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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:
```toml
[dependencies]
rhai = "0.10.2"
```
or simply:
```toml
[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:
```bash
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 (**R**ead-**E**val-**P**rint **L**oop).
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:
```bash
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`:
```rust
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:
```rust
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:
```rust
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:
```rust
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`:
```rust
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`, <br/>`u32`, `i32` (default for [`only_i32`]),<br/>`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`),<br/>`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.
```rust
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.
```rust
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.
```rust
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:
```rust
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`.
```rust
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.
```rust
// 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:
```rust
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.
```rust
#[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.
```rust
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.*
```rust
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.
```rust
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.
```rust
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:
```rust
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:
```rust
#[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:
```rust
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
--------
```rust
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).
```rust
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
-----------------
```rust
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
---------------
```rust
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
-----------------
```rust
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:
```rust
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:
```rust
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:
```rust
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`.
```rust
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.
```rust
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
----------------------------
```rust
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:
```rust
let my_str = "abc";
my_str += "ABC";
my_str += 12345;
my_str == "abcABC12345"
```
If
--
```rust
if true {
print("It's true!");
} else if true {
print("It's true again!");
} else {
print("It's false!");
}
```
While
-----
```rust
let x = 10;
while x > 0 {
print(x);
if x == 5 { break; }
x = x - 1;
}
```
Loop
----
```rust
let x = 10;
loop {
print(x);
x = x - 1;
if x == 0 { break; }
}
```
For
---
```rust
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
------
```rust
return; // equivalent to return ();
return 123 + 456;
```
Errors and Exceptions
---------------------
```rust
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.
```rust
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`]):
```rust
fn add(x, y) {
return x + y;
}
print(add(2, 3));
```
Just like in Rust, you can also use an implicit return.
```rust
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.
```rust
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.
```rust
// 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.
```rust
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
-------------------
```rust
let a = new_ts();
a.x = 500;
a.update();
```
`print` and `debug`
-------------------
```rust
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
```rust
// 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:
```rust
{
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:
```rust
{
let x = 999;
foo(42);
666
}
```
Constant propagation is used to remove dead code:
```rust
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:
```rust
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:
```rust
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_:
```rust
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.
```rust
let engine = rhai::Engine::new();
engine.set_optimization(false); // turn off the optimizer
```
[ChaiScript]: http://chaiscript.com/
[scripting languages for Rust]: https://github.com/rust-unofficial/awesome-rust#scripting
[awesome-rust]: https://github.com/rust-unofficial/awesome-rust
[`num-traits`]: https://crates.io/crates/num-traits/
[`debug_msgs`]: #optional-features
[`unchecked`]: #optional-features
[`no_stdlib`]: #optional-features
[`no_index`]: #optional-features
[`no_float`]: #optional-features
[`no_function`]: #optional-features
[`only_i32`]: #optional-features
[`only_i64`]: #optional-features
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