Rhai - Embedded Scripting for Rust

Rhai is an embedded scripting language for Rust that gives you a safe and easy way to add scripting to your applications.

Rhai's current feature set:

• Easy integration with Rust functions and data types
• Fairly efficient (1 mil iterations in 0.75 sec on my 5 year old laptop)
• Low compile-time overhead (~0.6 sec debug/~3 sec release for script runner app)
• Easy-to-use language similar to JS+Rust
• Support for overloaded functions
• No additional dependencies

Note: Currently, the version is 0.10.2, so the language and API's may change before they stabilize.

Installation

You can install Rhai using crates by adding this line to your dependencies:

[dependencies]
rhai = "0.10.2"

or simply:

[dependencies]
rhai = "*"

to use the latest version.

Beware that in order to use pre-releases (alpha and beta) you need to specify the exact version in your Cargo.toml.

Optional Features

debug_msgs

Print debug messages to stdout (using println!) related to function registrations and function calls.

no_stdlib

Exclude the standard library of utility functions in the build, and only include the minimum necessary functionalities.

Related

Other cool projects to check out:

• ChaiScript - A strong inspiration for Rhai. An embedded scripting language for C++ that I helped created many moons ago, now being lead by my cousin.
• You can also check out the list of scripting languages for Rust on awesome-rust

Examples

The repository contains several examples in the examples folder:

• arrays_and_structs demonstrates registering a new type to Rhai and the usage of arrays on it
• custom_types_and_methods shows how to register a type and methods for it
• hello simple example that evaluates an expression and prints the result
• reuse_scope evaluates two pieces of code in separate runs, but using a common scope
• rhai_runner runs each filename passed to it as a Rhai script
• simple_fn shows how to register a Rust function to a Rhai engine
• repl a simple REPL, see source code for what it can do at the moment

Examples can be run with the following command:

cargo run --example name

Example Scripts

We also have a few examples scripts that showcase Rhai's features, all stored in the scripts folder:

• array.rhai - arrays in Rhai
• assignment.rhai - variable declarations
• comments.rhai - just comments
• for1.rhai - for loops
• function_decl1.rhai - a function without parameters
• function_decl2.rhai - a function with two parameters
• function_decl3.rhai - a function with many parameters
• if1.rhai - if example
• loop.rhai - endless loop in Rhai, this example emulates a do..while cycle
• op1.rhai - just a simple addition
• op2.rhai - simple addition and multiplication
• op3.rhai - change evaluation order with parenthesis
• speed_test.rhai - a simple program to measure the speed of Rhai's interpreter
• string.rhai- string operations
• while.rhai - while loop

To run the scripts, you can either make your own tiny program, or make use of the rhai_runner example program:

cargo run --example rhai_runner scripts/any_script.rhai

Hello world

To get going with Rhai, you create an instance of the scripting engine and then run eval.

use rhai::Engine;

fn main() {
    let mut engine = Engine::new();

    if let Ok(result) = engine.eval::<i64>("40 + 2") {
        println!("Answer: {}", result);  // prints 42
    }
}

You can also evaluate a script file:

if let Ok(result) = engine.eval_file::<i64>("hello_world.rhai") { ... }

If you want to repeatedly evaluate a script, you can compile it first into an AST (abstract syntax tree) form:

// Compile to an AST and store it for later evaluations
let ast = Engine::compile("40 + 2").unwrap();

for _ in 0..42 {
    if let Ok(result) = engine.eval_ast::<i64>(&ast) {
        println!("Answer: {}", result);  // prints 42
    }
}

Compiling a script file is also supported:

let ast = Engine::compile_file("hello_world.rhai").unwrap();

Rhai also allows you to work backwards from the other direction - i.e. calling a Rhai-scripted function from Rust. You do this via call_fn, which takes a compiled AST (output from compile) and the function call arguments:

// Define a function in a script and compile to AST
let ast = Engine::compile("fn hello(x, y) { x.len() + y }")?;

// Evaluate the function in the AST, passing arguments into the script as a tuple
// (beware, arguments must be of the correct types because Rhai does not have built-in type conversions)
let result: i64 = engine.call_fn("hello", &ast, (&mut String::from("abc"), &mut 123_i64))?;

Values and types

The following primitive types are supported natively:

• Integer: i32, u32, i64 (default), u64
• Floating-point: f32, f64 (default)
• Character: char
• Boolean: bool
• Array: rhai::Array
• Dynamic (i.e. can be anything): rhai::Dynamic

Value conversions

All types are treated strictly separate by Rhai, meaning that i32 and i64 and u32 are completely different; you cannot even add them together.

