser for decimal number literals
    //
    // Parses numbers with a decimal point into a Decimal value.
    // The FHIRPath specification uses arbitrary precision decimal values,
    // represented here using the rust_decimal crate's Decimal type.
    //
    // Format: <digits>.<digits>
    // Example: 3.14159
    let number = any()
        .filter(|c: &char| c.is_ascii_digit())
        .repeated()
        .at_least(1) // Require at least one digit before the decimal
        .collect::<String>()
        .then(just('.')) // Require the decimal point
        .then(
            any()
                .filter(|c: &char| c.is_ascii_digit())
                .repeated()
                .at_least(1) // Require at least one digit after the decimal
                .collect::<String>(),
        )
        .try_map(|((i, _), d), span| {
            let num_str = format!("{}.{}", i, d);
            match Decimal::from_str(&num_str) {
                Ok(decimal) => Ok(Literal::Number(decimal)),
                Err(_) => Err(Rich::custom(span, format!("Invalid number: {}", num_str))),
            }
        })
        .padded(); // Allow whitespace around numbers

    // Parser for time format components
    //
    // Handles the FHIRPath time format: HH(:mm(:ss(.sss)?)?)?
    // This can be as simple as just hours (HH) or include minutes,
    // seconds, and milliseconds with the appropriate separators.
    //
    // Examples:
    // - 12 (just hours)
    // - 14:30 (hours and minutes)
    // - 09:45:30 (hours, minutes, seconds)
    // - 23:59:59.999 (hours, minutes, seconds, and milliseconds)
    let time_format = any()
        .filter(|c: &char| c.is_ascii_digit())
        .repeated()
        .at_least(2) // Hours: exactly 2 digits
        .at_most(2)
        .collect::<String>()
        .then(
            just(':') // Optional minutes part
                .ignore_then(
                    any()
                        .filter(|c: &char| c.is_ascii_digit())
                        .repeated()
                        .at_least(2) // Minutes: exactly 2 digits
                        .at_most(2)
                        .collect::<String>(),
                )
                .then(
                    just(':') // Optional seconds part
                        .ignore_then(
                            any()
                                .filter(|c: &char| c.is_ascii_digit())
                                .repeated()
                                .at_least(2) // Seconds: exactly 2 digits
                                .at_most(2)
                                .collect::<String>(),
                        )
                        .then(
                            just('.') // Optional milliseconds part
                                .ignore_then(
                                    any()
                                        .filter(|c: &char| c.is_ascii_digit())
                                        .repeated()
                                        .at_least(1) // Milliseconds: 1-3 digits
                                        .at_most(3)
                                        .collect::<String>(),
                                )
                                .or_not(),
                        )
                        .or_not(),
                )
                .or_not(),
        )
        .map(|(hours, rest_opt)| {
            // Combine all the parts into a single time string
            let mut result = hours;
            if let Some((minutes, seconds_part)) = rest_opt {
                result.push(':');
                result.push_str(&minutes);

                if let Some((seconds, milliseconds)) = seconds_part {
                    result.push(':');
                    result.push_str(&seconds);

                    // milliseconds is an Option<String>
                    if let Some(ms) = milliseconds {
                        result.push('.');
                        result.push_str(&ms);
                    }
                }
            }
            result
        });

    // Parser for timezone format
    //
    // Handles the two timezone formats defined in FHIRPath:
    // - 'Z' for UTC/Zulu time
    // - (+|-)HH:mm for timezone offset (e.g., +01:00, -05:30)
    //
    // This parser validates the format and produces a string
    // representation of the timezone.
    let timezone_format = just('Z')
        .to("Z".to_string()) // UTC/Zulu time
        .or(one_of("+-") // Or timezone offset
            .map(|c: char| c.to_string()) // Get sign as string
            .then(
                any()
                    .filter(|c: &char| c.is_ascii_digit())
                    .repeated()
                    .at_most(2) // Hours: exactly 2 digits
                    .at_least(2)
                    .collect::<String>(),
            )
            .then(just(':')) // Colon separator
            .then(
                any()
                    .filter(|c: &char| c.is_ascii_digit())
                    .repeated()
                    .at_most(2) // Minutes: exactly 2 digits
                    .at_least(2)
                    .collect::<String>(),
            )
            .map(|(((sign, hour), _), min)| format!("{}{}:{}", sign, hour, min)));

    // Parser for date format
    //
    // Handles the FHIRPath date format: YYYY(-MM(-DD)?)?
    // This parser supports multiple date precisions:
    // - Year only: YYYY (e.g., 2022)
    // - Year and month: YYYY-MM (e.g., 2022-01)
    // - Full date: YYYY-MM-DD (e.g., 2022-01-15)
    //
    // The parser validates the format and produces a string representation
    // of the date for use in Date literals.
    //
    // Examples:
    //
    // - 1972 (year only)
    // - 2015-12 (year and month)
    // - 1972-12-14 (full date)
    let date_format_str = any()
        .filter(|c: &char| c.is_ascii_digit())
        .repeated()
        .exactly(4) // Year: exactly 4 digits
        .collect::<String>()
        .then(
            just('-') // Optional month part
                .ignore_then(
                    any()
                        .filter(|c: &char| c.is_ascii_digit())
                        .repeated()
                        .exactly(2) // Month: exactly 2 digits
                        .collect::<String>()
                        .then(
                            just('-') // Optional day part
                                .ignore_then(
                                    any()
                                        .filter(|c: &char| c.is_ascii_digit())
                                        .repeated()
                                        .exactly(2) // Day: exactly 2 digits
                                        .collect::<String>(),
                                )
                                .or_not(),
                        ),
                )
                .or_not(),
        )
        .map(|(year, month_part)| {
            // Combine all the parts into a single date string
            let mut date_str = year;

            // month_part is Option<(month_str, Option<day_str>)>
            if let Some((month_str, day_part)) = month_part {
                date_str.push('-');
                date_str.push_str(&month_str);

                // day_part is Option<day_str>
                if let Some(day_str) = day_part {
                    date_str.push('-');
                    date_str.push_str(&day_str);
                }
            }

            date_str // Returns String
        })
        .boxed();

    // Parser for unit values in quantity literals
    //
    // Units in FHIRPath can be specified either as predefined time unit keywords
    // or as arbitrary string literals enclosed in single quotes.
    //
    // This parser handles both forms:
    // - Time unit keywords (year, month, day, hour, minute, second, etc.)
    // - String literal units ('mg', 'kg', 'cm', etc.)
    //
    // The parser returns the unit as a String regardless of which form was used.

