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
• Very few additional dependencies (right now only num-traits to do checked arithmetic operations)

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

Feature	Description
debug_msgs	Print debug messages to stdout related to function registrations and calls.
no_stdlib	Exclude the standard library of utility functions in the build, and only include the minimum necessary functionalities. Standard types are not affected.
unchecked	Exclude arithmetic checking (such as overflows and division by zero). Beware that a bad script may panic the entire system!
no_function	Disable script-defined functions if you don't need them.
no_index	Disable arrays and indexing features if you don't need them.
no_float	Disable floating-point numbers and math if you don't need them.
only_i32	Set the system integer type to i32 and disable all other integer types.
only_i64	Set the system integer type to i64 and disable all other integer types.

By default, Rhai includes all the standard functionalities in a small, tight package. Most features are here for you to opt-out of certain functionalities that you do not need. Excluding unneeded functionalities can result in smaller, faster builds as well as less bugs due to a more restricted language.

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

A number of examples can be found in the examples folder:

Example	Description
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, interactively evaluate statements from stdin

Examples can be run with the following command:

cargo run --example name

The repl example is a particularly good one as it allows you to interactively try out Rhai's language features in a standard REPL (Read-Eval-Print Loop).

Example Scripts

There are also a number of examples scripts that showcase Rhai's features, all in the scripts folder:

Language feature scripts	Description
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
string.rhai	string operations
while.rhai	while loop

Example scripts	Description
speed_test.rhai	a simple program to measure the speed of Rhai's interpreter
primes.rhai	use Sieve of Eratosthenes to find all primes smaller than a limit

To run the scripts, either make a tiny program or use of the rhai_runner example:

cargo run --example rhai_runner scripts/any_script.rhai

Hello world

To get going with Rhai, create an instance of the scripting engine and then call eval:

use rhai::{Engine, EvalAltResult};

fn main() -> Result<(), EvalAltResult>
{
    let mut engine = Engine::new();

    let result = engine.eval::<i64>("40 + 2")?;

    println!("Answer: {}", result);  // prints 42

    Ok(())
}

You can also evaluate a script file:

let 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:

use rhai::Engine;

let mut engine = Engine::new();

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

for _ in 0..42 {
    let result = engine.eval_ast::<i64>(&ast)?;

    println!("Answer: {}", result);  // prints 42
}

Compiling a script file is also supported:

use rhai::Engine;

let mut engine = Engine::new();

let ast = engine.compile_file("hello_world.rhai".into()).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:

use rhai::Engine;

let mut engine = Engine::new();

// Define a function in a script and load it into the Engine.
engine.consume(
    r"
        fn hello(x, y) {    // a function with two parameters: String and i64
            x.len() + y     // returning i64
        }

        fn hello(x) {       // functions can be overloaded: this one takes only one parameter
            x * 2           // returning i64
        }
    ", true)?;              // pass true to 'retain_functions' otherwise these functions
                            // will be cleared at the end of consume()

// Evaluate the function in the AST, passing arguments into the script as a tuple
// if there are more than one. Beware, arguments must be of the correct types because
// Rhai does not have built-in type conversions. If you pass in arguments of the wrong type,
// the Engine will not find the function.

let result: i64 = engine.call_fn("hello", &ast, ( String::from("abc"), 123_i64 ) )?;
//                                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ put arguments in a tuple

let result: i64 = engine.call_fn("hello", 123_i64)?
//                                        ^^^^^^^ calls 'hello' with one parameter (no need for tuple)

Values and types

The following primitive types are supported natively:

Category	Types
Integer	u8, i8, u16, i16, u32, i32 (default for only_i32), u64, i64 (default)
Floating-point (disabled with no_float)	f32, f64 (default)
Character	char
Boolean	bool
Array (disabled with no_index)	rhai::Array
Dynamic (i.e. can be anything)	rhai::Dynamic
System (current configuration)	rhai::INT (i32 or i64), rhai::FLOAT (f32 or f64)

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

The default integer type is i64. If you do not need any other integer type, you can enable the only_i64 feature.

If you only need 32-bit integers, you can enable the only_i32 feature and remove support for all integer types other than i32 including i64. This is useful on 32-bit systems where using 64-bit integers incurs a performance penalty.

