| .ci | ||
| .github/workflows | ||
| benches | ||
| examples | ||
| scripts | ||
| src | ||
| tests | ||
| _config.yml | ||
| .gitignore | ||
| .travis.yml | ||
| Cargo.toml | ||
| LICENSE.txt | ||
| README.md | ||
Rhai - Embedded Scripting for Rust
Rhai is an embedded scripting language and evaluation engine for Rust that gives a safe and easy way to add scripting to any application.
Rhai's current features set:
- Easy-to-use language similar to JS+Rust
- Easy integration with Rust native functions and types, including getter/setter/methods
- Easily call a script-defined function from Rust
- Freely pass variables/constants into a script via an external
Scope - Fairly efficient (1 million 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)
no-stdsupport- Support for function overloading
- Support for operator overloading
- Support for loading external modules
- Compiled script is optimized for repeat evaluations
- Support for minimal builds by excluding unneeded language features
- Very few additional dependencies (right now only
num-traitsto do checked arithmetic operations); forno-stdbuilds, a number of additional dependencies are pulled in to provide for functionalities that used to be instd.
Note: Currently, the version is 0.13.0, so the language and API's may change before they stabilize.
Installation
Install the Rhai crate by adding this line to dependencies:
[dependencies]
rhai = "0.13.0"
Use the latest released crate version on crates.io:
[dependencies]
rhai = "*"
Crate versions are released on crates.io infrequently, so if you want to track the
latest features, enhancements and bug fixes, pull directly from GitHub:
[dependencies]
rhai = { git = "https://github.com/jonathandturner/rhai" }
Beware that in order to use pre-releases (e.g. alpha and beta), the exact version must be specified in the Cargo.toml.
Optional features
| Feature | Description |
|---|---|
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 not needed. |
no_index |
Disable arrays and indexing features if not needed. |
no_object |
Disable support for custom types and objects. |
no_float |
Disable floating-point numbers and math if not needed. |
no_optimize |
Disable the script optimizer. |
no_module |
Disable modules. |
only_i32 |
Set the system integer type to i32 and disable all other integer types. INT is set to i32. |
only_i64 |
Set the system integer type to i64 and disable all other integer types. INT is set to i64. |
no_std |
Build for no-std. Notice that additional dependencies will be pulled in to replace std features. |
sync |
Restrict all values types to those that are Send + Sync. Under this feature, Engine, Scope and AST are all Send + Sync. |
By default, Rhai includes all the standard functionalities in a small, tight package. Most features are here to opt-out of certain functionalities that are not needed. Excluding unneeded functionalities can result in smaller, faster builds as well as less bugs due to a more restricted language.
Performance builds
Some features are for performance. For example, using only_i32 or only_i64 disables all other integer types (such as u16).
If only a single integer type is needed in scripts - most of the time this is the case - it is best to avoid registering
lots of functions related to other integer types that will never be used. As a result, performance will improve.
If only 32-bit integers are needed - again, most of the time this is the case - using only_i32 disables also i64.
On 64-bit targets this may not gain much, but on some 32-bit targets this improves performance due to 64-bit arithmetic
requiring more CPU cycles to complete.
Also, turning on no_float, and only_i32 makes the key Dynamic data type only 8 bytes small on 32-bit targets
while normally it can be up to 16 bytes (e.g. on x86/x64 CPU's) in order to hold an i64 or f64.
Making Dynamic small helps performance due to more caching efficiency.
Minimal builds
In order to compile a _minimal_build - i.e. a build optimized for size - perhaps for embedded targets, it is essential that
the correct linker flags are used in cargo.toml:
[profile.release]
opt-level = "z" # optimize for size
Opt out of as many features as possible, if they are not needed, to reduce code size because, remember, by default all code is compiled in as what a script requires cannot be predicted. If a language feature is not needed, omitting them via special features is a prudent strategy to optimize the build for size.
Omitting arrays (no_index) yields the most code-size savings, followed by floating-point support
(no_float), checked arithmetic (unchecked) and finally object maps and custom types (no_object).
Disable script-defined functions (no_function) only when the feature is not needed because code size savings is minimal.
Engine::new_raw creates a raw engine which does not register any utility functions.
This makes the scripting language quite useless as even basic arithmetic operators are not supported.
Selectively include the necessary operators by loading specific packages while minimizing the code footprint.
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 led by my cousin.
- 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 |
no_std |
example to test out no-std builds |
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 (1 million iterations) |
primes.rhai |
use Sieve of Eratosthenes to find all primes smaller than a limit |
fibonacci.rhai |
calculate the n-th Fibonacci number using a really dumb algorithm |
mat_mul.rhai |
matrix multiplication test to measure the speed of Rhai's interpreter |
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 via Engine::new and then call the eval method:
use rhai::{Engine, EvalAltResult};
fn main() -> Result<(), Box<EvalAltResult>>
{
let engine = Engine::new();
let result = engine.eval::<i64>("40 + 2")?;
println!("Answer: {}", result); // prints 42
Ok(())
}
EvalAltResult is a Rust enum containing all errors encountered during the parsing or evaluation process.
Script evaluation
The type parameter is used to specify the type of the return value, which must match the actual type or an error is returned.
Rhai is very strict here. Use Dynamic for uncertain return types.
There are two ways to specify the return type - turbofish notation, or type inference.
let result = engine.eval::<i64>("40 + 2")?; // return type is i64, specified using 'turbofish' notation
let result: i64 = engine.eval("40 + 2")?; // return type is inferred to be i64
result.is::<i64>() == true;
let result: Dynamic = engine.eval("boo()")?; // use 'Dynamic' if you're not sure what type it'll be!
let result = engine.eval::<String>("40 + 2")?; // returns an error because the actual return type is i64, not String
Evaluate a script file directly:
let result = engine.eval_file::<i64>("hello_world.rhai".into())?; // 'eval_file' takes a 'PathBuf'
Compiling scripts (to AST)
To repeatedly evaluate a script, 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")?;
for _ in 0..42 {
let result: i64 = engine.eval_ast(&ast)?;
println!("Answer #{}: {}", i, result); // prints 42
}
Compiling a script file is also supported:
let ast = engine.compile_file("hello_world.rhai".into())?;
Calling Rhai functions from Rust
Rhai also allows working backwards from the other direction - i.e. calling a Rhai-scripted function from Rust via call_fn.
// Define functions in a script.
let ast = engine.compile(true,
r"
// a function with two parameters: String and i64
fn hello(x, y) {
x.len() + y
}
// functions can be overloaded: this one takes only one parameter
fn hello(x) {
x * 2
}
// this one takes no parameters
fn hello() {
42
}
")?;
// A custom scope can also contain any variables/constants available to the functions
let mut scope = Scope::new();
// Evaluate a function defined in the script, 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.