    // Parser for time unit keywords
    // These are the predefined time unit keywords in FHIRPath
    let unit_keyword = choice((
        // Singular forms
        text::keyword("year").to("year".to_string()),
        text::keyword("month").to("month".to_string()),
        text::keyword("week").to("week".to_string()),
        text::keyword("day").to("day".to_string()),
        text::keyword("hour").to("hour".to_string()),
        text::keyword("minute").to("minute".to_string()),
        text::keyword("second").to("second".to_string()),
        text::keyword("millisecond").to("millisecond".to_string()),
        // Plural forms
        text::keyword("years").to("years".to_string()),
        text::keyword("months").to("months".to_string()),
        text::keyword("weeks").to("weeks".to_string()),
        text::keyword("days").to("days".to_string()),
        text::keyword("hours").to("hours".to_string()),
        text::keyword("minutes").to("minutes".to_string()),
        text::keyword("seconds").to("seconds".to_string()),
        text::keyword("milliseconds").to("milliseconds".to_string()),
    ));

    // Parser for string literal units
    // These are arbitrary units enclosed in single quotes
    let unit_string_literal = just('\'')
        .ignore_then(
            none_of("\\\'")
                .map(|c: char| c as u32)
                .or(esc)
                .repeated()
                .collect::<Vec<u32>>(),
        )
        .then_ignore(just('\''))
        .try_map(|codes, span| combine_string_code_units(codes).map_err(|e| Rich::custom(span, e)));

    // Combined parser for all unit forms
    let unit = choice((
        unit_keyword,        // Time unit keywords
        unit_string_literal, // String literal units
    ))
    .boxed() // Box for recursive definitions
    .padded(); // Allow whitespace around units

    // Define integer/number parsers specifically for quantity, without consuming trailing whitespace.
    let integer_for_quantity = any()
        .filter(|c: &char| c.is_ascii_digit())
        .repeated()
        .at_least(1)
        .collect::<String>()
        .try_map(|digits: String, span| match i64::from_str(&digits) {
            Ok(n) => Ok(n), // Return the i64 directly
            Err(_) => Err(Rich::custom(span, format!("Invalid integer: {}", digits))),
        });

    let number_for_quantity = any()
        .filter(|c: &char| c.is_ascii_digit())
        .repeated()
        .at_least(1)
        .collect::<String>()
        .then(just('.'))
        .then(
            any()
                .filter(|c: &char| c.is_ascii_digit())
                .repeated()
                .at_least(1)
                .collect::<String>(),
        )
        .try_map(|((i, _), d), span| {
            let num_str = format!("{}.{}", i, d);
            match Decimal::from_str(&num_str) {
                Ok(decimal) => Ok(decimal), // Return the Decimal directly
                Err(_) => Err(Rich::custom(span, format!("Invalid number: {}", num_str))),
            }
        });

    // Quantity parser: (integer_for_quantity | number_for_quantity) + required whitespace + unit
    // This parser consumes the whole quantity structure.
    let quantity = choice((
        // Try integer quantity first
        integer_for_quantity
            .then_ignore(text::whitespace().at_least(1)) // Require whitespace
            .then(unit.clone()) // Parse the unit string
            .map(|(i, u_str)| Literal::Quantity(Decimal::from(i), u_str)), // Create Literal::Quantity with Decimal and String unit
        // Then try decimal quantity
        number_for_quantity
            .then_ignore(text::whitespace().at_least(1)) // Require whitespace
            .then(unit.clone()) // Parse the unit string
            .map(|(d, u_str)| Literal::Quantity(d, u_str)), // Create Literal::Quantity with Decimal and String unit
    ));

    // Removed unused emit_error helper function

    // Parser for DateTime: @Date T Time [Timezone]
    let datetime_literal = just('@')
        .ignore_then(date_format_str.clone())
        .then_ignore(just('T'))
        .then(time_format)
        .then(timezone_format.clone().or_not())
        .try_map(|((date_str, time_str), tz_opt), span| {
            let full_str = if let Some(tz) = tz_opt {
                format!("{}T{}{}", date_str, time_str, tz)
            } else {
                format!("{}T{}", date_str, time_str)
            };

            helios_fhir::PrecisionDateTime::parse(&full_str)
                .ok_or_else(|| Rich::custom(span, format!("Invalid datetime format: {}", full_str)))
                .map(Literal::DateTime)
        });

    // Parser for Partial DateTime: @Date T
    let partial_datetime_literal = just('@')
        .ignore_then(date_format_str.clone())
        .then_ignore(just('T'))
        .try_map(|date_str, span| {
            let full_str = format!("{}T", date_str);
            helios_fhir::PrecisionDateTime::parse(&full_str)
                .ok_or_else(|| {
                    Rich::custom(
                        span,
                        format!("Invalid partial datetime format: {}", full_str),
                    )
                })
                .map(Literal::DateTime)
        });

    // Parser for Time: @ T Time (strictly no timezone)
    // Uses try_map to fail parsing if a timezone is present.
    let time_literal = just('@')
        .ignore_then(
            just('T')
                .ignore_then(time_format)
                .then(timezone_format.or_not()), // Parse time and optional timezone
        )
        .try_map(|(time_str, tz_opt), span| {
            // Validate that timezone is not present
            if tz_opt.is_some() {
                Err(Rich::custom(
                    span,
                    "Time literal cannot have a timezone offset",
                ))
            } else {
                helios_fhir::PrecisionTime::parse(&time_str)
                    .ok_or_else(|| Rich::custom(span, format!("Invalid time format: {}", time_str)))
                    .map(Literal::Time)
            }
        });

    // Parser for Date: @ Date
    let date_literal = just('@')
        .ignore_then(date_format_str.clone())
        .try_map(|date_str, span| {
            helios_fhir::PrecisionDate::parse(&date_str)
                .ok_or_else(|| Rich::custom(span, format!("Invalid date format: {}", date_str)))
                .map(Literal::Date)
        });

    // Order matters: try quantity before plain number/integer.
    // Specific date/time formats should be tried before more general ones if there's ambiguity,
    // though the new structure aims to make them distinct.
    let literal = choice((
        null,
        boolean,
        string,
        quantity,                          // Try quantity first
        number,                            // Then number (requires '.')
        integer,                           // Then integer
        padded!(datetime_literal),         // @Date T Time [TZ]
        padded!(partial_datetime_literal), // @Date T
        padded!(time_literal),             // @ T Time (will fail if TZ present)
        padded!(date_literal),             // @Date
    ))
    .map(Term::Literal);

    // IDENTIFIER: ([A-Za-z] | '_')([A-Za-z0-9] | '_')*
    let standard_identifier = any()
        .filter(|c: &char| c.is_ascii_alphabetic() || *c == '_')
        .then(
            any()
                .filter(|c: &char| c.is_ascii_alphanumeric() || *c == '_')
                .repeated()
                .collect::<Vec<_>>(),
        )
        .map(|(first, rest): (char, Vec<char>)| {
            let mut s = first.to_string();
            s.extend(rest);
            s
        })
        .padded();

