Exprimo

Exprimo is a reliable and JavaScript-compliant expression evaluator written in Rust, designed for rule-based engines and dynamic expression evaluation. Inspired by angular-expressions, it's built to be simple, fast, and production-ready.

Description

Exprimo parses and evaluates JavaScript expressions efficiently and securely. It utilizes the power of Rust and its excellent memory safety guarantees to provide a reliable and fast JavaScript expression evaluator with proper error handling and JavaScript semantics compliance.

Perfect for:

• Rule-based engines
• Dynamic configuration
• Conditional logic evaluation
• Template expressions
• Business rule processing

Features

✅ JavaScript-Compliant - Follows JavaScript semantics for intuitive expression writing
✅ Robust Error Handling - Gracefully handles edge cases (division by zero, NaN, Infinity)
✅ Type Coercion - Supports both loose (==) and strict (===) equality with proper type coercion
✅ Rich Type Support - Numbers, strings, booleans, arrays, objects, null, NaN, Infinity
✅ Custom Functions - Extend with your own Rust functions
✅ Built-in Methods - Array and object methods (.length, .includes(), .hasOwnProperty())
✅ String Escapes - Proper handling of escape sequences (\n, \t, \\, etc.)
✅ Production-Ready - Comprehensive test coverage (43+ tests)

Installation

Add Exprimo to your Cargo.toml:

[dependencies]
exprimo = "*"

Or install via cargo:

cargo add exprimo

Optional Logging

Enable trace logging for debugging:

[dependencies]
exprimo = { version = "*", features = ["logging"] }

This will install Scribe Rust. Set LOG_LEVEL=TRACE in environment variables for logs to output.

Quick Start

use exprimo::Evaluator;
use serde_json::Value;
use std::collections::HashMap;

// Set up context with variables
let mut context = HashMap::new();
context.insert("user_age".to_string(), Value::Number(30.into()));
context.insert("user_status".to_string(), Value::String("active".to_string()));
context.insert("user_premium".to_string(), Value::Bool(true));

// Create evaluator
let evaluator = Evaluator::new(context, HashMap::new());

// Evaluate expressions
let result = evaluator.evaluate("user_age >= 18 && user_status === 'active'").unwrap();
assert_eq!(result, Value::Bool(true));

// Ternary operator
let result = evaluator.evaluate("user_premium ? 'VIP' : 'Standard'").unwrap();
assert_eq!(result, Value::String("VIP".to_string()));

// Type coercion with ==
let result = evaluator.evaluate("user_premium == 1").unwrap();
assert_eq!(result, Value::Bool(true)); // true == 1 with type coercion

Supported Operations

Arithmetic Operators

a + b    // Addition (also string concatenation)
a - b    // Subtraction
a * b    // Multiplication
a / b    // Division (returns Infinity for division by zero)
a % b    // Modulo
-a       // Unary negation
+a       // Unary plus

Comparison Operators

a == b   // Loose equality (with type coercion)
a != b   // Loose inequality
a === b  // Strict equality (no type coercion)
a !== b  // Strict inequality
a > b    // Greater than
a < b    // Less than
a >= b   // Greater than or equal
a <= b   // Less than or equal

Logical Operators

a && b   // Logical AND
a || b   // Logical OR
!a       // Logical NOT

Ternary Operator

condition ? valueIfTrue : valueIfFalse

Grouping

(a + b) * c

Type Coercion

Exprimo implements JavaScript-compliant type coercion for the == operator:

// String to number coercion
evaluator.evaluate("'5' == 5").unwrap();        // true
evaluator.evaluate("'5' === 5").unwrap();       // false (strict)

// Boolean to number coercion
evaluator.evaluate("true == 1").unwrap();       // true
evaluator.evaluate("false == 0").unwrap();      // true

// Empty string to number
evaluator.evaluate("'' == 0").unwrap();         // true

Truthiness Rules

Exprimo follows JavaScript truthiness semantics:

Value	Truthy/Falsy	Example
true	Truthy	true ? 'yes' : 'no' → 'yes'
false	Falsy	false ? 'yes' : 'no' → 'no'
null	Falsy	null ? 'yes' : 'no' → 'no'
0	Falsy	0 ? 'yes' : 'no' → 'no'
NaN	Falsy	NaN ? 'yes' : 'no' → 'no'
"" (empty string)	Falsy	"" ? 'yes' : 'no' → 'no'
Non-zero numbers	Truthy	42 ? 'yes' : 'no' → 'yes'
Non-empty strings	Truthy	"hello" ? 'yes' : 'no' → 'yes'
All arrays	Truthy	[] ? 'yes' : 'no' → 'yes'
All objects	Truthy	{} ? 'yes' : 'no' → 'yes'

Note: Arrays and objects are always truthy, even if empty (JavaScript standard behavior).