// If arguments of the wrong types are passed, the Engine will not find the function.
let result: i64 = engine.call_fn(&mut scope, &ast, "hello", ( String::from("abc"), 123_i64 ) )?;
// ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
// put arguments in a tuple
let result: i64 = engine.call_fn(&mut scope, &ast, "hello", (123_i64,) )?
// ^^^^^^^^^^ tuple of one
let result: i64 = engine.call_fn(&mut scope, &ast, "hello", () )?
// ^^ unit = tuple of zero
Creating Rust anonymous functions from Rhai script
It is possible to further encapsulate a script in Rust such that it becomes a normal Rust function.
Such an anonymous function is basically a boxed closure, very useful as call-back functions.
Creating them is accomplished via the Func trait which contains create_from_script
(as well as its companion method create_from_ast):
use rhai::{Engine, Func}; // use 'Func' for 'create_from_script'
let engine = Engine::new(); // create a new 'Engine' just for this
let script = "fn calc(x, y) { x + y.len() < 42 }";
// Func takes two type parameters:
// 1) a tuple made up of the types of the script function's parameters
// 2) the return type of the script function
//
// 'func' will have type Box<dyn Fn(i64, String) -> Result<bool, Box<EvalAltResult>>> and is callable!
let func = Func::<(i64, String), bool>::create_from_script(
// ^^^^^^^^^^^^^ function parameter types in tuple
engine, // the 'Engine' is consumed into the closure
script, // the script, notice number of parameters must match
"calc" // the entry-point function name
)?;
func(123, "hello".to_string())? == false; // call the anonymous function
schedule_callback(func); // pass it as a callback to another function
// Although there is nothing you can't do by manually writing out the closure yourself...
let engine = Engine::new();
let ast = engine.compile(script)?;
schedule_callback(Box::new(move |x: i64, y: String| -> Result<bool, Box<EvalAltResult>> {
engine.call_fn(&mut Scope::new(), &ast, "calc", (x, y))
}));
Raw Engine
Engine::new creates a scripting Engine with common functionalities (e.g. printing to the console via print).
In many controlled embedded environments, however, these are not needed.
Use Engine::new_raw to create a raw Engine, in which nothing is added, not even basic arithmetic and logic operators!
Packages
Rhai functional features are provided in different packages that can be loaded via a call to load_package.
Packages reside under rhai::packages::* and the trait rhai::packages::Package must be imported in order for
packages to be used.
use rhai::Engine;
use rhai::packages::Package // load the 'Package' trait to use packages
use rhai::packages::CorePackage; // the 'core' package contains basic functionalities (e.g. arithmetic)
let mut engine = Engine::new_raw(); // create a 'raw' Engine
let package = CorePackage::new(); // create a package - can be shared among multiple `Engine` instances
engine.load_package(package.get()); // load the package manually
The follow packages are available:
| Package | Description | In CorePackage |
In StandardPackage |
|---|---|---|---|
ArithmeticPackage |
Arithmetic operators (e.g. +, -, *, /) |
Yes | Yes |
BasicIteratorPackage |
Numeric ranges (e.g. range(1, 10)) |
Yes | Yes |
LogicPackage |
Logic and comparison operators (e.g. ==, >) |
Yes | Yes |
BasicStringPackage |
Basic string functions | Yes | Yes |
BasicTimePackage |
Basic time functions (e.g. timestamps) | Yes | Yes |
MoreStringPackage |
Additional string functions | No | Yes |
BasicMathPackage |
Basic math functions (e.g. sin, sqrt) |
No | Yes |
BasicArrayPackage |
Basic array functions | No | Yes |
BasicMapPackage |
Basic object map functions | No | Yes |
CorePackage |
Basic essentials | ||
StandardPackage |
Standard library |
Evaluate expressions only
Sometimes a use case does not require a full-blown scripting language, but only needs to evaluate expressions.
In these cases, use the compile_expression and eval_expression methods or their _with_scope variants.
let result = engine.eval_expression::<i64>("2 + (10 + 10) * 2")?;
When evaluation expressions, no full-blown statement (e.g. if, while, for) - not even variable assignments -
is supported and will be considered parse errors when encountered.
// The following are all syntax errors because the script is not an expression.
engine.eval_expression::<()>("x = 42")?;
let ast = engine.compile_expression("let x = 42")?;
let result = engine.eval_expression_with_scope::<i64>(&mut scope, "if x { 42 } else { 123 }")?;
Values and types
The following primitive types are supported natively:
| Category | Equivalent Rust types | type_of() |
to_string() |
|---|---|---|---|
| Integer number | u8, i8, u16, i16, u32, i32 (default for only_i32),u64, i64 (default) |
"i32", "u64" etc. |
"42", "123" etc. |
Floating-point number (disabled with no_float) |
f32, f64 (default) |
"f32" or "f64" |
"123.4567" etc. |
| Boolean value | bool |
"bool" |
"true" or "false" |
| Unicode character | char |
"char" |
"A", "x" etc. |
| Unicode string | String (not &str) |
"string" |
"hello" etc. |
Array (disabled with no_index) |
rhai::Array |
"array" |
"[ ?, ?, ? ]" |
Object map (disabled with no_object) |
rhai::Map |
"map" |
#{ "a": 1, "b": 2 } |
Timestamp (implemented in the BasicTimePackage) |
std::time::Instant |
"timestamp" |
not supported |
| Dynamic value (i.e. can be anything) | rhai::Dynamic |
the actual type | actual value |
| System integer (current configuration) | rhai::INT (i32 or i64) |
"i32" or "i64" |
"42", "123" etc. |
System floating-point (current configuration, disabled with no_float) |
rhai::FLOAT (f32 or f64) |
"f32" or "f64" |
"123.456" etc. |
| Nothing/void/nil/null (or whatever you want to call it) | () |
"()" |
"" (empty string) |
All types are treated strictly separate by Rhai, meaning that i32 and i64 and u32 are completely different -
they even cannot be added together. This is very similar to Rust.
The default integer type is i64. If other integer types are not needed, it is possible to exclude them and make a
smaller build with the only_i64 feature.
If only 32-bit integers are needed, enabling the only_i32 feature will remove support for all integer types other than i32, including i64.
This is useful on some 32-bit targets where using 64-bit integers incur a performance penalty.
If no floating-point is needed or supported, use the no_float feature to remove it.
The to_string function converts a standard type into a string for display purposes.
The type_of function detects the actual type of a value. This is useful because all variables are Dynamic in nature.
// Use 'type_of()' to get the actual types of values
type_of('c') == "char";
type_of(42) == "i64";
let x = 123;
x.type_of() == "i64"; // method-call style is also OK
type_of(x) == "i64";
x = 99.999;
type_of(x) == "f64";
x = "hello";
if type_of(x) == "string" {
do_something_with_string(x);
}
Dynamic values
A Dynamic value can be any type. However, if the sync feature is used, then all types must be Send + Sync.