    // DELIMITEDIDENTIFIER: '`' (ESC | .)*? '`'
    let delimited_identifier = just('`')
        .ignore_then(
            none_of("`")
                .map(|c: char| c as u32)
                .or(esc)
                .repeated()
                .collect::<Vec<u32>>(),
        )
        .then_ignore(just('`'))
        .try_map(|codes, span| combine_string_code_units(codes).map_err(|e| Rich::custom(span, e)))
        .padded();

    // Combined identifier parser - allow true/false as identifiers
    // Also allow keywords used in specific contexts (like 'as', 'is') to be parsed as identifiers
    // when they appear where an identifier is expected (e.g., in function calls or member access).
    // The context of the grammar will differentiate their use.
    let identifier = choice((
        standard_identifier,
        delimited_identifier,
        // Allow keywords to be parsed as identifiers if they appear in identifier positions
        text::keyword("as").to(String::from("as")),
        text::keyword("contains").to(String::from("contains")),
        text::keyword("in").to(String::from("in")),
        text::keyword("is").to(String::from("is")),
        text::keyword("true").to(String::from("true")), // Allow 'true' as identifier
        text::keyword("false").to(String::from("false")), // Allow 'false' as identifier
                                                        // Add other keywords if they can appear as identifiers in some contexts
    ));

    // Qualified identifier (for type specifiers)
    // Handles all these patterns:
    // - Single identifier: Boolean, Patient, etc.
    // - Namespace.Type: System.Boolean, FHIR.Patient
    // - Backtick quoted: `System`.`Boolean`, FHIR.`Patient`
    let qualified_identifier = {
        // First try to handle explicit namespace.type pattern
        let explicit_namespace_type = identifier
            .clone()
            .then(just('.').ignore_then(identifier.clone()))
            .map(|(namespace, type_name)| {
                // Clean both parts (removing backticks if present)
                let clean_ns = clean_backtick_identifier(&namespace);
                let clean_type = clean_backtick_identifier(&type_name);
                TypeSpecifier::QualifiedIdentifier(clean_ns, Some(clean_type))
            });

        // Then handle standalone identifiers (which might themselves contain dots)
        let standalone_type = identifier.clone().map(|id| {
            let clean_id = clean_backtick_identifier(&id);

            // Check if this identifier already contains dots (like "System.Boolean")
            if clean_id.contains('.') {
                // This might be a pre-qualified identifier typed directly
                // Split at the last dot to get namespace and type
                if let Some(last_dot_pos) = clean_id.rfind('.') {
                    let namespace = clean_id[..last_dot_pos].to_string();
                    let type_name = clean_id[last_dot_pos + 1..].to_string();
                    TypeSpecifier::QualifiedIdentifier(namespace, Some(type_name))
                } else {
                    // Shouldn't happen if contains('.') returned true, but just in case
                    TypeSpecifier::QualifiedIdentifier(clean_id, None)
                }
            } else {
                // Simple unqualified type name
                TypeSpecifier::QualifiedIdentifier(clean_id, None)
            }
        });

        // Try explicit namespace.type first, then fallback to standalone identifier
        choice((explicit_namespace_type.boxed(), standalone_type.boxed())).boxed()
    };
    let qualified_identifier = padded!(qualified_identifier);

    // Create a separate string parser for external constants
    let string_for_external = just('\'')
        .ignore_then(
            none_of("\'\\")
                .map(|c: char| c as u32)
                .or(esc)
                .repeated()
                .collect::<Vec<u32>>(),
        )
        .then_ignore(just('\''))
        .try_map(|codes, span| combine_string_code_units(codes).map_err(|e| Rich::custom(span, e)))
        .padded();

    // External constants
    let external_constant = just('%')
        .ignore_then(choice((identifier.clone(), string_for_external)))
        .map(Term::ExternalConstant)
        .padded();

    // Use explicit boxing to prevent infinite type recursion in chumsky 0.10

    recursive(|expr| {
        // Atom: the most basic elements like literals, identifiers, parenthesized expressions.
        let atom = choice((
            // Box each branch individually to ensure type uniformity for choice
            literal.clone().map(Expression::Term).boxed(), // Map literal Term to Expression here
            external_constant.clone().map(Expression::Term).boxed(),
            // Function call: identifier(...) - Try this *before* simple identifier
            identifier
                .clone()
                .then(
                    expr.clone()
                        .separated_by(just(',').padded())
                        .allow_trailing()
                        .collect::<Vec<_>>()
                        // Ensure parentheses are padded to handle potential whitespace
                        .delimited_by(just('(').padded(), just(')').padded()),
                )
                // Directly create the Expression::Term(Term::Invocation(...)) structure
                .map(|(name, params)| {
                    Expression::Term(Term::Invocation(Invocation::Function(name, params)))
                })
                .boxed(),
            // Instance selector: TypeName { field: expr, ... }
            identifier
                .clone()
                .then(
                    identifier
                        .clone()
                        .then_ignore(just(':').padded())
                        .then(expr.clone().boxed())
                        .separated_by(just(',').padded())
                        .allow_trailing()
                        .collect::<Vec<_>>()
                        .delimited_by(just('{').padded(), just('}').padded()),
                )
                .map(|(type_name, fields)| {
                    Expression::InstanceSelector(
                        type_name,
                        fields.into_iter().map(|(k, v)| (k, Box::new(v))).collect(),
                    )
                })
                .boxed(),
            // Simple identifier, $this, $index, $total (parsed if not a function call)
            choice((
                identifier.clone().map(Invocation::Member),
                just("$this").to(Invocation::This),
                just("$index").to(Invocation::Index),
                just("$total").to(Invocation::Total),
            ))
            .map(Term::Invocation) // Map these simple invocations to Term
            .map(Expression::Term) // Map Term to Expression
            .boxed(),
            // Parenthesized expression - add extra boxing to break recursion
            expr.clone()
                .boxed()
                .delimited_by(just('(').padded(), just(')').padded())
                .boxed(),
        ))
        .padded();

        // Postfix operators: . (member/function invocation) and [] (indexer)
        let postfix_op = choice((
            // Member/Function Invocation: '.' followed by identifier, optionally followed by args (...)
            just('.')
                .ignore_then(
                    identifier.clone().then(
                        // Optionally parse arguments
                        expr.clone()
                            .boxed()
                            .separated_by(just(',').padded())
                            .allow_trailing()
                            .collect::<Vec<_>>()
                            .delimited_by(just('(').padded(), just(')').padded())
                            .or_not(), // Make arguments optional
                    ),
                )
                .map(|(name, params_opt)| {
                    // Create the correct Invocation based on whether params were found
                    let invocation = match params_opt {
                        Some(params) => Invocation::Function(name, params),
                        None => Invocation::Member(name),
                    };
                    // Return the closure
                    Box::new(move |left: Expression| {
                        Expression::Invocation(Box::new(left), invocation.clone())
                    }) as Box<dyn Fn(Expression) -> Expression>
                }),
            // Indexer
            expr.clone()
                .delimited_by(just('[').padded(), just(']').padded())
                .map(|idx| {
                    Box::new(move |left: Expression| {
                        Expression::Indexer(Box::new(left), Box::new(idx.clone()))
                    }) as Box<dyn Fn(Expression) -> Expression>
                }),
        ))
        .boxed(); // Box the choice result

        let atom_with_postfix = atom
            .clone()
            .then(postfix_op.repeated().collect::<Vec<_>>())
            .map(|(left, ops)| ops.into_iter().fold(left, |acc, op| op(acc)));