There is a to_float function to convert a supported number to an f64, and a to_int function to convert a supported number to i64 and that's about it. For other conversions you can register your own conversion functions.

There is also a type_of function to detect the type of a value.

let x = 42;
let y = x * 100.0;              // error: cannot multiply i64 with f64
let y = x.to_float() * 100.0;   // works
let z = y.to_int() + x;         // works

let c = 'X';                    // character
print("c is '" + c + "' and its code is " + c.to_int());    // prints "c is 'X' and its code is 88"

// Use 'type_of' to get the type of variables
type_of(c) == "char";
type_of(x) == "i64";
y.type_of() == "f64";

if z.type_of() == "string" {
    do_something_with_strong(z);
}

Working with functions

Rhai's scripting engine is very lightweight. It gets its ability from the functions in your program. To call these functions, you need to register them with the scripting engine.

use rhai::Engine;
use rhai::RegisterFn;                       // include the `RegisterFn` trait to use `register_fn`
use rhai::{Dynamic, RegisterDynamicFn};     // include the `RegisterDynamicFn` trait to use `register_dynamic_fn`

// Normal function
fn add(x: i64, y: i64) -> i64 {
    x + y
}

// Function that returns a Dynamic value
fn get_an_any() -> Dynamic {
    Box::new(42_i64)
}

fn main() {
    let mut engine = Engine::new();

    engine.register_fn("add", add);

    if let Ok(result) = engine.eval::<i64>("add(40, 2)") {
       println!("Answer: {}", result);  // prints 42
    }

    // Functions that return Dynamic values must use register_dynamic_fn()
    engine.register_dynamic_fn("get_an_any", get_an_any);

    if let Ok(result) = engine.eval::<i64>("get_an_any()") {
       println!("Answer: {}", result);  // prints 42
    }
}

To return a Dynamic value, simply Box it and return it.

fn decide(yes_no: bool) -> Dynamic {
    if yes_no {
        Box::new(42_i64)
    } else {
        Box::new("hello world!".to_string())    // remember &str is not supported
    }
}

Generic functions

Generic functions can be used in Rhai, but you'll need to register separate instances for each concrete type:

use std::fmt::Display;

use rhai::{Engine, RegisterFn};

fn showit<T: Display>(x: &mut T) -> () {
    println!("{}", x)
}

fn main() {
    let mut engine = Engine::new();

    engine.register_fn("print", showit as fn(x: &mut i64)->());
    engine.register_fn("print", showit as fn(x: &mut bool)->());
    engine.register_fn("print", showit as fn(x: &mut String)->());
}

You can also see in this example how you can register multiple functions (or in this case multiple instances of the same function) to the same name in script. This gives you a way to overload functions and call the correct one, based on the types of the arguments, from your script.

Fallible functions

If your function is fallible (i.e. it returns a Result<_, Error>), you can register it with register_result_fn (using the RegisterResultFn trait).

Your function must return Result<_, EvalAltResult>. EvalAltResult implements From<&str> and From<String> etc. and the error text gets converted into EvalAltResult::ErrorRuntime.

use rhai::{Engine, EvalAltResult, Position};
use rhai::RegisterResultFn;     // include the `RegisterResultFn` trait to use `register_result_fn`

// Function that may fail
fn safe_divide(x: i64, y: i64) -> Result<i64, EvalAltResult> {
    if y == 0 {
        // Return an error if y is zero
        Err("Division by zero detected!".into())    // short-cut to create EvalAltResult
    } else {
        Ok(x / y)
    }
}

fn main() {
    let mut engine = Engine::new();

    // Fallible functions that return Result values must use register_result_fn()
    engine.register_result_fn("divide", safe_divide);

    if let Err(error) = engine.eval::<i64>("divide(40, 0)") {
       println!("Error: {:?}", error);  // prints ErrorRuntime("Division by zero detected!", (1, 1)")
    }
}

Overriding built-in functions

Any similarly-named function defined in a script overrides any built-in function.

// Override the built-in function 'to_int'
fn to_int(num) {
    print("Ha! Gotcha!" + num);
}

print(to_int(123));     // what will happen?

