Extend Rhai with Custom Syntax

For the ultimate adventurous, there is a built-in facility to extend the Rhai language with custom-defined syntax.

But before going off to define the next weird statement type, heed this warning:

Don’t Do It™

Stick with standard language syntax as much as possible.

Having to learn Rhai is bad enough, no sane user would ever want to learn yet another obscure language syntax just to do something.

Try custom operators first. A custom syntax should be considered a last resort.

Where this might be useful

  • Where an operation is used a LOT and a custom syntax saves a lot of typing.

  • Where a custom syntax significantly simplifies the code and significantly enhances understanding of the code’s intent.

  • Where certain logic cannot be easily encapsulated inside a function.

  • Where you just want to confuse your user and make their lives miserable, because you can.

Disable custom syntax

Custom syntax can be disabled via the no_custom_syntax feature.

How to Do It

Step One – Design The Syntax

A custom syntax is simply a list of symbols.

These symbol types can be used:

  • Standard keywords
  • Standard operators
  • Reserved symbols.
  • Identifiers following the variable naming rules.
  • $expr$ – any valid expression, statement or statements block.
  • $block$ – any valid statements block (i.e. must be enclosed by {}).
  • $ident$ – any variable name.
  • $symbol$ – any symbol, active or reserved.
  • $bool$ – a boolean value.
  • $int$ – an integer number.
  • $float$ – a floating-point number (if not no_float).
  • $string$ – a string literal.

The first symbol must be an identifier

There is no specific limit on the combination and sequencing of each symbol type, except the first symbol which must be a custom keyword that follows the naming rules of variables.

The first symbol also cannot be a normal keyword unless it is disabled. Any valid identifier that is not an active keyword works fine, even if it is a reserved keyword.

The first symbol must be unique

Rhai uses the first symbol as a clue to parse custom syntax.

Therefore, at any one time, there can only be one custom syntax starting with each unique symbol.

Any new custom syntax definition using the same first symbol simply overwrites the previous one.


exec [ $ident$ $symbol$ $int$ ] <- $expr$ : $block$

The above syntax is made up of a stream of symbols:

PositionInput slotSymbolDescription
1execcustom keyword
2[the left bracket symbol
20$ident$a variable name
31$symbol$the operator
42$int$an integer number
5]the right bracket symbol
6<-the left-arrow symbol (which is a reserved symbol in Rhai).
73$expr$an expression, which may be enclosed with {}, or not.
8:the colon symbol
94$block$a statements block, which must be enclosed with {}.

This syntax matches the following sample code and generates five inputs (one for each non-keyword):

// Assuming the 'exec' custom syntax implementation declares the variable 'hello':
let x = exec [hello < 42] <- foo(1, 2) : {
            hello += bar(hello);

print(x);       // variable 'x'  has a value returned by the custom syntax

print(hello);   // variable declared by a custom syntax persists!

Step Two – Implementation

Any custom syntax must include an implementation of it.

Function signature

The signature of an implementation function is as follows.

Fn(context: &mut EvalContext, inputs: &[Expression]) -> Result<Dynamic, Box<EvalAltResult>>


context&mut EvalContextmutable reference to the current evaluation context
inputs&[Expression]a list of input expression trees

and EvalContext is a type that encapsulates the current evaluation context.

Return value

Return value is the result of evaluating the custom syntax expression.

Access arguments

The most important argument is inputs where the matched identifiers ($ident$), expressions/statements ($expr$) and statements blocks ($block$) are provided.