Because type_of() a Dynamic value returns the type of the actual value, it is usually used to perform type-specific
actions based on the actual value's type.
let mystery = get_some_dynamic_value();
if type_of(mystery) == "i64" {
print("Hey, I got an integer here!");
} else if type_of(mystery) == "f64" {
print("Hey, I got a float here!");
} else if type_of(mystery) == "string" {
print("Hey, I got a string here!");
} else if type_of(mystery) == "bool" {
print("Hey, I got a boolean here!");
} else if type_of(mystery) == "array" {
print("Hey, I got an array here!");
} else if type_of(mystery) == "map" {
print("Hey, I got an object map here!");
} else if type_of(mystery) == "TestStruct" {
print("Hey, I got the TestStruct custom type here!");
} else {
print("I don't know what this is: " + type_of(mystery));
}
In Rust, sometimes a Dynamic forms part of a returned value - a good example is an array with Dynamic elements,
or an object map with Dynamic property values. To get the real values, the actual value types must be known in advance.
There is no easy way for Rust to decide, at run-time, what type the Dynamic value is (short of using the type_name
function and match against the name).
A Dynamic value's actual type can be checked via the is method.
The cast method then converts the value into a specific, known type.
Alternatively, use the try_cast method which does not panic but returns None when the cast fails.
let list: Array = engine.eval("...")?; // return type is 'Array'
let item = list[0]; // an element in an 'Array' is 'Dynamic'
item.is::<i64>() == true; // 'is' returns whether a 'Dynamic' value is of a particular type
let value = item.cast::<i64>(); // if the element is 'i64', this succeeds; otherwise it panics
let value: i64 = item.cast(); // type can also be inferred
let value = item.try_cast::<i64>().unwrap(); // 'try_cast' does not panic when the cast fails, but returns 'None'
The type_name method gets the name of the actual type as a static string slice, which you may match against.
let list: Array = engine.eval("...")?; // return type is 'Array'
let item = list[0]; // an element in an 'Array' is 'Dynamic'
match item.type_name() { // 'type_name' returns the name of the actual Rust type
"i64" => ...
"alloc::string::String" => ...
"bool" => ...
"path::to::module::TestStruct" => ...
}
The following conversion traits are implemented for Dynamic:
From<i64>(i32ifonly_i32)From<f64>(if notno_float)From<bool>From<String>From<char>From<Vec<T>>(into an array)From<HashMap<String, T>>(into an object map).
Value conversions
The to_float function converts a supported number to FLOAT (f32 or f64),
and the to_int function converts a supported number to INT (i32 or i64).
That's about it. For other conversions, register custom conversion functions.
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"
Traits
A number of traits, under the rhai:: module namespace, provide additional functionalities.
| Trait | Description | Methods |
|---|---|---|
RegisterFn |
Trait for registering functions | register_fn |
RegisterDynamicFn |
Trait for registering functions returning Dynamic |
register_dynamic_fn |
RegisterResultFn |
Trait for registering fallible functions returning Result<T, Box<EvalAltResult>> |
register_result_fn |
Func |
Trait for creating anonymous functions from script | create_from_ast, create_from_script |
Working with functions
Rhai's scripting engine is very lightweight. It gets most of its abilities from functions.
To call these functions, they need to be registered with the Engine.
use rhai::{Dynamic, 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 {
Dynamic::from(42_i64)
}
fn main() -> Result<(), Box<EvalAltResult>>
{
let 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 from a Rust function, use the Dynamic::from method.
use rhai::Dynamic;
fn decide(yes_no: bool) -> Dynamic {
if yes_no {
Dynamic::from(42_i64)
} else {
Dynamic::from(String::from("hello!")) // remember &str is not supported by Rhai
}
}
Generic functions
Rust generic functions can be used in Rhai, but separate instances for each concrete type must be registered separately. This is essentially function overloading (Rhai does not natively support generics).
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 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)->());
}
This example shows how to register multiple functions (or, in this case, multiple overloaded versions of the same function) under the same name. This enables function overloading based on the number and types of parameters.
Fallible functions
If a function is fallible (i.e. it returns a Result<_, Error>), it can be registered with register_result_fn
(using the RegisterResultFn trait).
The function must return Result<_, Box<EvalAltResult>>. Box<EvalAltResult> implements From<&str> and From<String> etc.
and the error text gets converted into Box<EvalAltResult::ErrorRuntime>.
The error values are Box-ed in order to reduce memory footprint of the error path, which should be hit rarely.
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, Box<EvalAltResult>> {
if y == 0 {
// Return an error if y is zero
Err("Division by zero!".into()) // short-cut to create Box<EvalAltResult::ErrorRuntime>
} else {
Ok(x / y)
}
}
fn main()
{
let 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?
Operator overloading
In Rhai, a lot of functionalities are actually implemented as functions, including basic operations such as arithmetic calculations.
For example, in the expression "a + b", the + operator is not built-in, but calls a function named "+" instead!
let x = a + b;
let x = +(a, b); // <- the above is equivalent to this function call
Similarly, comparison operators including ==, != etc. are all implemented as functions, with the stark exception of && and ||.
Because they short-circuit, && and || are handled specially and not via a function; as a result,
overriding them has no effect at all.
Operator functions cannot be defined as a script function (because operators syntax are not valid function names).
However, operator functions can be registered to the Engine via register_fn, register_result_fn etc.
When a custom operator function is registered with the same name as an operator, it overloads (or overrides) the built-in version.
use rhai::{Engine, EvalAltResult, RegisterFn};
let mut engine = Engine::new();
fn strange_add(a: i64, b: i64) -> i64 { (a + b) * 42 }
engine.register_fn("+", strange_add); // overload '+' operator for two integers!
let result: i64 = engine.eval("1 + 0"); // the overloading version is used
println!("result: {}", result); // prints 42
let result: f64 = engine.eval("1.0 + 0.0"); // '+' operator for two floats not overloaded
println!("result: {}", result); // prints 1.0
Use operator overloading for custom types (described below) only. Be very careful when overloading built-in operators because script writers expect standard operators to behave in a consistent and predictable manner, and will be annoyed if a calculation for '+' turns into a subtraction, for example.
Operator overloading also impacts script optimization when using OptimizationLevel::Full.
See the relevant section for more details.
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 {
field: i64
}
impl TestStruct {
fn update(&mut self) {
self.field += 41;
}
fn new() -> Self {
TestStruct { field: 1 }
}
}
fn main() -> Result<(), Box<EvalAltResult>>
{
let 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.field); // prints 42
Ok(())
}
All custom types must implement Clone. This allows the Engine to pass by value.