Custom types and methods

Here's an more complete example of working with Rust. First the example, then we'll break it into parts:

use rhai::{Engine, RegisterFn};

#[derive(Clone)]
struct TestStruct {
    x: i64
}

impl TestStruct {
    fn update(&mut self) {
        self.x += 1000;
    }

    fn new() -> TestStruct {
        TestStruct { x: 1 }
    }
}

fn main() {
    let mut engine = Engine::new();

    engine.register_type::<TestStruct>();

    engine.register_fn("update", TestStruct::update);
    engine.register_fn("new_ts", TestStruct::new);

    if let Ok(result) = engine.eval::<TestStruct>("let x = new_ts(); x.update(); x") {
        println!("result: {}", result.x); // prints 1001
    }
}

First, for each type we use with the engine, we need to be able to Clone. This allows the engine to pass by value and still keep its own state.

#[derive(Clone)]
struct TestStruct {
    x: i64
}

Next, we create a few methods that we'll later use in our scripts. Notice that we register our custom type with the engine.

impl TestStruct {
    fn update(&mut self) {
        self.x += 1000;
    }

    fn new() -> TestStruct {
        TestStruct { x: 1 }
    }
}

let mut engine = Engine::new();

engine.register_type::<TestStruct>();

To use methods and functions with the engine, we need to register them. There are some convenience functions to help with this. Below I register update and new with the engine.

Note: the engine follows the convention that methods use a &mut first parameter so that invoking methods can update the value in memory.

engine.register_fn("update", TestStruct::update);
engine.register_fn("new_ts", TestStruct::new);

Finally, we call our script. The script can see the function and method we registered earlier. We need to get the result back out from script land just as before, this time casting to our custom struct type.

if let Ok(result) = engine.eval::<TestStruct>("let x = new_ts(); x.update(); x") {
    println!("result: {}", result.x); // prints 1001
}

In fact, any function with a first argument (either by copy or via a &mut reference) can be used as a method-call on that type because internally they are the same thing: methods on a type is implemented as a functions taking an first argument.

fn foo(ts: &mut TestStruct) -> i64 {
    ts.x
}

engine.register_fn("foo", foo);

if let Ok(result) = engine.eval::<i64>("let x = new_ts(); x.foo()") {
    println!("result: {}", result); // prints 1
}

type_of works fine with custom types and returns the name of the type:

let x = new_ts();
print(x.type_of());     // prints "foo::bar::TestStruct"

If you use register_type_with_name to register the custom type with a special pretty-print name, type_of will return that instead.

Getters and setters

Similarly, you can work with members of your custom types. This works by registering a 'get' or a 'set' function for working with your struct.

For example:

#[derive(Clone)]
struct TestStruct {
    x: i64
}

impl TestStruct {
    fn get_x(&mut self) -> i64 {
        self.x
    }

    fn set_x(&mut self, new_x: i64) {
        self.x = new_x;
    }

    fn new() -> TestStruct {
        TestStruct { x: 1 }
    }
}

let mut engine = Engine::new();

engine.register_type::<TestStruct>();

engine.register_get_set("x", TestStruct::get_x, TestStruct::set_x);
engine.register_fn("new_ts", TestStruct::new);

if let Ok(result) = engine.eval::<i64>("let a = new_ts(); a.x = 500; a.x") {
    println!("result: {}", result);
}

WARNING: Gotcha's with Getters

When you get a property, the value is cloned. Any update to it downstream will NOT be reflected back to the custom type.

This can introduce subtle bugs. For example:

fn change(s) {
    s = 42;
}

let a = new_ts();
a.x = 500;
a.x.change();   // Only a COPY of 'a.x' is changed. 'a.x' is NOT changed.
a.x == 500;

Initializing and maintaining state

By default, Rhai treats each engine invocation as a fresh one, persisting only the functions that have been defined but no top-level state. This gives each one a fairly clean starting place. Sometimes, though, you want to continue using the same top-level state from one invocation to the next.