        // Prefix operators (Polarity)
        let prefix_op = choice((just('+').to('+'), just('-').to('-'))).padded();

        let term_with_polarity = prefix_op
            .repeated()
            .collect::<Vec<_>>()
            .then(atom_with_postfix)
            .map(|(ops, right)| {
                ops.into_iter()
                    .rev()
                    .fold(right, |acc, op| Expression::Polarity(op, Box::new(acc)))
            });

        // Infix operators with precedence levels (from high to low)

        // Level 1: Multiplicative (*, /, div, mod) - Left associative
        let op_mul = choice((
            just('*').to("*"),
            just('/').to("/"),
            text::keyword("div").to("div"),
            text::keyword("mod").to("mod"),
        ))
        .padded();
        let multiplicative = term_with_polarity
            .clone()
            .then(
                op_mul
                    .then(term_with_polarity)
                    .repeated()
                    .collect::<Vec<_>>(),
            )
            .map(|(left, ops)| {
                ops.into_iter().fold(left, |acc, (op_str, right)| {
                    Expression::Multiplicative(Box::new(acc), op_str.to_string(), Box::new(right))
                })
            });

        // Level 2: Additive (+, -, &) - Left associative
        let op_add = choice((just('+').to("+"), just('-').to("-"), just('&').to("&"))).padded();
        let additive = multiplicative
            .clone()
            .then(op_add.then(multiplicative).repeated().collect::<Vec<_>>())
            .map(|(left, ops)| {
                ops.into_iter().fold(left, |acc, (op_str, right)| {
                    Expression::Additive(Box::new(acc), op_str.to_string(), Box::new(right))
                })
            });

        // Level 3: Type (is, as) - Left associative
        // Per FHIRPath spec, type operations bind tighter than union
        let op_type = choice((text::keyword("is").to("is"), text::keyword("as").to("as"))).padded();
        let type_expr = additive
            .clone()
            .then(
                op_type
                    .then(qualified_identifier.clone())
                    .repeated()
                    .collect::<Vec<_>>(),
            ) // Type specifier follows 'is'/'as'
            .map(|(left, ops)| {
                ops.into_iter().fold(left, |acc, (op_str, type_spec)| {
                    Expression::Type(Box::new(acc), op_str.to_string(), type_spec)
                })
            });

        // Level 4: Union (|) - Left associative
        let op_union = just('|').padded();
        let union = type_expr
            .clone()
            .then(op_union.then(type_expr).repeated().collect::<Vec<_>>())
            .map(|(left, ops)| {
                ops.into_iter().fold(left, |acc, (_, right)| {
                    Expression::Union(Box::new(acc), Box::new(right))
                })
            });

        // Level 5: Inequality (<, <=, >, >=) - Left associative
        let op_ineq = choice((
            just("<=").to("<="),
            just("<").to("<"),
            just(">=").to(">="),
            just(">").to(">"),
        ))
        .padded();
        let inequality = union
            .clone()
            .then(op_ineq.then(union).repeated().collect::<Vec<_>>())
            .map(|(left, ops)| {
                ops.into_iter().fold(left, |acc, (op_str, right)| {
                    Expression::Inequality(Box::new(acc), op_str.to_string(), Box::new(right))
                })
            });

        // Level 6: Equality (=, ~, !=, !~) - Left associative
        let op_eq = choice((
            just("=").to("="),
            just("~").to("~"),
            just("!=").to("!="),
            just("!~").to("!~"),
        ))
        .padded();
        let equality = inequality
            .clone()
            .boxed()
            .then(
                op_eq
                    .then(inequality.clone().boxed())
                    .repeated()
                    .collect::<Vec<_>>(),
            )
            .map(|(left, ops)| {
                ops.into_iter().fold(left, |acc, (op_str, right)| {
                    Expression::Equality(Box::new(acc), op_str.to_string(), Box::new(right))
                })
            });

        // Level 7: Membership (in, contains) - Left associative
        let op_mem = choice((
            text::keyword("in").to("in"),
            text::keyword("contains").to("contains"),
        ))
        .padded();
        let membership = equality
            .clone()
            .boxed()
            .then(
                op_mem
                    .then(equality.clone().boxed())
                    .repeated()
                    .collect::<Vec<_>>(),
            )
            .map(|(left, ops)| {
                ops.into_iter().fold(left, |acc, (op_str, right)| {
                    Expression::Membership(Box::new(acc), op_str.to_string(), Box::new(right))
                })
            });

        // Level 8: Logical AND (and) - Left associative
        let op_and = text::keyword("and").padded();
        let logical_and = membership
            .clone()
            .boxed()
            .then(
                op_and
                    .then(membership.clone().boxed())
                    .repeated()
                    .collect::<Vec<_>>(),
            )
            .map(|(left, ops)| {
                ops.into_iter().fold(left, |acc, (_, right)| {
                    Expression::And(Box::new(acc), Box::new(right))
                })
            });

        // Level 9: Logical OR/XOR (or, xor) - Left associative
        let op_or = choice((text::keyword("or").to("or"), text::keyword("xor").to("xor"))).padded();
        let logical_or = logical_and
            .clone()
            .boxed()
            .then(
                op_or
                    .then(logical_and.clone().boxed())
                    .repeated()
                    .collect::<Vec<_>>(),
            )
            .map(|(left, ops)| {
                ops.into_iter().fold(left, |acc, (op_str, right)| {
                    Expression::Or(Box::new(acc), op_str.to_string(), Box::new(right))
                })
            });

        // Level 10: Implies (implies) - Right associative
        let op_implies = text::keyword("implies").padded();
        logical_or
            .clone()
            .boxed()
            .then(
                op_implies
                    .then(logical_or.clone().boxed())
                    .repeated()
                    .collect::<Vec<_>>(),
            )
            .map(|(left, ops)| {
                ops.into_iter().fold(left, |acc, (_, right)| {
                    Expression::Implies(Box::new(acc), Box::new(right))
                })
            })
    }) // Close the recursive closure here
    .then_ignore(end()) // Ensure the entire input is consumed after the expression
}