You can turn off support for custom types via the no_object feature.
#[derive(Clone)]
struct TestStruct {
field: 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.field += 41;
}
fn new() -> Self {
TestStruct { field: 1 }
}
}
let engine = Engine::new();
engine.register_type::<TestStruct>();
To use native types, methods and functions with the Engine, we need to register them.
There are some convenience functions to help with these. Below, the update and new methods are registered with the Engine.
Note: 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); // registers 'update(&mut TestStruct)'
engine.register_fn("new_ts", TestStruct::new); // registers '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.field); // prints 42
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 a &mut first argument.
fn foo(ts: &mut TestStruct) -> i64 {
ts.field
}
engine.register_fn("foo", foo);
let result = engine.eval::<i64>("let x = new_ts(); x.foo()")?;
println!("result: {}", result); // prints 1
If the no_object feature is turned on, however, the method style of function calls
(i.e. calling a function as an object-method) is no longer supported.
// Below is a syntax error under 'no_object' because 'len' cannot be called in method style.
let result = engine.eval::<i64>("let x = [1, 2, 3]; x.len()")?;
type_of() works fine with custom types and returns the name of the type.
If register_type_with_name is used to register the custom type
with a special "pretty-print" name, type_of() will return that name instead.
engine.register_type::<TestStruct>();
engine.register_fn("new_ts", TestStruct::new);
let x = new_ts();
print(x.type_of()); // prints "path::to::module::TestStruct"
engine.register_type_with_name::<TestStruct>("Hello");
engine.register_fn("new_ts", TestStruct::new);
let x = new_ts();
print(x.type_of()); // prints "Hello"
Getters and setters
Similarly, custom types can expose members by registering a get and/or set function.
#[derive(Clone)]
struct TestStruct {
field: i64
}
impl TestStruct {
fn get_field(&mut self) -> i64 {
self.field
}
fn set_field(&mut self, new_val: i64) {
self.field = new_val;
}
fn new() -> Self {
TestStruct { field: 1 }
}
}
let engine = Engine::new();
engine.register_type::<TestStruct>();
engine.register_get_set("xyz", TestStruct::get_field, TestStruct::set_field);
engine.register_fn("new_ts", TestStruct::new);
let result = engine.eval::<i64>("let a = new_ts(); a.xyz = 42; a.xyz")?;
println!("Answer: {}", result); // prints 42
Needless to say, register_type, register_type_with_name, register_get, register_set and register_get_set
are not available when the no_object feature is turned on.
Scope - 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 global state. This gives each evaluation a clean starting slate. In order to continue using the same global state
from one invocation to the next, such a state must be manually created and passed in.
All Scope variables are Dynamic, meaning they can store values of any type. If the sync feature is used, however,
then only types that are Send + Sync are supported, and the entire Scope itself will also be Send + Sync.
This is extremely useful in multi-threaded applications.
In this example, a global state object (a Scope) is created with a few initialized variables, then the same state is
threaded through multiple invocations:
use rhai::{Engine, Scope, EvalAltResult};
fn main() -> Result<(), Box<EvalAltResult>>
{
let engine = Engine::new();
// First create the state
let mut scope = Scope::new();
// Then push (i.e. add) some initialized variables into the state.
// 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", 42_i64);
scope.push("z", 999_i64);
// 'set_value' adds a variable when one doesn't exist
scope.set_value("s", "hello, world!".to_string()); // remember to use 'String', not '&str'
// First invocation
engine.eval_with_scope::<()>(&mut scope, r"
let x = 4 + 5 - y + z + s.len();
y = 1;
")?;
// Second invocation using the same state
let result = engine.eval_with_scope::<i64>(&mut scope, "x")?;
println!("result: {}", result); // prints 979
// Variable y is changed in the script - read it with 'get_value'
assert_eq!(scope.get_value::<i64>("y").expect("variable y should exist"), 1);
// We can modify scope variables directly with 'set_value'
scope.set_value("y", 42_i64);
assert_eq!(scope.get_value::<i64>("y").expect("variable y should exist"), 42);
Ok(())
}
Engine configuration options
| Method | Description |
|---|---|
set_optimization_level |
Set the amount of script optimizations performed. See script optimization. |
set_max_call_levels |
Set the maximum number of function call levels (default 50) to avoid infinite recursion. |
Rhai Language Guide
Comments
Comments are C-style, including '/* ... */' pairs and '//' for comments to the end of the line.
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 nesting needs with nested comments:
/*/*/*/*/**/*/*/*/*/
*/
Statements
Statements are terminated by semicolons ';' - they are mandatory, except for the last statement where it can be omitted.
A statement can be used anywhere where an expression is expected. The last statement of a statement block
(enclosed by '{' .. '}' pairs) is always the return value of the statement. If a statement has no return value
(e.g. variable definitions, assignments) then the value will be ().
let a = 42; // normal assignment statement
let a = foo(42); // normal function call statement
foo < 42; // normal expression as statement
let a = { 40 + 2 }; // 'a' is set to the value of the statement block, which is the value of the last statement
// ^ notice that the last statement does not require a terminating semicolon (although it also works with it)
// ^ notice that a semicolon is required here to terminate the assignment statement; it is syntax error without it
4 * 10 + 2 // this is also a statement, which is an expression, with no ending semicolon because
// it is the last statement of the whole block
Variables
Variables in Rhai follow normal C naming rules (i.e. must contain only ASCII letters, digits and underscores '_').
Variable names must start with an ASCII letter or an underscore '_', must contain at least one ASCII letter,
and must start with an ASCII letter before a digit.
Therefore, names like '_', '_42', '3a' etc. are not legal variable names, but '_c3po' and 'r2d2' are.
Variable names are also case sensitive.
Variables are defined using the let keyword. A variable defined within a statement block is local to that block.
let x = 3; // ok
let _x = 42; // ok
let x_ = 42; // also ok
let _x_ = 42; // still ok
let _ = 123; // <- syntax error: illegal variable name
let _9 = 9; // <- syntax error: illegal variable name
let x = 42; // variable is 'x', lower case
let X = 123; // variable is 'X', upper case
x == 42;
X == 123;
{
let x = 999; // local variable 'x' shadows the 'x' in parent block
x == 999; // access to local 'x'
}
x == 42; // the parent block's 'x' is not changed
Constants
Constants can be defined using the const keyword and are immutable. Constants follow the same naming rules as variables.
const x = 42;
print(x * 2); // prints 84
x = 123; // <- syntax error: cannot assign to constant
Constants must be assigned a value, not an expression.
const x = 40 + 2; // <- syntax error: cannot assign expression to constant
Numbers
Integer numbers follow C-style format with support for decimal, binary ('0b'), octal ('0o') and hex ('0x') notations.