In this example, we first create a state with a few initialized variables, then thread the same state through multiple invocations:

use rhai::{Engine, Scope};

fn main() {
    let mut engine = Engine::new();

    // First create the state
    let mut scope = Scope::new();

    // Then push some initialized variables into the state
    // NOTE: Remember the default numbers used by Rhai are i64 and f64.
    //       Better stick to them or it gets hard to work with other variables in the script.
    scope.push("y".into(), 42_i64);
    scope.push("z".into(), 999_i64);

    // First invocation
    engine.eval_with_scope::<()>(&mut scope, r"
        let x = 4 + 5 - y + z;
        y = 1;
    ").expect("y and z not found?");

    // Second invocation using the same state
    if let Ok(result) = engine.eval_with_scope::<i64>(&mut scope, "x") {
       println!("result: {}", result);  // should print 966
    }

    // Variable y is changed in the script
    assert_eq!(scope.get_value::<i64>("y").unwrap(), 1);
}

Rhai Language guide

Variables

let x = 3;

Numeric operators

let x = (1 + 2) * (6 - 4) / 2;

Comparison operators

You can compare most values of the same data type. If you compare two values of different data types, the result is always false.

42 == 42;           // true
42 > 42;            // false
"hello" > "foo";    // true
"42" == 42;         // false
42 == 42.0;         // false - i64 is different from f64

Boolean operators

Double boolean operators && and || short-circuit, meaning that the second operand will not be evaluated if the first one already proves the condition wrong.

Single boolean operators & and | always evaluate both operands.

this() || that();   // that() is not evaluated if this() is true
this() && that();   // that() is not evaluated if this() is false

this() | that();    // both this() and that() are evaluated
this() & that();    // both this() and that() are evaluated

If

if true {
    print("It's true!");
} else if true {
    print("It's true again!");
} else {
    print("It's false!");
}

While

let x = 10;

while x > 0 {
    print(x);
    if x == 5 { break; }
    x = x - 1;
}

Loop

let x = 10;

loop {
    print(x);
    x = x - 1;
    if x == 0 { break; }
}

Functions

Rhai supports defining functions in script:

fn add(x, y) {
    return x + y;
}

print(add(2, 3));

Just like in Rust, you can also use an implicit return.

fn add(x, y) {
    x + y
}

print(add(2, 3));

Remember that functions defined in script always take Dynamic arguments (i.e. the arguments can be of any type).

Arguments are passed by value, so all functions are pure (i.e. they never modify their arguments).

Furthermore, functions can only be defined at the top level, never inside a block or another function.

// Top level is OK
fn add(x, y) {
    x + y
}

// The following will not compile
fn do_addition(x) {
    fn add_y(n) {   // functions cannot be defined inside another function
        n + y
    }

    add_y(x)
}

Return

return;

return 123 + 456;

Errors and Exceptions

if error != "" {
    throw error;  // 'throw' takes a string to form the exception text
}

throw;  // no exception text

All of Engine's evaluation/consuming methods return Result<T, rhai::EvalAltResult> with EvalAltResult holding error information.

Exceptions thrown via throw in the script can be captured by matching Err(EvalAltResult::ErrorRuntime(reason, position)) with the exception text captured by the reason parameter.

let result = engine.eval::<i64>(&mut scope, r#"
    let x = 42;

    if x > 0 {
        throw x + " is too large!";
    }
"#);

println!(result);   // prints "Runtime error: 42 is too large! (line 5, position 15)"

Arrays

You can create arrays of values, and then access them with numeric indices.

The following functions (defined in the standard library but excluded if you use the no_stdlib feature) operate on arrays:

• push - inserts an element at the end
• pop - removes the last element and returns it (() if empty)
• shift - removes the first element and returns it (() if empty)
• len - returns the number of elements
• pad - pads the array with an element until a specified length
• clear - empties the array
• truncate - cuts off the array at exactly a specified length (discarding all subsequent elements)

let y = [1, 2, 3];      // 3 elements
y[1] = 42;

print(y[1]);            // prints 42

ts.list = y;            // arrays can be assigned completely (by value copy)
let foo = ts.list[1];
foo == 42;

let foo = [1, 2, 3][0];
foo == 1;

fn abc() { [42, 43, 44] }

let foo = abc()[0];
foo == 42;

let foo = y[0];
foo == 1;

y.push(4);              // 4 elements
y.push(5);              // 5 elements

print(y.len());         // prints 5

let first = y.shift();  // remove the first element, 4 elements remaining
first == 1;

let last = y.pop();     // remove the last element, 3 elements remaining
last == 5;

print(y.len());         // prints 3

y.pad(10, "hello");     // pad the array up to 10 elements

print(y.len());         // prints 10

y.truncate(5);          // truncate the array to 5 elements

print(y.len());         // prints 5

y.clear();              // empty the array

print(y.len());         // prints 0

push and pad are only defined for standard built-in types. If you want to use them with your own custom type, you need to define a specific override:

engine.register_fn("push",
    |list: &mut Array, item: MyType| list.push(Box::new(item))
);

The type of a Rhai array is rhai::Array. type_of() returns "array".