/// Creates a parser for FHIRPath expressions that includes span (position + length) information.
///
/// This parser produces `SpannedExpression` nodes, each annotated with their position and length
/// in the source text. It is used for the `parseDebugTree` validation output and debug-trace features.
///
/// The parser structure mirrors `parser()` exactly but wraps each node with span information
/// using chumsky's `map_with` combinator.
pub fn spanned_parser<'src>()
-> impl Parser<'src, &'src str, SpannedExpression, extra::Err<Rich<'src, char>>> + Clone + 'src {
    // Parser for escape sequences (same as parser()). Emits a raw code unit
    // so surrogate pairs can be combined in `combine_string_code_units`.
    let esc = just('\\').ignore_then(choice((
        just('`').to('`' as u32),
        just('\'').to('\'' as u32),
        just('\\').to('\\' as u32),
        just('/').to('/' as u32),
        just('f').to(0x000C_u32),
        just('n').to('\n' as u32),
        just('r').to('\r' as u32),
        just('t').to('\t' as u32),
        just('"').to('"' as u32),
        just('u').ignore_then(
            any()
                .filter(|c: &char| c.is_ascii_hexdigit())
                .repeated()
                .exactly(4)
                .collect::<String>()
                .try_map(
                    |digits: String, span| match u32::from_str_radix(&digits, 16) {
                        Ok(code) => Ok(code),
                        Err(_) => Err(Rich::custom(span, "Invalid hex digits")),
                    },
                ),
        ),
    )));

    // LITERAL PARSERS

    let null = just('{').then(just('}')).to(Literal::Null);

    let boolean = choice((
        text::keyword("true").to(Literal::Boolean(true)),
        text::keyword("false").to(Literal::Boolean(false)),
    ))
    .boxed();

    let string = just('\'')
        .ignore_then(
            none_of("\\\'")
                .map(|c: char| c as u32)
                .or(esc)
                .repeated()
                .collect::<Vec<u32>>(),
        )
        .then_ignore(just('\''))
        .try_map(|codes, span| combine_string_code_units(codes).map_err(|e| Rich::custom(span, e)))
        .map(Literal::String)
        .boxed();

    let integer = any()
        .filter(|c: &char| c.is_ascii_digit())
        .repeated()
        .at_least(1)
        .collect::<String>()
        .try_map(|digits: String, span| match i64::from_str(&digits) {
            Ok(n) => Ok(Literal::Integer(n)),
            Err(_) => Err(Rich::custom(span, format!("Invalid integer: {}", digits))),
        });
    let integer = custom_padded(integer);

    let number = any()
        .filter(|c: &char| c.is_ascii_digit())
        .repeated()
        .at_least(1)
        .collect::<String>()
        .then(just('.'))
        .then(
            any()
                .filter(|c: &char| c.is_ascii_digit())
                .repeated()
                .at_least(1)
                .collect::<String>(),
        )
        .try_map(|((i, _), d), span| {
            let num_str = format!("{}.{}", i, d);
            match Decimal::from_str(&num_str) {
                Ok(decimal) => Ok(Literal::Number(decimal)),
                Err(_) => Err(Rich::custom(span, format!("Invalid number: {}", num_str))),
            }
        })
        .padded();

    let time_format = any()
        .filter(|c: &char| c.is_ascii_digit())
        .repeated()
        .at_least(2)
        .at_most(2)
        .collect::<String>()
        .then(
            just(':')
                .ignore_then(
                    any()
                        .filter(|c: &char| c.is_ascii_digit())
                        .repeated()
                        .at_least(2)
                        .at_most(2)
                        .collect::<String>(),
                )
                .then(
                    just(':')
                        .ignore_then(
                            any()
                                .filter(|c: &char| c.is_ascii_digit())
                                .repeated()
                                .at_least(2)
                                .at_most(2)
                                .collect::<String>(),
                        )
                        .then(
                            just('.')
                                .ignore_then(
                                    any()
                                        .filter(|c: &char| c.is_ascii_digit())
                                        .repeated()
                                        .at_least(1)
                                        .at_most(3)
                                        .collect::<String>(),
                                )
                                .or_not(),
                        )
                        .or_not(),
                )
                .or_not(),
        )
        .map(|(hours, rest_opt)| {
            let mut result = hours;
            if let Some((minutes, seconds_part)) = rest_opt {
                result.push(':');
                result.push_str(&minutes);
                if let Some((seconds, milliseconds)) = seconds_part {
                    result.push(':');
                    result.push_str(&seconds);
                    if let Some(ms) = milliseconds {
                        result.push('.');
                        result.push_str(&ms);
                    }
                }
            }
            result
        });

    let timezone_format = just('Z').to("Z".to_string()).or(one_of("+-")
        .map(|c: char| c.to_string())
        .then(
            any()
                .filter(|c: &char| c.is_ascii_digit())
                .repeated()
                .at_most(2)
                .at_least(2)
                .collect::<String>(),
        )
        .then(just(':'))
        .then(
            any()
                .filter(|c: &char| c.is_ascii_digit())
                .repeated()
                .at_most(2)
                .at_least(2)
                .collect::<String>(),
        )
        .map(|(((sign, hour), _), min)| format!("{}{}:{}", sign, hour, min)));

    let date_format_str = any()
        .filter(|c: &char| c.is_ascii_digit())
        .repeated()
        .exactly(4)
        .collect::<String>()
        .then(
            just('-')
                .ignore_then(
                    any()
                        .filter(|c: &char| c.is_ascii_digit())
                        .repeated()
                        .exactly(2)
                        .collect::<String>()
                        .then(
                            just('-')
                                .ignore_then(
                                    any()
                                        .filter(|c: &char| c.is_ascii_digit())
                                        .repeated()
                                        .exactly(2)
                                        .collect::<String>(),
                                )
                                .or_not(),
                        ),
                )
                .or_not(),
        )
        .map(|(year, month_part)| {
            let mut date_str = year;
            if let Some((month_str, day_part)) = month_part {
                date_str.push('-');
                date_str.push_str(&month_str);
                if let Some(day_str) = day_part {
                    date_str.push('-');
                    date_str.push_str(&day_str);
                }
            }
            date_str
        })
        .boxed();

    let unit_keyword = choice((
        text::keyword("year").to("year".to_string()),
        text::keyword("month").to("month".to_string()),
        text::keyword("week").to("week".to_string()),
        text::keyword("day").to("day".to_string()),
        text::keyword("hour").to("hour".to_string()),
        text::keyword("minute").to("minute".to_string()),
        text::keyword("second").to("second".to_string()),
        text::keyword("millisecond").to("millisecond".to_string()),
        text::keyword("years").to("years".to_string()),
        text::keyword("months").to("months".to_string()),
        text::keyword("weeks").to("weeks".to_string()),
        text::keyword("days").to("days".to_string()),
        text::keyword("hours").to("hours".to_string()),
        text::keyword("minutes").to("minutes".to_string()),
        text::keyword("seconds").to("seconds".to_string()),
        text::keyword("milliseconds").to("milliseconds".to_string()),
    ));