The default system integer type (also aliased to INT) is i64. It can be turned into i32 via the only_i32 feature.
Floating-point numbers are also supported if not disabled with no_float. The default system floating-point type is i64
(also aliased to FLOAT).
'_' separators can be added freely and are ignored within a number.
| Format | Type |
|---|---|
123_345, -42 |
i64 in decimal |
0o07_76 |
i64 in octal |
0xabcd_ef |
i64 in hex |
0b0101_1001 |
i64 in binary |
123_456.789 |
f64 |
Numeric operators
Numeric operators generally follow C styles.
| Operator | Description | Integers only |
|---|---|---|
+ |
Plus | |
- |
Minus | |
* |
Multiply | |
/ |
Divide (integer division if acting on integer types) | |
% |
Modulo (remainder) | |
~ |
Power | |
& |
Binary And bit-mask | Yes |
| |
Binary Or bit-mask | Yes |
^ |
Binary Xor bit-mask | Yes |
<< |
Left bit-shift | Yes |
>> |
Right bit-shift | Yes |
let x = (1 + 2) * (6 - 4) / 2; // arithmetic, with parentheses
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
| Operator | Description |
|---|---|
+ |
Plus |
- |
Negative |
let number = -5;
number = -5 - +5;
Numeric functions
The following standard functions (defined in the [BasicMathPackage] but excluded if using a raw Engine) 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 BasicMathPackage but excluded if using a raw Engine) 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
String and char literals follow C-style formatting, with support for Unicode ('\uxxxx' or '\Uxxxxxxxx') and
hex ('\xxx') escape sequences.
Hex sequences map to ASCII characters, while '\u' maps to 16-bit common Unicode code points and '\U' maps the full,
32-bit extended Unicode code points.
Standard escape sequences:
| Escape sequence | Meaning |
|---|---|
\\ |
back-slash \ |
\t |
tab |
\r |
carriage-return CR |
\n |
line-feed LF |
\" |
double-quote " in strings |
\' |
single-quote ' in characters |
\xxx |
Unicode in 2-digit hex |
\uxxxx |
Unicode in 4-digit hex |
\Uxxxxxxxx |
Unicode in 8-digit hex |
Internally Rhai strings are stored as UTF-8 just like Rust (they are Rust String's!), but there are major differences.
In Rhai a string is the same as an array of Unicode characters and can be directly indexed (unlike Rust).
This is similar to most other languages where strings are internally represented not as UTF-8 but as arrays of multi-byte
Unicode characters.
Individual characters within a Rhai string can also be replaced just as if the string is an array of Unicode characters.
In Rhai, there is also no separate concepts of String and &str as in Rust.
Strings can be built up from other strings and types via the + operator (provided by the MoreStringPackage
but excluded if using a raw Engine). This is particularly useful when printing output.
type_of() a string returns "string".
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";
// Unlike Rust, Rhai strings can be indexed to get a character
// (disabled with 'no_index')
let c = record[4];
c == 'C';
ts.s = record; // custom type properties can take strings
let c = ts.s[4];
c == 'C';
let c = "foo"[0]; // indexing also works on string literals...
c == 'f';
let c = ("foo" + "bar")[5]; // ... and expressions returning strings
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 directly modified character-by-character
// (disabled with 'no_index')
record[4] = '\x58'; // 0x58 = 'X'
record == "Bob X. Davis: age 42 ❤\n";
// Use 'in' to test if a substring (or character) exists in a string
"Davis" in record == true;
'X' in record == true;
'C' in record == false;
Built-in functions
The following standard methods (defined in the MoreStringPackage but excluded if using a raw Engine) operate on strings:
| Function | Parameter(s) | Description |
|---|---|---|
len |
none | returns the number of characters (not number of bytes) in the string |
pad |
character to pad, target length | pads the string with an character to at least a specified length |
append |
character/string to append | Adds a character or a string to the end of another string |
clear |
none | empties the string |
truncate |
target length | cuts off the string at exactly a specified number of characters |
contains |
character/sub-string to search for | checks if a certain character or sub-string occurs in the string |
index_of |
character/sub-string to search for, start index (optional) | returns the index that a certain character or sub-string occurs in the string, or -1 if not found |
sub_string |
start index, length (optional) | extracts a sub-string (to the end of the string if length is not specified) |
crop |
start index, length (optional) | retains only a portion of the string (to the end of the string if length is not specified) |
replace |
target sub-string, replacement string | replaces a sub-string with another |
trim |
none | trims the string of whitespace at the beginning and end |
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$$$";
let n = full_name.index_of('$');
n == 12;
full_name.index_of("$$", n + 1) == 13;
full_name.sub_string(n, 3) == "$$$";
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.crop(5);
full_name == "C.";
full_name.crop(0, 1);
full_name == "C";
full_name.clear();
full_name.len() == 0;
Arrays
Arrays are first-class citizens in Rhai. Like C, arrays are accessed with zero-based, non-negative integer indices.
Array literals are built within square brackets '[' ... ']' and separated by commas ','.
All elements stored in an array are Dynamic, and the array can freely grow or shrink with elements added or removed.
The Rust type of a Rhai array is rhai::Array. type_of() an array returns "array".
Arrays are disabled via the no_index feature.
Built-in functions
The following methods (defined in the BasicArrayPackage but excluded if using a raw Engine) operate on arrays:
| Function | Parameter(s) | Description |
|---|---|---|
push |
element to insert | inserts an element at the end |
append |
array to append | concatenates the second array to the end of the first |
+ operator |
first array, second array | concatenates the first array with the second |
insert |
element to insert, position (beginning if <= 0, end if >= length) |
insert an element at a certain index |
pop |
none | removes the last element and returns it (() if empty) |
shift |
none | removes the first element and returns it (() if empty) |
remove |
index | removes an element at a particular index and returns it, or returns () if the index is not valid |
len |
none | returns the number of elements |
pad |
element to pad, target length | pads the array with an element to at least a specified length |
clear |
none | empties the array |
truncate |
target length | cuts off the array at exactly a specified length (discarding all subsequent elements) |
Examples
let y = [2, 3]; // array literal with 2 elements
y.insert(0, 1); // insert element at the beginning
y.insert(999, 4); // insert element at the end
y.len() == 4;
y[0] == 1;
y[1] == 2;
y[2] == 3;
y[3] == 4;
(1 in y) == true; // use 'in' to test if an item exists in the array
(42 in y) == false; // 'in' uses the '==' operator (which users can override)
// to check if the target item exists in the array
y[1] = 42; // array elements can be reassigned
(42 in y) == true;
y.remove(2) == 3; // remove element
y.len() == 3;
y[2] == 4; // elements after the removed element are shifted
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] // a function returning an array
}
let foo = abc()[0];
foo == 42;
let foo = y[0];
foo == 1;
y.push(4); // 4 elements
y.push(5); // 5 elements
y.len() == 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;
y.len() == 3;
for item in y { // arrays can be iterated with a 'for' statement
print(item);
}
y.pad(10, "hello"); // pad the array up to 10 elements
y.len() == 10;
y.truncate(5); // truncate the array to 5 elements
y.len() == 5;
y.clear(); // empty the array
y.len() == 0;
push and pad are only defined for standard built-in types. For custom types, type-specific versions must be registered:
engine.register_fn("push", |list: &mut Array, item: MyType| list.push(Box::new(item)) );
Object maps
Object maps are dictionaries. Properties are all Dynamic and can be freely added and retrieved.