For loops

let array = [1, 3, 5, 7, 9, 42];

for x in array {
    print(x);
    if x == 42 { break; }
}

// The range function allows iterating from first..last-1
for x in range(0,50) {
    print(x);
    if x == 42 { break; }
}

Members and methods

let a = new_ts();
a.x = 500;
a.update();

Numbers

let x = 123;            // i64
let x = 123.4;          // f64
let x = 123_456_789;    // separators can be put anywhere inside the number

let x = 0x12abcd;       // i64 in hex
let x = 0o777;          // i64 in oct
let x = 0b1010_1111;    // i64 in binary

Conversion functions (defined in the standard library but excluded if you use the no_stdlib feature):

• to_int - converts an f32 or f64 to i64
• to_float - converts an integer type to f64

Strings and Chars

let name = "Bob";
let middle_initial = 'C';
let last = "Davis";

let full_name = name + " " + middle_initial + ". " + last;
full_name == "Bob C. Davis";

// String building with different types (not available if 'no_stdlib' features is used)
let age = 42;
let record = full_name + ": age " + age;
record == "Bob C. Davis: age 42";

// Strings can be indexed to get a character
let c = record[4];
c == 'C';

ts.s = record;

let c = ts.s[4];
c == 'C';

let c = "foo"[0];
c == 'f';

let c = ("foo" + "bar")[5];
c == 'r';

// Escape sequences in strings
record += " \u2764\n";                  // escape sequence of '❤' in Unicode 
record == "Bob C. Davis: age 42 ❤\n";   // '\n' = new-line

// Unlike Rust, Rhai strings can be modified
record[4] = '\x58'; // 0x58 = 'X'
record == "Bob X. Davis: age 42 ❤\n";

The following standard functions (defined in the standard library but excluded if you use the no_stdlib feature) operate on strings:

• len - returns the number of characters (not number of bytes) in the string
• pad - pads the string with an character until a specified number of characters
• append - Adds a character or a string to the end of another string
• clear - empties the string
• truncate - cuts off the string at exactly a specified number of characters
• contains - checks if a certain character or sub-string occurs in the string
• replace - replaces a substring with another
• trim - trims the string

let full_name == " Bob C. Davis ";
full_name.len() == 14;

full_name.trim();
full_name.len() == 12;
full_name == "Bob C. Davis";

full_name.pad(15, '$');
full_name.len() == 15;
full_name == "Bob C. Davis$$$";

full_name.truncate(6);
full_name.len() == 6;
full_name == "Bob C.";

full_name.replace("Bob", "John");
full_name.len() == 7;
full_name = "John C.";

full_name.contains('C') == true;
full_name.contains("John") == true;

full_name.clear();
full_name.len() == 0;

Print and Debug

print("hello");         // prints hello to stdout
print(1 + 2 + 3);       // prints 6 to stdout
print("hello" + 42);    // prints hello42 to stdout
debug("world!");        // prints "world!" to stdout using debug formatting

Overriding Print and Debug with Callback functions

// Any function that takes a &str argument can be used to override print and debug
engine.on_print(|x| println!("hello: {}", x));
engine.on_debug(|x| println!("DEBUG: {}", x));

// Redirect logging output to somewhere else
let mut log: Vec<String> = Vec::new();
engine.on_print(|x| log.push(format!("log: {}", x)));
engine.on_debug(|x| log.push(format!("DEBUG: {}", x)));
            :
        eval script
            :
println!("{:?}", log);   // 'log' captures all the 'print' and 'debug' results.

Comments

let /* intruder comment */ name = "Bob";
// This is a very important comment
/* This comment spans
   multiple lines, so it
   only makes sense that
   it is even more important */

/* Fear not, Rhai satisfies all your nesting
   needs with nested comments:
   /*/*/*/*/**/*/*/*/*/
*/

Unary operators

let number = -5;
number = -5 - +5;
let booly = !true;

Compound assignment operators

let number = 5;
number += 4;
number -= 3;
number *= 2;
number /= 1;
number %= 3;
number <<= 2;
number >>= 1;

The += operator can also be used to build strings:

let my_str = "abc";
my_str += "ABC";
my_str += 12345;

my_str == "abcABC12345"