    let unit_string_literal = just('\'')
        .ignore_then(
            none_of("\\\'")
                .map(|c: char| c as u32)
                .or(esc)
                .repeated()
                .collect::<Vec<u32>>(),
        )
        .then_ignore(just('\''))
        .try_map(|codes, span| combine_string_code_units(codes).map_err(|e| Rich::custom(span, e)));

    let unit = choice((unit_keyword, unit_string_literal)).boxed().padded();

    let integer_for_quantity = any()
        .filter(|c: &char| c.is_ascii_digit())
        .repeated()
        .at_least(1)
        .collect::<String>()
        .try_map(|digits: String, span| match i64::from_str(&digits) {
            Ok(n) => Ok(n),
            Err(_) => Err(Rich::custom(span, format!("Invalid integer: {}", digits))),
        });

    let number_for_quantity = any()
        .filter(|c: &char| c.is_ascii_digit())
        .repeated()
        .at_least(1)
        .collect::<String>()
        .then(just('.'))
        .then(
            any()
                .filter(|c: &char| c.is_ascii_digit())
                .repeated()
                .at_least(1)
                .collect::<String>(),
        )
        .try_map(|((i, _), d), span| {
            let num_str = format!("{}.{}", i, d);
            match Decimal::from_str(&num_str) {
                Ok(decimal) => Ok(decimal),
                Err(_) => Err(Rich::custom(span, format!("Invalid number: {}", num_str))),
            }
        });

    let quantity = choice((
        integer_for_quantity
            .then_ignore(text::whitespace().at_least(1))
            .then(unit.clone())
            .map(|(i, u_str)| Literal::Quantity(Decimal::from(i), u_str)),
        number_for_quantity
            .then_ignore(text::whitespace().at_least(1))
            .then(unit.clone())
            .map(|(d, u_str)| Literal::Quantity(d, u_str)),
    ));

    let datetime_literal = just('@')
        .ignore_then(date_format_str.clone())
        .then_ignore(just('T'))
        .then(time_format)
        .then(timezone_format.clone().or_not())
        .try_map(|((date_str, time_str), tz_opt), span| {
            let full_str = if let Some(tz) = tz_opt {
                format!("{}T{}{}", date_str, time_str, tz)
            } else {
                format!("{}T{}", date_str, time_str)
            };
            helios_fhir::PrecisionDateTime::parse(&full_str)
                .ok_or_else(|| Rich::custom(span, format!("Invalid datetime format: {}", full_str)))
                .map(Literal::DateTime)
        });

    let partial_datetime_literal = just('@')
        .ignore_then(date_format_str.clone())
        .then_ignore(just('T'))
        .try_map(|date_str, span| {
            let full_str = format!("{}T", date_str);
            helios_fhir::PrecisionDateTime::parse(&full_str)
                .ok_or_else(|| {
                    Rich::custom(
                        span,
                        format!("Invalid partial datetime format: {}", full_str),
                    )
                })
                .map(Literal::DateTime)
        });

    let time_literal = just('@')
        .ignore_then(
            just('T')
                .ignore_then(time_format)
                .then(timezone_format.or_not()),
        )
        .try_map(|(time_str, tz_opt), span| {
            if tz_opt.is_some() {
                Err(Rich::custom(
                    span,
                    "Time literal cannot have a timezone offset",
                ))
            } else {
                helios_fhir::PrecisionTime::parse(&time_str)
                    .ok_or_else(|| Rich::custom(span, format!("Invalid time format: {}", time_str)))
                    .map(Literal::Time)
            }
        });

    let date_literal = just('@')
        .ignore_then(date_format_str.clone())
        .try_map(|date_str, span| {
            helios_fhir::PrecisionDate::parse(&date_str)
                .ok_or_else(|| Rich::custom(span, format!("Invalid date format: {}", date_str)))
                .map(Literal::Date)
        });

    let literal = choice((
        null,
        boolean,
        string,
        quantity,
        number,
        integer,
        custom_padded(datetime_literal),
        custom_padded(partial_datetime_literal),
        custom_padded(time_literal),
        custom_padded(date_literal),
    ))
    .map(Term::Literal);

    // IDENTIFIER PARSERS

    let standard_identifier = any()
        .filter(|c: &char| c.is_ascii_alphabetic() || *c == '_')
        .then(
            any()
                .filter(|c: &char| c.is_ascii_alphanumeric() || *c == '_')
                .repeated()
                .collect::<Vec<_>>(),
        )
        .map(|(first, rest): (char, Vec<char>)| {
            let mut s = first.to_string();
            s.extend(rest);
            s
        })
        .padded();

    let delimited_identifier = just('`')
        .ignore_then(
            none_of("`")
                .map(|c: char| c as u32)
                .or(esc)
                .repeated()
                .collect::<Vec<u32>>(),
        )
        .then_ignore(just('`'))
        .try_map(|codes, span| combine_string_code_units(codes).map_err(|e| Rich::custom(span, e)))
        .padded();

    let identifier = choice((
        standard_identifier,
        delimited_identifier,
        text::keyword("as").to(String::from("as")),
        text::keyword("contains").to(String::from("contains")),
        text::keyword("in").to(String::from("in")),
        text::keyword("is").to(String::from("is")),
        text::keyword("true").to(String::from("true")),
        text::keyword("false").to(String::from("false")),
    ));

    let qualified_identifier = {
        let explicit_namespace_type = identifier
            .clone()
            .then(just('.').ignore_then(identifier.clone()))
            .map(|(namespace, type_name)| {
                let clean_ns = clean_backtick_identifier(&namespace);
                let clean_type = clean_backtick_identifier(&type_name);
                TypeSpecifier::QualifiedIdentifier(clean_ns, Some(clean_type))
            });

        let standalone_type = identifier.clone().map(|id| {
            let clean_id = clean_backtick_identifier(&id);
            if clean_id.contains('.') {
                if let Some(last_dot_pos) = clean_id.rfind('.') {
                    let namespace = clean_id[..last_dot_pos].to_string();
                    let type_name = clean_id[last_dot_pos + 1..].to_string();
                    TypeSpecifier::QualifiedIdentifier(namespace, Some(type_name))
                } else {
                    TypeSpecifier::QualifiedIdentifier(clean_id, None)
                }
            } else {
                TypeSpecifier::QualifiedIdentifier(clean_id, None)
            }
        });

        choice((explicit_namespace_type.boxed(), standalone_type.boxed())).boxed()
    };
    let qualified_identifier = custom_padded(qualified_identifier);

    let string_for_external = just('\'')
        .ignore_then(
            none_of("\'\\")
                .map(|c: char| c as u32)
                .or(esc)
                .repeated()
                .collect::<Vec<u32>>(),
        )
        .then_ignore(just('\''))
        .try_map(|codes, span| combine_string_code_units(codes).map_err(|e| Rich::custom(span, e)))
        .padded();

    let external_constant = just('%')
        .ignore_then(choice((identifier.clone(), string_for_external)))
        .map(Term::ExternalConstant)
        .padded();