Object map literals are built within braces '#{' ... '}' (name : value syntax similar to Rust)
and separated by commas ','. The property name can be a simple variable name following the same
naming rules as variables, or an arbitrary string literal.
Property values can be accessed via the dot notation (object . property) or index notation (object [ property ]).
The dot notation allows only property names that follow the same naming rules as variables.
The index notation allows setting/getting properties of arbitrary names (even the empty string).
Important: Trying to read a non-existent property returns () instead of causing an error.
The Rust type of a Rhai object map is rhai::Map. type_of() an object map returns "map".
Object maps are disabled via the no_object feature.
Built-in functions
The following methods (defined in the BasicMapPackage but excluded if using a raw Engine) operate on object maps:
| Function | Parameter(s) | Description |
|---|---|---|
has |
property name | does the object map contain a property of a particular name? |
len |
none | returns the number of properties |
clear |
none | empties the object map |
remove |
property name | removes a certain property and returns it (() if the property does not exist) |
mixin |
second object map | mixes in all the properties of the second object map to the first (values of properties with the same names replace the existing values) |
+ operator |
first object map, second object map | merges the first object map with the second |
keys |
none | returns an array of all the property names (in random order), not available under no_index |
values |
none | returns an array of all the property values (in random order), not available under no_index |
Examples
let y = #{ // object map literal with 3 properties
a: 1,
bar: "hello",
"baz!$@": 123.456, // like JS, you can use any string as property names...
"": false, // even the empty string!
a: 42 // <- syntax error: duplicated property name
};
y.a = 42; // access via dot notation
y.baz!$@ = 42; // <- syntax error: only proper variable names allowed in dot notation
y."baz!$@" = 42; // <- syntax error: strings not allowed in dot notation
y.a == 42;
y["baz!$@"] == 123.456; // access via index notation
"baz!$@" in y == true; // use 'in' to test if a property exists in the object map
("z" in y) == false;
ts.obj = y; // object maps can be assigned completely (by value copy)
let foo = ts.list.a;
foo == 42;
let foo = #{ a:1, b:2, c:3 }["a"];
foo == 1;
fn abc() {
#{ a:1, b:2, c:3 } // a function returning an object map
}
let foo = abc().b;
foo == 2;
let foo = y["a"];
foo == 42;
y.has("a") == true;
y.has("xyz") == false;
y.xyz == (); // a non-existing property returns '()'
y["xyz"] == ();
y.len() == 3;
y.remove("a") == 1; // remove property
y.len() == 2;
y.has("a") == false;
for name in keys(y) { // get an array of all the property names via the 'keys' function
print(name);
}
for val in values(y) { // get an array of all the property values via the 'values' function
print(val);
}
y.clear(); // empty the object map
y.len() == 0;
Parsing from JSON
The syntax for an object map is extremely similar to JSON, with the exception of null values which can
technically be mapped to (). A valid JSON string does not start with a hash character # while a
Rhai object map does - that's the major difference!
JSON numbers are all floating-point while Rhai supports integers (INT) and floating-point (FLOAT) if
the no_float feature is not turned on. Most common generators of JSON data distinguish between
integer and floating-point values by always serializing a floating-point number with a decimal point
(i.e. 123.0 instead of 123 which is assumed to be an integer). This style can be used successfully
with Rhai object maps.
Use the parse_json method to parse a piece of JSON into an object map:
// JSON string - notice that JSON property names are always quoted
// notice also that comments are acceptable within the JSON string
let json = r#"{
"a": 1, // <- this is an integer number
"b": true,
"c": 123.0, // <- this is a floating-point number
"$d e f!": "hello", // <- any text can be a property name
"^^^!!!": [1,42,"999"], // <- value can be array or another hash
"z": null // <- JSON 'null' value
}
"#;
// Parse the JSON expression as an object map
// Set the second boolean parameter to true in order to map 'null' to '()'
let map = engine.parse_json(json, true)?;
map.len() == 6; // 'map' contains all properties in the JSON string
// Put the object map into a 'Scope'
let mut scope = Scope::new();
scope.push("map", map);
let result = engine.eval_with_scope::<INT>(r#"map["^^^!!!"].len()"#)?;
result == 3; // the object map is successfully used in the script
timestamp's
Timestamps are provided by the BasicTimePackage (excluded if using a raw Engine) via the timestamp
function.
The Rust type of a timestamp is std::time::Instant. type_of() a timestamp returns "timestamp".
Built-in functions
The following methods (defined in the BasicTimePackage but excluded if using a raw Engine) operate on timestamps:
| Function | Parameter(s) | Description |
|---|---|---|
elapsed |
none | returns the number of seconds since the timestamp |
- operator |
later timestamp, earlier timestamp | returns the number of seconds between the two timestamps |
Examples
let now = timestamp();
// Do some lengthy operation...
if now.elapsed() > 30.0 {
print("takes too long (over 30 seconds)!")
}
Comparison operators
Comparing most values of the same data type work out-of-the-box for standard types supported by the system.
However, if using a raw Engine, comparisons can only be made between restricted system types -
INT (i64 or i32 depending on only_i32 and only_i64), f64 (if not no_float), string, array, bool, char.
42 == 42; // true
42 > 42; // false
"hello" > "foo"; // true
"42" == 42; // false
Comparing two values of different data types, or of unknown data types, always results in false.
42 == 42.0; // false - i64 is different from f64
42 > "42"; // false - i64 is different from string
42 <= "42"; // false again
let ts = new_ts(); // custom type
ts == 42; // false - types are not the same
Boolean operators
| Operator | Description |
|---|---|
! |
Boolean Not |
&& |
Boolean And (short-circuits) |
|| |
Boolean Or (short-circuits) |
& |
Boolean And (doesn't short-circuit) |
| |
Boolean Or (doesn't short-circuit) |
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 statements
if foo(x) {
print("It's true!");
} else if bar == baz {
print("It's true again!");
} else if ... {
:
} else if ... {
:
} else {
print("It's finally false!");
}
All branches of an if statement must be enclosed within braces '{' .. '}', even when there is only one statement.