If you do not need floating-point, enable the no_float feature to remove support.

Value conversions

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, EvalAltResult};
use rhai::RegisterFn;                       // use `RegisterFn` trait for `register_fn`
use rhai::{Dynamic, RegisterDynamicFn};     // use `RegisterDynamicFn` trait for `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() -> Result<(), EvalAltResult>
{
    let mut engine = Engine::new();

    engine.register_fn("add", add);

    let 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);

    let result = engine.eval::<i64>("get_an_any()")?;

    println!("Answer: {}", result);  // prints 42

    Ok(())
}

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 show_it<T: Display>(x: &mut T) -> () {
    println!("put up a good show: {}!", x)
}

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

    engine.register_fn("print", show_it as fn(x: &mut i64)->());
    engine.register_fn("print", show_it as fn(x: &mut bool)->());
    engine.register_fn("print", show_it 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 the correct one, based on the types of the parameters, 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;     // use `RegisterResultFn` trait for `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 happens?

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, EvalAltResult};
use rhai::RegisterFn;

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

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

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

fn main() -> Result<(), EvalAltResult>
{
    let mut engine = Engine::new();

    engine.register_type::<TestStruct>();

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

    let result = engine.eval::<TestStruct>("let x = new_ts(); x.update(); x")?;

    println!("result: {}", result.x); // prints 1001

    Ok(())
}

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.

let 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);

let 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);

let result = engine.eval::<i64>("let a = new_ts(); a.x = 500; a.x")?;

println!("result: {}", result);

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, EvalAltResult};

fn main() -> Result<(), EvalAltResult>
{
    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 system number types in Rhai are i64 (i32 if 'only_i32') ond f64.
    //       Better stick to them or it gets hard working with 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;
    ")?;

    // Second invocation using the same state
    let 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")?, 1);

    Ok(())
}

Rhai Language guide

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:
   /*/*/*/*/**/*/*/*/*/
*/

Variables

Variables in Rhai follow normal naming rules (i.e. must contain only ASCII letters, digits and '_' underscores).

let x = 3;

Numbers

Format	Type
123_345, -42	i64 in decimal, '_' separators are ignored
0o07_76	i64 in octal, '_' separators are ignored
0xabcd_ef	i64 in hex, '_' separators are ignored
0b0101_1001	i64 in binary, '_' separators are ignored
123_456.789	f64, '_' separators are ignored

Numeric operators

let x = (1 + 2) * (6 - 4) / 2;  // arithmetic
let reminder = 42 % 10;         // modulo
let power = 42 ~ 2;             // power (i64 and f64 only)
let left_shifted = 42 << 3;     // left shift
let right_shifted = 42 >> 3;    // right shift
let bit_op = 42 | 99;           // bit masking

Unary operators

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

Numeric functions

The following standard functions (defined in the standard library but excluded if no_stdlib) operate on i8, i16, i32, i64, f32 and f64 only:

Function	Description
abs	absolute value
to_float	converts an integer type to f64

Floating-point functions

The following standard functions (defined in the standard library but excluded if no_stdlib) operate on f64 only:

Category	Functions
Trigonometry	sin, cos, tan, sinh, cosh, tanh in degrees
Arc-trigonometry	asin, acos, atan, asinh, acosh, atanh in degrees
Square root	sqrt
Exponential	exp (base e)
Logarithmic	ln (base e), log10 (base 10), log (any base)
Rounding	floor, ceiling, round, int, fraction
Conversion	to_int
Testing	is_nan, is_finite, is_infinite

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
let age = 42;
let record = full_name + ": age " + age;
record == "Bob C. Davis: age 42";

// Strings can be indexed to get a character
// (disabled with the 'no_index' feature)
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 no_stdlib) operate on strings:

Function	Description
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

Examples:

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;

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 no_stdlib) operate on arrays:

Function	Description
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)

Examples:

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 register a type-specific version:

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".

Arrays are disabled via the no_index feature.