To access a particular argument, use the following patterns:

Argument typePattern (n = slot in inputs)Result typeDescription
$ident$inputs[n].get_string_value().unwrap()&strvariable name
$symbol$inputs[n].get_literal_value::<ImmutableString>().unwrap()ImmutableStringsymbol literal
$expr$&inputs[n]&Expressionan expression tree
$block$&inputs[n]&Expressionan expression tree
$bool$inputs[n].get_literal_value::<bool>().unwrap()boolboolean value
$int$inputs[n].get_literal_value::<INT>().unwrap()INTinteger number
$float$inputs[n].get_literal_value::<FLOAT>().unwrap()FLOATfloating-point number


string text

Get literal constants

Several argument types represent literal constants that can be obtained directly via Expression::get_literal_value<T> or Expression::get_string_value (for strings).

let expression = &inputs[0];

// Use 'get_literal_value' with a turbo-fish type to extract the value
let string_value = expression.get_literal_value::<ImmutableString>().unwrap();
let string_slice = expression.get_string_value().unwrap();

let float_value = expression.get_literal_value::<FLOAT>().unwrap();

// Or assign directly to a variable with type...
let int_value: i64 = expression.get_literal_value().unwrap();

// Or use type inference!
let bool_value = expression.get_literal_value().unwrap();

if bool_value { ... }       // 'bool_value' inferred to be 'bool'

Evaluate an expression tree

Use the EvalContext::eval_expression_tree method to evaluate an arbitrary expression tree within the current evaluation context.

let expression = &inputs[0];
let result = context.eval_expression_tree(expression)?;

Retain variables in block scope

When an expression tree actually contains a statements block (i.e. $block), local variables/constants defined within that block are usually removed at the end of the block.

Sometimes it is useful to retain these local variables/constants for further processing (e.g. collecting new variables into an object map).

As such, evaluate the expression tree using the EvalContext::eval_expression_tree_raw method which contains a parameter to control whether the statements block should be rewound.

// Assume 'expression' contains a statements block with local variable definitions
let expression = &inputs[0];
let result = context.eval_expression_tree_raw(expression, false)?;

// Variables defined within 'expression' persist in context.scope()

Declare variables

New variables/constants maybe declared (usually with a variable name that is passed in via $ident$).

It can simply be pushed into the Scope.

let var_name = inputs[0].get_string_value().unwrap();
let expression = &inputs[1];

context.scope_mut().push(var_name, 0_i64);      // declare new variable

let result = context.eval_expression_tree(expression)?;

Step Three – Register the Custom Syntax

Use Engine::register_custom_syntax to register a custom syntax.

Again, beware that the first symbol must be unique. If there already exists a custom syntax starting with that symbol, the previous syntax will be overwritten.

The syntax is passed simply as a slice of &str.

// Custom syntax implementation
fn implementation_func(context: &mut EvalContext, inputs: &[Expression]) -> Result<Dynamic, Box<EvalAltResult>> {
    let var_name = inputs[0].get_string_value().unwrap();
    let stmt = &inputs[1];
    let condition = &inputs[2];

    // Push new variable into the scope BEFORE 'context.eval_expression_tree'
    context.scope_mut().push(var_name.to_string(), 0_i64);

    let mut count = 0_i64;

    loop {
        // Evaluate the statements block

        count += 1;

        // Declare a new variable every three turns...
        if count % 3 == 0 {
            context.scope_mut().push(format!("{}{}", var_name, count), count);

        // Evaluate the condition expression
        let expr_result = !context.eval_expression_tree(condition)?;

        match expr_result.as_bool() {
            Ok(true) => (),
            Ok(false) => break,
            Err(err) => return Err(EvalAltResult::ErrorMismatchDataType(


// Register the custom syntax (sample): exec<x> -> { x += 1 } while x < 0
    [ "exec", "<", "$ident$", ">", "->", "$block$", "while", "$expr$" ], // the custom syntax
    true,  // variables declared within this custom syntax

Remember that a custom syntax acts as an expression, so it can show up practically anywhere:

// Use as an expression:
let foo = (exec<x> -> { x += 1 } while x < 42) * 100;

// New variables are successfully declared...
x == 42;
x3 == 3;
x6 == 6;

// Use as a function call argument:
do_something(exec<x> -> { x += 1 } while x < 42, 24, true);

// Use as a statement:
exec<x> -> { x += 1 } while x < 0;
//                               ^ terminate statement with ';' unless the custom
//                                 syntax already ends with '}'

Step Four – Disable Unneeded Statement Types

When a DSL needs a custom syntax, most likely than not it is extremely specialized. Therefore, many statement types actually may not make sense under the same usage scenario.