Like Rust, there is no ambiguity regarding which if clause a statement belongs to.
if (decision) print("I've decided!");
// ^ syntax error, expecting '{' in statement block
Like Rust, if statements can also be used as expressions, replacing the ? : conditional operators in other C-like languages.
// The following is equivalent to C: int x = 1 + (decision ? 42 : 123) / 2;
let x = 1 + if decision { 42 } else { 123 } / 2;
x == 22;
let x = if decision { 42 }; // no else branch defaults to '()'
x == ();
while loops
let x = 10;
while x > 0 {
x = x - 1;
if x < 6 { continue; } // skip to the next iteration
print(x);
if x == 5 { break; } // break out of while loop
}
Infinite loop
let x = 10;
loop {
x = x - 1;
if x > 5 { continue; } // skip to the next iteration
print(x);
if x == 0 { break; } // break out of loop
}
for loops
Iterating through a range or an array is provided by the for ... in loop.
let array = [1, 3, 5, 7, 9, 42];
// Iterate through array
for x in array {
if x > 10 { continue; } // skip to the next iteration
print(x);
if x == 42 { break; } // break out of for loop
}
// The 'range' function allows iterating from first to last-1
for x in range(0, 50) {
if x > 10 { continue; } // skip to the next iteration
print(x);
if x == 42 { break; } // break out of for loop
}
// The 'range' function also takes a step
for x in range(0, 50, 3) { // step by 3
if x > 10 { continue; } // skip to the next iteration
print(x);
if x == 42 { break; } // break out of for loop
}
// Iterate through object map
let map = #{a:1, b:3, c:5, d:7, e:9};
// Property names are returned in random order
for x in keys(map) {
if x > 10 { continue; } // skip to the next iteration
print(x);
if x == 42 { break; } // break out of for loop
}
// Property values are returned in random order
for val in values(map) {
print(val);
}
return-ing values
return; // equivalent to return ();
return 123 + 456; // returns 579
Errors and throw-ing exceptions
All of Engine's evaluation/consuming methods return Result<T, Box<rhai::EvalAltResult>> with EvalAltResult
holding error information. To deliberately return an error during an evaluation, use the throw keyword.
if some_bad_condition_has_happened {
throw error; // 'throw' takes a string as the exception text
}
throw; // defaults to empty exception text: ""
Exceptions thrown via throw in the script can be captured by matching Err(EvalAltResult::ErrorRuntime( reason , position ))
with the exception text captured by the first parameter.
let result = engine.eval::<i64>(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));
Implicit return
Just like in Rust, an implicit return can be used. In fact, the last statement of a block is always the block's return value
regardless of whether it is terminated with a semicolon ';'. This is different from Rust.
fn add(x, y) { // implicit return:
x + y; // value of the last statement (no need for ending semicolon)
// is used as the return value
}
fn add2(x) {
return x + 2; // explicit return
}
print(add(2, 3)); // prints 5
print(add2(42)); // prints 44
No access to external scope
Functions are not closures. They do not capture the calling environment and can only access their own parameters. They cannot access variables external to the function itself.
let x = 42;
fn foo() { x } // <- syntax error: variable 'x' doesn't exist
Passing arguments by value
Functions defined in script always take Dynamic parameters (i.e. the parameter can be of any type).
It is important to remember that all arguments are passed by value, so all functions are pure
(i.e. they never modifytheir arguments).
Any update to an argument will not be reflected back to the caller. This can introduce subtle bugs, if not careful.
fn change(s) { // 's' is passed by value
s = 42; // only a COPY of 's' is changed
}
let x = 500;
x.change(); // de-sugars to change(x)
x == 500; // 'x' is NOT changed!
Global definitions only
Functions can only be defined at the global level, never inside a block or another function.
// Global level is OK
fn add(x, y) {
x + y
}
// The following will not compile
fn do_addition(x) {
fn add_y(n) { // <- syntax error: functions cannot be defined inside another function
n + y
}
add_y(x)
}
Unlike C/C++, functions can be defined anywhere within the global level. A function does not need to be defined prior to being used in a script; a statement in the script can freely call a function defined afterwards. This is similar to Rust and many other modern languages.
Function overloading
Functions can be overloaded and are resolved purely upon the function's name and the number of parameters
(but not parameter types, since all parameters are the same type - Dynamic).
New definitions overwrite previous definitions of the same name and number of parameters.
fn foo(x,y,z) { print("Three!!! " + x + "," + y + "," + z) }
fn foo(x) { print("One! " + x) }
fn foo(x,y) { print("Two! " + x + "," + y) }
fn foo() { print("None.") }
fn foo(x) { print("HA! NEW ONE! " + x) } // overwrites previous definition
foo(1,2,3); // prints "Three!!! 1,2,3"
foo(42); // prints "HA! NEW ONE! 42"
foo(1,2); // prints "Two!! 1,2"
foo(); // prints "None."
Members and methods
Properties and methods in a Rust custom type registered with the Engine can be called just like in Rust.
let a = new_ts(); // constructor function
a.field = 500; // property access
a.update(); // method call, 'a' can be changed
update(a); // this works, but 'a' is unchanged because only
// a COPY of 'a' is passed to 'update' by VALUE
Custom types, properties and methods can be disabled via the no_object feature.
print and debug
The print and debug functions default to printing to stdout, with debug using standard debug formatting.
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
When embedding Rhai into an application, it is usually necessary to trap print and debug output
(for logging into a tracking log, for example).
// 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 logbook = Arc::new(RwLock::new(Vec::<String>::new()));
// Redirect print/debug output to 'log'
let log = logbook.clone();
engine.on_print(move |s| log.write().unwrap().push(format!("entry: {}", s)));
let log = logbook.clone();
engine.on_debug(move |s| log.write().unwrap().push(format!("DEBUG: {}", s)));
// Evaluate script
engine.eval::<()>(script)?;
// 'logbook' captures all the 'print' and 'debug' output
for entry in logbook.read().unwrap().iter() {
println!("{}", entry);
}
Using external modules
Rhai allows organizing code (functions and variables) into modules. A module is a single script file
with export statements that exports certain global variables and functions as contents of the module.
Everything exported as part of a module is constant and read-only.
A module can be imported via the import statement, and its members accessed via '::' similar to C++.
import "crypto" as crypto; // import the script file 'crypto.rhai' as a module
crypto::encrypt(secret); // use functions defined under the module via '::'
print(crypto::status); // module variables are constants
crypto::hash::sha256(key); // sub-modules are also supported
import statements are scoped, meaning that they are only accessible inside the scope that they're imported.
let mod = "crypto";
if secured { // new block scope
import mod as crypto; // import module (the path needs not be a constant string)
crypto::encrypt(key); // use a function in the module
} // the module disappears at the end of the block scope
crypto::encrypt(others); // <- this causes a run-time error because the 'crypto' module
// is no longer available!