    // RECURSIVE EXPRESSION PARSER (produces SpannedExpression)

    recursive(
        |spanned_expr: Recursive<
            dyn Parser<'src, &'src str, SpannedExpression, extra::Err<Rich<'src, char>>> + 'src,
        >| {
            // Helper to create a SpannedExpression from kind and chumsky span
            #[inline]
            fn make_spanned(kind: SpannedExprKind, start: usize, end: usize) -> SpannedExpression {
                SpannedExpression {
                    kind,
                    span: ExprSpan {
                        position: start,
                        length: end - start,
                    },
                }
            }

            // Atom: basic elements with span tracking
            let atom = choice((
                // Literal atom
                literal
                    .clone()
                    .map_with(|term, extra| {
                        let s = extra.span();
                        let spanned_term = match term {
                            Term::Literal(l) => SpannedTerm::Literal(l),
                            _ => unreachable!(),
                        };
                        make_spanned(SpannedExprKind::Term(spanned_term), s.start, s.end)
                    })
                    .boxed(),
                // External constant
                external_constant
                    .clone()
                    .map_with(|term, extra| {
                        let s = extra.span();
                        let spanned_term = match term {
                            Term::ExternalConstant(name) => SpannedTerm::ExternalConstant(name),
                            _ => unreachable!(),
                        };
                        make_spanned(SpannedExprKind::Term(spanned_term), s.start, s.end)
                    })
                    .boxed(),
                // Function call: identifier(...)
                identifier
                    .clone()
                    .then(
                        spanned_expr
                            .clone()
                            .separated_by(just(',').padded())
                            .allow_trailing()
                            .collect::<Vec<_>>()
                            .delimited_by(just('(').padded(), just(')').padded()),
                    )
                    .map_with(|(name, params), extra| {
                        let s = extra.span();
                        make_spanned(
                            SpannedExprKind::Term(SpannedTerm::Invocation(
                                SpannedInvocation::Function(name, params),
                            )),
                            s.start,
                            s.end,
                        )
                    })
                    .boxed(),
                // Instance selector: TypeName { field: expr, ... }
                identifier
                    .clone()
                    .then(
                        identifier
                            .clone()
                            .then_ignore(just(':').padded())
                            .then(spanned_expr.clone().boxed())
                            .separated_by(just(',').padded())
                            .allow_trailing()
                            .collect::<Vec<_>>()
                            .delimited_by(just('{').padded(), just('}').padded()),
                    )
                    .map_with(|(type_name, fields), extra| {
                        let s = extra.span();
                        make_spanned(
                            SpannedExprKind::InstanceSelector(
                                type_name,
                                fields.into_iter().map(|(k, v)| (k, Box::new(v))).collect(),
                            ),
                            s.start,
                            s.end,
                        )
                    })
                    .boxed(),
                // Simple invocation ($this, $index, $total, identifiers)
                choice((
                    identifier.clone().map(SpannedInvocation::Member),
                    just("$this").to(SpannedInvocation::This),
                    just("$index").to(SpannedInvocation::Index),
                    just("$total").to(SpannedInvocation::Total),
                ))
                .map_with(|inv, extra| {
                    let s = extra.span();
                    make_spanned(
                        SpannedExprKind::Term(SpannedTerm::Invocation(inv)),
                        s.start,
                        s.end,
                    )
                })
                .boxed(),
                // Parenthesized expression
                spanned_expr
                    .clone()
                    .boxed()
                    .delimited_by(just('(').padded(), just(')').padded())
                    .map_with(|inner, extra| {
                        let s = extra.span();
                        make_spanned(
                            SpannedExprKind::Term(SpannedTerm::Parenthesized(Box::new(inner))),
                            s.start,
                            s.end,
                        )
                    })
                    .boxed(),
            ))
            .padded();

            // Postfix operators: . (invocation) and [] (indexer)
            let postfix_op = choice((
                // Member/Function Invocation
                just('.')
                    .ignore_then(
                        identifier.clone().then(
                            spanned_expr
                                .clone()
                                .boxed()
                                .separated_by(just(',').padded())
                                .allow_trailing()
                                .collect::<Vec<_>>()
                                .delimited_by(just('(').padded(), just(')').padded())
                                .or_not(),
                        ),
                    )
                    .map_with(|(name, params_opt), extra| {
                        let op_end = extra.span().end;
                        let invocation = match params_opt {
                            Some(params) => SpannedInvocation::Function(name, params),
                            None => SpannedInvocation::Member(name),
                        };
                        Box::new(move |left: SpannedExpression| {
                            let start = left.span.position;
                            make_spanned(
                                SpannedExprKind::Invocation(Box::new(left), invocation.clone()),
                                start,
                                op_end,
                            )
                        })
                            as Box<dyn Fn(SpannedExpression) -> SpannedExpression>
                    }),
                // Indexer
                spanned_expr
                    .clone()
                    .delimited_by(just('[').padded(), just(']').padded())
                    .map_with(|idx, extra| {
                        let op_end = extra.span().end;
                        Box::new(move |left: SpannedExpression| {
                            let start = left.span.position;
                            make_spanned(
                                SpannedExprKind::Indexer(Box::new(left), Box::new(idx.clone())),
                                start,
                                op_end,
                            )
                        })
                            as Box<dyn Fn(SpannedExpression) -> SpannedExpression>
                    }),
            ))
            .boxed();

            let atom_with_postfix = atom
                .clone()
                .then(postfix_op.repeated().collect::<Vec<_>>())
                .map(|(left, ops)| ops.into_iter().fold(left, |acc, op| op(acc)));

            // Prefix operators (Polarity)
            let prefix_op = choice((just('+').to('+'), just('-').to('-'))).padded();

            let term_with_polarity = prefix_op
                .repeated()
                .collect::<Vec<_>>()
                .then(atom_with_postfix)
                .map_with(|(ops, right), extra| {
                    if ops.is_empty() {
                        right
                    } else {
                        let full_start = extra.span().start;
                        ops.into_iter().rev().fold(right, |acc, op| {
                            let end = acc.span.position + acc.span.length;
                            make_spanned(
                                SpannedExprKind::Polarity(op, Box::new(acc)),
                                full_start,
                                end,
                            )
                        })
                    }
                });