So, while at it, better disable those built-in keywords and operators that should not be used by the user. The would leave only the bare minimum language surface exposed, together with the custom syntax that is tailor-designed for the scenario.

A keyword or operator that is disabled can still be used in a custom syntax.

In an extreme case, it is possible to disable every keyword in the language, leaving only custom syntax (plus possibly expressions). But again, Don’t Do It™ – unless you are certain of what you’re doing.

Step Five – Document

For custom syntax, documentation is crucial.

Make sure there are lots of examples for users to follow.

Step Six – Profit!

Practical Example – Matrix Literal

Say you’d want to use something like ndarray to manipulate matrices.

However, you’d like to write matrix literals in a more intuitive syntax than an array of arrays.

In other words, you’d like to turn:

// Array of arrays
let matrix = [ [  a, b,     0 ],
               [ -b, a,     0 ],
               [  0, 0, c * d ] ];


// Directly parse to an ndarray::Array (look ma, no commas!)
let matrix = @|  a   b   0  |
              | -b   a   0  |
              |  0   0  c*d |;

This can easily be done via a custom syntax, which yields a syntax that is more pleasing.

    ["@", "|", "$expr$", "$expr$", "$expr$", "|", 
          "|", "$expr$", "$expr$", "$expr$", "|",
          "|", "$expr$", "$expr$", "$expr$", "|" 
    |context, inputs| {
        use ndarray::arr2;

        let mut values = [[0.0; 3]; 3];

        for y in 0..2 {
            for x in 0..2 {
                let offset = y * 3 + x;

                match context.eval_expression_tree(&inputs[offset])?.as_float() {
                    Ok(v) => values[y][x] = v,
                    Err(typ) => return Err(Box::new(EvalAltResult::ErrorMismatchDataType(
                                            "float".to_string(), typ.to_string(),

        let matrix = arr2(&values);


For matrices of flexible dimensions, check out custom syntax parsers.

Practical Example – Recreating C’s Ternary Operator

Rhai has if-expressions, but sometimes a C-style ternary operator is more concise.

// A custom syntax must start with a unique symbol, so we use 'iff'.
// Register the custom syntax: iff condition ? true-value : false-value
    ["iff", "$expr$", "?", "$expr$", ":", "$expr$"],
    |context, inputs| match context.eval_expression_tree(&inputs[0])?.as_bool() {
        Ok(true) => context.eval_expression_tree(&inputs[1]),
        Ok(false) => context.eval_expression_tree(&inputs[2]),
        Err(typ) => Err(Box::new(EvalAltResult::ErrorMismatchDataType(
            "bool".to_string(), typ.to_string(), inputs[0].position()

Tip: Custom syntax performance

The code in the example above is essentially what the if statement does internally, and since custom syntax is pre-parsed, there really is no performance penalty!

Practical Example – Recreating JavaScript’s var Statement

The following example recreates a statement similar to the var variable declaration syntax in JavaScript, which creates a global variable if one doesn’t already exist. There is currently no equivalent in Rhai.

// Register the custom syntax: var x = ???
engine.register_custom_syntax([ "var", "$ident$", "=", "$expr$" ], true, |context, inputs| {
    let var_name = inputs[0].get_string_value().unwrap().to_string();
    let expr = &inputs[1];

    // Evaluate the expression
    let value = context.eval_expression_tree(expr)?;

    // Push a new variable into the scope if it doesn't already exist.
    // Otherwise just set its value.
    if !context.scope().is_constant(var_name).unwrap_or(false) {
        context.scope_mut().set_value(var_name.to_string(), value);
    } else {
        Err(format!("variable {} is constant", var_name).into())