Call Rhai Functions from Rust
Rhai also allows working backwards from the other direction – i.e. calling a Rhai-scripted
function from Rust via Engine::call_fn
.
┌─────────────┐
│ Rhai script │
└─────────────┘
import "process" as proc; // this is evaluated every time
fn hello(x, y) {
// hopefully 'my_var' is in scope when this is called
x.len + y + my_var
}
fn hello(x) {
// hopefully 'my_string' is in scope when this is called
x * my_string.len()
}
fn hello() {
// hopefully 'MY_CONST' is in scope when this is called
if MY_CONST {
proc::process_data(42); // can access imported module
}
}
┌──────┐
│ Rust │
└──────┘
// Compile the script to AST
let ast = engine.compile(script)?;
// Create a custom 'Scope'
let mut scope = Scope::new();
// A custom 'Scope' can also contain any variables/constants available to
// the functions
scope.push("my_var", 42_i64);
scope.push("my_string", "hello, world!");
scope.push_constant("MY_CONST", true);
// 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.
//
// Variables/constants pushed into the custom 'Scope'
// (i.e. 'my_var', 'my_string', 'MY_CONST') are visible to the function.
let result: i64 = engine.call_fn(&mut scope, &ast, "hello", ( "abc", 123_i64 ) )?;
// ^^^ ^^^^^^^^^^^^^^^^^^
// return type must be specified 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
When using Engine::call_fn
, the AST
is always evaluated before the function is called.
This is usually desirable in order to import the necessary external modules that are needed by the function.
All new variables/constants introduced are, by default, not retained inside the Scope
.
In other words, the Scope
is rewound before each call.
If these default behaviors are not desirable, use Engine::call_fn_raw
.
FuncArgs
Trait
Rhai implements FuncArgs
for tuples and Vec<T>
.
Engine::call_fn
takes a parameter of any type that implements the FuncArgs
trait,
which is used to parse a data type into individual argument values for the function call.
Custom types (e.g. structures) can also implement FuncArgs
so they can be used for
calling Engine::call_fn
.
use std::iter::once;
use rhai::FuncArgs;
// A struct containing function arguments
struct Options {
pub foo: bool,
pub bar: String,
pub baz: i64
}
impl FuncArgs for Options {
fn parse<C: Extend<Dynamic>>(self, container: &mut C) {
container.extend(once(self.foo.into()));
container.extend(once(self.bar.into()));
container.extend(once(self.baz.into()));
}
}
let options = Options { foo: true, bar: "world", baz: 42 };
// The type 'Options' can now be used as arguments to 'call_fn'!
let result: i64 = engine.call_fn(&mut scope, &ast, "hello", options)?;
Low-Level API – Engine::call_fn_raw
For more control, construct all arguments as Dynamic
values and use Engine::call_fn_raw
,
passing it anything that implements AsMut<[Dynamic]>
(such as a simple array or a Vec<Dynamic>
):
let result = engine.call_fn_raw(
&mut scope, // scope to use
&ast, // AST containing the functions
false, // false = do not evaluate the AST
false, // false = do not rewind the scope (i.e. keep new variables)
"hello", // function entry-point
None, // 'this' pointer, if any
[ "abc".into(), 123_i64.into() ] // arguments
)?;
Engine::call_fn_raw
extends control to the following:
- Whether to skip evaluation of the
AST
before calling the target function - Whether to rewind the custom
Scope
at the end of the function call - Whether to bind the
this
pointer to a specific value
Skip evaluation of the AST
By default, the AST
is evaluated before calling the target function.
A parameter can be passed to skip this evaluation.
Keep new variables/constants
By default, the Engine
rewinds the custom Scope
after each call to the initial size,
so any new variable/constant defined are cleared and will not spill into the custom Scope
.
This keeps the Scope
from being continuously polluted by new variables and is usually the
expected intuitive behavior.
A parameter can be passed to keep new variables/constants within the custom Scope
.
This allows the function to easily pass values back to the caller by leaving them inside the
custom Scope
.
┌─────────────┐
│ Rhai script │
└─────────────┘
fn initialize() {
let x = 42; // 'x' is retained
let y = x * 2; // 'y' is retained
// Use a new statements block to define temp variables
{
let temp = x + y; // 'temp' is NOT retained
foo = temp * temp; // 'foo' is visible in the scope
}
}
let foo = 123; // 'foo' is retained
// Use a new statements block to define temp variables
{
let bar = foo / 2; // 'bar' is NOT retained
foo = bar * bar;
}
┌──────┐
│ Rust │
└──────┘
engine.call_fn_raw(&mut scope, &ast, true, false, "initialize", None, [])?;
// ^^^^ evaluate AST before call
// ^^^^^ do not rewind scope
// At this point, 'scope' contains these variables: 'foo', 'x', 'y'
Bind the this
pointer
Engine::call_fn_raw
can also bind a value to the this
pointer of a script-defined function.
It is possible, then, to call a function that uses this
.
let ast = engine.compile("fn action(x) { this += x; }")?;
let mut value: Dynamic = 1_i64.into();
engine.call_fn_raw(
&mut scope,
&ast,
false,
false,
"action",
Some(&mut value), // binding the 'this' pointer
[ 41_i64.into() ]
)?;
assert_eq!(value.as_int()?, 42);