Special Values

Infinity and NaN

Exprimo properly handles Infinity and NaN:

// Division by zero returns Infinity
evaluator.evaluate("5 / 0").unwrap();     // Infinity (no error!)
evaluator.evaluate("-5 / 0").unwrap();    // -Infinity

// Invalid conversions return NaN
evaluator.evaluate("'abc' * 2").unwrap(); // NaN (no error!)

// NaN comparisons
evaluator.evaluate("NaN == NaN").unwrap();  // false (JavaScript behavior)
evaluator.evaluate("NaN === NaN").unwrap(); // false

// Infinity identifier
evaluator.evaluate("Infinity > 1000000").unwrap(); // true

Note: Due to serde_json::Number limitations, NaN and Infinity are represented as best-effort approximations. For production use with heavy NaN/Infinity usage, consider a custom Value type.

Undefined

The undefined identifier returns null (closest JSON equivalent):

evaluator.evaluate("undefined").unwrap(); // null

String Escape Sequences

Exprimo processes common escape sequences:

evaluator.evaluate("'line1\\nline2'").unwrap();      // "line1\nline2"
evaluator.evaluate("'col1\\tcol2'").unwrap();        // "col1\tcol2"
evaluator.evaluate("'quote: \\'hello\\''").unwrap(); // "quote: 'hello'"
evaluator.evaluate("\"quote: \\\"hello\\\"\"").unwrap(); // "quote: \"hello\""
evaluator.evaluate("'path\\\\to\\\\file'").unwrap(); // "path\to\file"

Supported escapes: \n, \t, \r, \\, \', \", \0

Type Conversions

String to Number

evaluator.evaluate("'42' * 2").unwrap();      // 84
evaluator.evaluate("'abc' * 2").unwrap();     // NaN
evaluator.evaluate("'' * 2").unwrap();        // 0 (empty string → 0)
evaluator.evaluate("'Infinity' > 100").unwrap(); // true

Array to Number

evaluator.evaluate("[] * 5").unwrap();        // 0 (empty array → 0)
evaluator.evaluate("[42] * 2").unwrap();      // 84 (single element)
evaluator.evaluate("[1, 2] * 2").unwrap();    // NaN (multiple elements)

Object to Number

evaluator.evaluate("{} * 2").unwrap();        // NaN (objects → NaN)

Built-in Properties and Methods

Arrays

Arrays are represented by serde_json::Value::Array.

.length

Returns the number of elements in an array.

let mut context = HashMap::new();
context.insert("myArray".to_string(), Value::Array(vec![
    Value::Number(10.into()),
    Value::Number(20.into()),
    Value::Number(30.into()),
]));
let evaluator = Evaluator::new(context, HashMap::new());

evaluator.evaluate("myArray.length").unwrap(); // 3

.includes(valueToFind)

Checks if an array contains valueToFind using SameValueZero comparison (strict equality where NaN equals NaN).

evaluator.evaluate("[1, 'foo', null].includes('foo')").unwrap(); // true
evaluator.evaluate("[1, 2, 3].includes(4)").unwrap();            // false
evaluator.evaluate("[NaN].includes(NaN)").unwrap();              // true (special case)

Objects

Objects are represented by serde_json::Value::Object.

.hasOwnProperty(key)

Checks if an object contains the specified key as its own direct property.

let mut context = HashMap::new();
let mut obj = serde_json::Map::new();
obj.insert("name".to_string(), Value::String("Alice".to_string()));
obj.insert("age".to_string(), Value::Number(30.into()));
context.insert("myObject".to_string(), Value::Object(obj));
let evaluator = Evaluator::new(context, HashMap::new());

evaluator.evaluate("myObject.hasOwnProperty('name')").unwrap();   // true
evaluator.evaluate("myObject.hasOwnProperty('gender')").unwrap(); // false
evaluator.evaluate("myObject.hasOwnProperty(123)").unwrap();      // false (coerced to "123")

Custom Functions

Extend Exprimo with your own Rust functions by implementing the CustomFunction trait.