            // Infix operators with precedence levels

            // Level 1: Multiplicative
            let op_mul = choice((
                just('*').to("*"),
                just('/').to("/"),
                text::keyword("div").to("div"),
                text::keyword("mod").to("mod"),
            ))
            .padded();
            let multiplicative = term_with_polarity
                .clone()
                .then(
                    op_mul
                        .then(term_with_polarity)
                        .repeated()
                        .collect::<Vec<_>>(),
                )
                .map(|(left, ops)| {
                    ops.into_iter().fold(left, |acc, (op_str, right)| {
                        let start = acc.span.position;
                        let end = right.span.position + right.span.length;
                        make_spanned(
                            SpannedExprKind::Multiplicative(
                                Box::new(acc),
                                op_str.to_string(),
                                Box::new(right),
                            ),
                            start,
                            end,
                        )
                    })
                });

            // Level 2: Additive
            let op_add = choice((just('+').to("+"), just('-').to("-"), just('&').to("&"))).padded();
            let additive = multiplicative
                .clone()
                .then(op_add.then(multiplicative).repeated().collect::<Vec<_>>())
                .map(|(left, ops)| {
                    ops.into_iter().fold(left, |acc, (op_str, right)| {
                        let start = acc.span.position;
                        let end = right.span.position + right.span.length;
                        make_spanned(
                            SpannedExprKind::Additive(
                                Box::new(acc),
                                op_str.to_string(),
                                Box::new(right),
                            ),
                            start,
                            end,
                        )
                    })
                });

            // Level 3: Type (is, as)
            let op_type =
                choice((text::keyword("is").to("is"), text::keyword("as").to("as"))).padded();
            let type_expr = additive
                .clone()
                .then(
                    op_type
                        .then(qualified_identifier.clone())
                        .repeated()
                        .collect::<Vec<_>>(),
                )
                .map_with(|(left, ops), extra| {
                    if ops.is_empty() {
                        left
                    } else {
                        let full_end = extra.span().end;
                        ops.into_iter().fold(left, |acc, (op_str, type_spec)| {
                            let start = acc.span.position;
                            make_spanned(
                                SpannedExprKind::Type(Box::new(acc), op_str.to_string(), type_spec),
                                start,
                                full_end,
                            )
                        })
                    }
                });

            // Level 4: Union (|)
            let op_union = just('|').padded();
            let union = type_expr
                .clone()
                .then(op_union.then(type_expr).repeated().collect::<Vec<_>>())
                .map(|(left, ops)| {
                    ops.into_iter().fold(left, |acc, (_, right)| {
                        let start = acc.span.position;
                        let end = right.span.position + right.span.length;
                        make_spanned(
                            SpannedExprKind::Union(Box::new(acc), Box::new(right)),
                            start,
                            end,
                        )
                    })
                });

            // Level 5: Inequality (<, <=, >, >=)
            let op_ineq = choice((
                just("<=").to("<="),
                just("<").to("<"),
                just(">=").to(">="),
                just(">").to(">"),
            ))
            .padded();
            let inequality = union
                .clone()
                .then(op_ineq.then(union).repeated().collect::<Vec<_>>())
                .map(|(left, ops)| {
                    ops.into_iter().fold(left, |acc, (op_str, right)| {
                        let start = acc.span.position;
                        let end = right.span.position + right.span.length;
                        make_spanned(
                            SpannedExprKind::Inequality(
                                Box::new(acc),
                                op_str.to_string(),
                                Box::new(right),
                            ),
                            start,
                            end,
                        )
                    })
                });

            // Level 6: Equality (=, ~, !=, !~)
            let op_eq = choice((
                just("=").to("="),
                just("~").to("~"),
                just("!=").to("!="),
                just("!~").to("!~"),
            ))
            .padded();
            let equality = inequality
                .clone()
                .boxed()
                .then(
                    op_eq
                        .then(inequality.clone().boxed())
                        .repeated()
                        .collect::<Vec<_>>(),
                )
                .map(|(left, ops)| {
                    ops.into_iter().fold(left, |acc, (op_str, right)| {
                        let start = acc.span.position;
                        let end = right.span.position + right.span.length;
                        make_spanned(
                            SpannedExprKind::Equality(
                                Box::new(acc),
                                op_str.to_string(),
                                Box::new(right),
                            ),
                            start,
                            end,
                        )
                    })
                });

            // Level 7: Membership (in, contains)
            let op_mem = choice((
                text::keyword("in").to("in"),
                text::keyword("contains").to("contains"),
            ))
            .padded();
            let membership = equality
                .clone()
                .boxed()
                .then(
                    op_mem
                        .then(equality.clone().boxed())
                        .repeated()
                        .collect::<Vec<_>>(),
                )
                .map(|(left, ops)| {
                    ops.into_iter().fold(left, |acc, (op_str, right)| {
                        let start = acc.span.position;
                        let end = right.span.position + right.span.length;
                        make_spanned(
                            SpannedExprKind::Membership(
                                Box::new(acc),
                                op_str.to_string(),
                                Box::new(right),
                            ),
                            start,
                            end,
                        )
                    })
                });

            // Level 8: Logical AND
            let op_and = text::keyword("and").padded();
            let logical_and = membership
                .clone()
                .boxed()
                .then(
                    op_and
                        .then(membership.clone().boxed())
                        .repeated()
                        .collect::<Vec<_>>(),
                )
                .map(|(left, ops)| {
                    ops.into_iter().fold(left, |acc, (_, right)| {
                        let start = acc.span.position;
                        let end = right.span.position + right.span.length;
                        make_spanned(
                            SpannedExprKind::And(Box::new(acc), Box::new(right)),
                            start,
                            end,
                        )
                    })
                });

            // Level 9: Logical OR/XOR
            let op_or =
                choice((text::keyword("or").to("or"), text::keyword("xor").to("xor"))).padded();
            let logical_or = logical_and
                .clone()
                .boxed()
                .then(
                    op_or
                        .then(logical_and.clone().boxed())
                        .repeated()
                        .collect::<Vec<_>>(),
                )
                .map(|(left, ops)| {
                    ops.into_iter().fold(left, |acc, (op_str, right)| {
                        let start = acc.span.position;
                        let end = right.span.position + right.span.length;
                        make_spanned(
                            SpannedExprKind::Or(Box::new(acc), op_str.to_string(), Box::new(right)),
                            start,
                            end,
                        )
                    })
                });

            // Level 10: Implies
            let op_implies = text::keyword("implies").padded();
            logical_or
                .clone()
                .boxed()
                .then(
                    op_implies
                        .then(logical_or.clone().boxed())
                        .repeated()
                        .collect::<Vec<_>>(),
                )
                .map(|(left, ops)| {
                    ops.into_iter().fold(left, |acc, (_, right)| {
                        let start = acc.span.position;
                        let end = right.span.position + right.span.length;
                        make_spanned(
                            SpannedExprKind::Implies(Box::new(acc), Box::new(right)),
                            start,
                            end,
                        )
                    })
                })
        },
    )
    .then_ignore(end())
}