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

Compound assignment operators

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

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

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

my_str == "abcABC12345"

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; }
}

For

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

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

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

Return

return;     // equivalent to return ();

return 123 + 456;

Errors and Exceptions

if some_bad_condition_has_happened {
    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)"

Functions

Rhai supports defining functions in script (unless disabled with no_function):

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));

Functions defined in script always take Dynamic parameters (i.e. the parameter can be of any type). It is important to remember that all parameters are passed by value, so all functions are pure (i.e. they never modify their parameters). Any update to an argument will not be reflected back to the caller. This can introduce subtle bugs, if you are not careful.

fn change(s) {
    s = 42;     // only a COPY of 'x' is changed
}

let x = 500;
x.change();
x == 500;       // 'x' is NOT changed!

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)
}

Functions can be overloaded based on the number of parameters (but not parameter types, since all parameters are Dynamic). New definitions of the same name and number of parameters overwrite previous definitions.

fn abc(x,y,z) { print("Three!!! " + x + "," + y + "," + z) }
fn abc(x) { print("One! " + x) }
fn abc(x,y) { print("Two! " + x + "," + y) }
fn abc() { print("None.") }
fn abc(x) { print("HA! NEW ONE! " + x) }    // overwrites previous definition

abc(1,2,3);     // prints "Three!!! 1,2,3"
abc(42);        // prints "HA! NEW ONE! 42"
abc(1,2);       // prints "Two!! 1,2"
abc();          // prints "None."

Members and methods

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

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

// Example: quick-'n-dirty logging
let mut log: Vec<String> = Vec::new();

// Redirect print/debug output to 'log'
engine.on_print(|s| log.push(format!("entry: {}", s)));
engine.on_debug(|s| log.push(format!("DEBUG: {}", s)));

// Evaluate script
engine.eval::<()>(script)?;

// 'log' captures all the 'print' and 'debug' output
for entry in log {
    println!("{}", entry);
}

Optimizations

Rhai includes an optimizer that tries to optimize a script after parsing. This can reduce resource utilization and increase execution speed.

For example, in the following:

{
    let x = 999;        // NOT eliminated - Rhai doesn't check yet whether a variable is used later on
    123;                // eliminated - no effect
    "hello";            // eliminated - no effect
    [1, 2, x, x*2, 5];  // eliminated - no effect
    foo(42);            // NOT eliminated - the function 'foo' may have side effects
    666                 // NOT eliminated - this is the return value of the block,
                        //                  and the block is the last one
                        //                  so this is the return value of the whole script
}

Rhai attempts to eliminate dead code (i.e. code that does nothing, for example an expression by itself as a statement, which is allowed in Rhai). The above script optimizes to:

{
    let x = 999;
    foo(42);
    666
}

Constant propagation is used to remove dead code:

if true || some_work() { print("done!"); }  // since '||' short-circuits, 'some_work' is never called
if true { print("done!"); }                 // <-- the line above is equivalent to this
print("done!");                             // <-- the line above is further simplified to this
                                            //     because the condition is always true

These are quite effective for template-based machine-generated scripts where certain constant values are spliced into the script text in order to turn on/off certain sections.

Beware, however, that most operators are actually function calls, and those functions can be overridden, so they are not optimized away:

if 1 == 1 { ... }       // '1==1' is NOT optimized away because you can define
                        // your own '==' function to override the built-in default!

Here be dragons!

Some optimizations can be quite aggressive and can alter subtle semantics of the script. For example:

if true {       // <-- condition always true
    123.456;    // <-- eliminated
    hello;      // <-- eliminated, EVEN THOUGH the variable doesn't exist!
    foo(42)     // <-- promoted up-level
}

// The above optimizes to:

foo(42)

Nevertheless, if you would be evaluating the original script, it would have been an error - the variable hello doesn't exist, so the script would have been terminated at that point with an error return.

In fact, any errors inside a statement that has been eliminated will silently go away:

print("start!");
if my_decision { /* do nothing... */ }  // <-- eliminated due to no effect
print("end!");

// The above optimizes to:

print("start!");
print("end!");

In the script above, if my_decision holds anything other than a boolean value, the script should have been terminated due to a type error. However, after optimization, the entire if statement is removed, thus the script silently runs to completion without errors.

It is usually a bad idea to depend on a script failing or such kind of subtleties, but if it turns out to be necessary (why? I would never guess), there is a setting in Engine to turn off optimizations.

let engine = rhai::Engine::new();
engine.set_optimization(false);     // turn off the optimizer