Example: toUpperCase Function

use exprimo::{Evaluator, CustomFunction, CustomFuncError};
use serde_json::Value;
use std::collections::HashMap;
use std::sync::Arc;

#[derive(Debug)]
struct ToUpperCase;

impl CustomFunction for ToUpperCase {
    fn call(&self, args: &[Value]) -> Result<Value, CustomFuncError> {
        if args.len() != 1 {
            return Err(CustomFuncError::ArityError { expected: 1, got: args.len() });
        }
        match &args[0] {
            Value::String(s) => Ok(Value::String(s.to_uppercase())),
            _ => Err(CustomFuncError::ArgumentError("Argument must be a string".to_string())),
        }
    }
}

fn main() {
    let context = HashMap::new();
    let mut custom_functions: HashMap<String, Arc<dyn CustomFunction>> = HashMap::new();
    custom_functions.insert("toUpperCase".to_string(), Arc::new(ToUpperCase));
    
    let evaluator = Evaluator::new(context, custom_functions);
    
    let result = evaluator.evaluate("toUpperCase('hello world')").unwrap();
    assert_eq!(result, Value::String("HELLO WORLD".to_string()));
}

Custom Function Requirements

• Implement exprimo::CustomFunction trait (also requires std::fmt::Debug)
• Implement the call method with signature: fn call(&self, args: &[Value]) -> Result<Value, CustomFuncError>
• Handle argument validation (count and types)
• Return Ok(Value) on success or Err(CustomFuncError) on failure
• Wrap in Arc::new() before inserting into the custom functions map

Real-World Example: Rule Engine

use exprimo::Evaluator;
use serde_json::Value;
use std::collections::HashMap;

// Rule engine context
let mut context = HashMap::new();
context.insert("user_age".to_string(), Value::Number(25.into()));
context.insert("user_country".to_string(), Value::String("US".to_string()));
context.insert("user_score".to_string(), Value::Number(850.into()));
context.insert("user_verified".to_string(), Value::Bool(true));
context.insert("user_tags".to_string(), Value::Array(vec![
    Value::String("premium".to_string()),
    Value::String("verified".to_string()),
]));

let evaluator = Evaluator::new(context, HashMap::new());

// Rule 1: Eligibility check
let eligible = evaluator.evaluate(
    "user_age >= 18 && user_verified && user_score >= 700"
).unwrap();
assert_eq!(eligible, Value::Bool(true));

// Rule 2: Discount calculation
let discount = evaluator.evaluate(
    "user_tags.includes('premium') ? 20 : 10"
).unwrap();
assert_eq!(discount, Value::Number(20.into()));

// Rule 3: Complex condition
let approved = evaluator.evaluate(
    "(user_country === 'US' || user_country === 'CA') && user_score > 800"
).unwrap();
assert_eq!(approved, Value::Bool(true));

Error Handling

Exprimo provides detailed error types:

use exprimo::EvaluationError;

let result = evaluator.evaluate("unknown_variable");
match result {
    Ok(value) => println!("Result: {}", value),
    Err(EvaluationError::Node(e)) => println!("Node error: {}", e),
    Err(EvaluationError::TypeError(e)) => println!("Type error: {}", e),
    Err(EvaluationError::CustomFunction(e)) => println!("Function error: {}", e),
}

Known Limitations

1. serde_json::Number Constraints

   • NaN and Infinity don't serialize perfectly to JSON
   • Workarounds are in place, but consider a custom Value type for production

2. Complex Literals

   • Only empty array [] and empty object {} literals are supported
   • Complex literals like [1, 2, 3] or {a: 1, b: 2} are not yet implemented
   • Workaround: Pass complex structures via context

3. Object Literal Ambiguity

   • {} in expression context is parsed as a block statement (JavaScript quirk)
   • Workaround: Use variables or wrap in parentheses (future support)

Testing

Run the test suite:

cargo test

Run with logging:

LOG_LEVEL=TRACE cargo test --features "logging"

Run examples:

cargo run --example basic
LOG_LEVEL=TRACE cargo run --features "logging" --example basic

Performance

Exprimo is designed for performance:

• Minimal allocations
• Efficient AST traversal
• Zero-copy where possible
• Rust's memory safety without runtime overhead

Changelog

See CHANGELOG.md for version history.

Recent Improvements (Latest Version)

• ✅ Division by zero returns Infinity/NaN instead of errors
• ✅ Invalid type conversions return NaN instead of errors
• ✅ Proper NaN comparison semantics (NaN != NaN)
• ✅ Arrays and objects are now truthy (JavaScript standard)
• ✅ Type coercion for == operator
• ✅ Separate strict equality === without coercion
• ✅ Infinity, NaN, undefined identifiers
• ✅ String escape sequence processing
• ✅ Array to number conversion
• ✅ SameValueZero for Array.includes()

Contributing

Contributions are welcome! Please:

1. Fork the repository
2. Create a feature branch
3. Add tests for new functionality
4. Ensure all tests pass
5. Submit a pull request

License

Exprimo is licensed under the MIT license. See the LICENSE file for details.

Credits

Inspired by angular-expressions.

Built with ❤️ using Rust and rslint_parser.