Script optimization
Rhai includes an optimizer that tries to optimize a script after parsing.
This can reduce resource utilization and increase execution speed.
Script optimization can be turned off via the no_optimize feature.
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
}
Constants propagation is used to remove dead code:
const ABC = true;
if ABC || some_work() { print("done!"); } // 'ABC' is constant so it is replaced by 'true'...
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.
For fixed script texts, the constant values can be provided in a user-defined Scope object
to the Engine for use in compilation and evaluation.
Beware, however, that most operators are actually function calls, and those functions can be overridden, so they are not optimized away:
const DECISION = 1;
if DECISION == 1 { // NOT optimized away because you can define
: // your own '==' function to override the built-in default!
:
} else if DECISION == 2 { // same here, NOT optimized away
:
} else if DECISION == 3 { // same here, NOT optimized away
:
} else {
:
}
because no operator functions will be run (in order not to trigger side effects) during the optimization process
(unless the optimization level is set to OptimizationLevel::Full). So, instead, do this:
const DECISION_1 = true;
const DECISION_2 = false;
const DECISION_3 = false;
if DECISION_1 {
: // this branch is kept and promoted to the parent level
} else if DECISION_2 {
: // this branch is eliminated
} else if DECISION_3 {
: // this branch is eliminated
} else {
: // this branch is eliminated
}
In general, boolean constants are most effective for the optimizer to automatically prune
large if-else branches because they do not depend on operators.
Alternatively, turn the optimizer to OptimizationLevel::Full
Here be dragons!
Optimization levels
There are actually three levels of optimizations: None, Simple and Full.
-
Noneis obvious - no optimization on the AST is performed. -
Simple(default) performs relatively safe optimizations without causing side effects (i.e. it only relies on static analysis and will not actually perform any function calls). -
Fullis much more aggressive, including running functions on constant arguments to determine their result. One benefit to this is that many more optimization opportunities arise, especially with regards to comparison operators.
An Engine's optimization level is set via a call to set_optimization_level:
// Turn on aggressive optimizations
engine.set_optimization_level(rhai::OptimizationLevel::Full);
If it is ever needed to re-optimize an AST, use the optimize_ast method:
// Compile script to AST
let ast = engine.compile("40 + 2")?;
// Create a new 'Scope' - put constants in it to aid optimization if using 'OptimizationLevel::Full'
let scope = Scope::new();
// Re-optimize the AST
let ast = engine.optimize_ast(&scope, &ast, OptimizationLevel::Full);
When the optimization level is OptimizationLevel::Full, the Engine assumes all functions to be pure and will eagerly
evaluated all function calls with constant arguments, using the result to replace the call. This also applies to all operators
(which are implemented as functions). For instance, the same example above:
// When compiling the following with OptimizationLevel::Full...
const DECISION = 1;
// this condition is now eliminated because 'DECISION == 1'
if DECISION == 1 { // is a function call to the '==' function, and it returns 'true'
print("hello!"); // this block is promoted to the parent level
} else {
print("boo!"); // this block is eliminated because it is never reached
}
print("hello!"); // <- the above is equivalent to this
// ('print' and 'debug' are handled specially)
Because of the eager evaluation of functions, many constant expressions will be evaluated and replaced by the result.
This does not happen with OptimizationLevel::Simple which doesn't assume all functions to be pure.
// When compiling the following with OptimizationLevel::Full...
let x = (1+2)*3-4/5%6; // <- will be replaced by 'let x = 9'
let y = (1>2) || (3<=4); // <- will be replaced by 'let y = true'
Function side effect considerations
All of Rhai's built-in functions (and operators which are implemented as functions) are pure (i.e. they do not mutate state
nor cause side any effects, with the exception of print and debug which are handled specially) so using
OptimizationLevel::Full is usually quite safe unless you register your own types and functions.
If custom functions are registered, they may be called (or maybe not, if the calls happen to lie within a pruned code block).
If custom functions are registered to replace built-in operators, they will also be called when the operators are used
(in an if statement, for example) and cause side-effects.
Function volatility considerations
Even if a custom function does not mutate state nor cause side effects, it may still be volatile, i.e. it depends
on the external environment and is not pure. A perfect example is a function that gets the current time -
obviously each run will return a different value! The optimizer, when using OptimizationLevel::Full, assumes that
all functions are pure, so when it finds constant arguments it will eagerly execute the function call.
This causes the script to behave differently from the intended semantics because essentially the result of the function call
will always be the same value.
Therefore, avoid using OptimizationLevel::Full if you intend to register non-pure custom types and/or functions.
Subtle semantic changes
Some optimizations 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
}
foo(42) // <- the above optimizes to this
Nevertheless, if the original script were evaluated instead, 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 disappear:
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 (because an access to my_decision produces
no side effects), thus the script silently runs to completion without errors.
Turning off optimizations
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), turn it off by setting the optimization level to OptimizationLevel::None.
let engine = rhai::Engine::new();
// Turn off the optimizer
engine.set_optimization_level(rhai::OptimizationLevel::None);
eval - or "How to Shoot Yourself in the Foot even Easier"
Saving the best for last: in addition to script optimizations, there is the ever-dreaded... eval function!
let x = 10;
fn foo(x) { x += 12; x }
let script = "let y = x;"; // build a script
script += "y += foo(y);";
script += "x + y";
let result = eval(script); // <- look, JS, we can also do this!
print("Answer: " + result); // prints 42
print("x = " + x); // prints 10: functions call arguments are passed by value
print("y = " + y); // prints 32: variables defined in 'eval' persist!
eval("{ let z = y }"); // to keep a variable local, use a statement block
print("z = " + z); // <- error: variable 'z' not found
"print(42)".eval(); // <- nope... method-call style doesn't work
Script segments passed to eval execute inside the current Scope, so they can access and modify everything,
including all variables that are visible at that position in code! It is almost as if the script segments were
physically pasted in at the position of the eval call. But because of this, new functions cannot be defined
within an eval call, since functions can only be defined at the global level, not inside a function call!
let script = "x += 32";
let x = 10;
eval(script); // variable 'x' in the current scope is visible!
print(x); // prints 42
// The above is equivalent to:
let script = "x += 32";
let x = 10;
x += 32;
print(x);
For those who subscribe to the (very sensible) motto of "eval is evil",
disable eval by overriding it, probably with something that throws.
fn eval(script) { throw "eval is evil! I refuse to run " + script }
let x = eval("40 + 2"); // 'eval' here throws "eval is evil! I refuse to run 40 + 2"
Or override it from Rust:
fn alt_eval(script: String) -> Result<(), EvalAltResult> {
Err(format!("eval is evil! I refuse to run {}", script).into())
}
engine.register_result_fn("eval", alt_eval);