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.

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 statement block.
  • $block$ – any valid statement 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 statement 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 statement 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)?;

Declare variables

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

It can simply be pushed into the Scope.

However, beware that all new variables must be declared prior to evaluating any expression tree. In other words, any Scope calls that change the list of must come before any EvalContext::eval_expression_tree calls.

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

context.scope_mut().push(var_name, 0_i64);      // do this BEFORE 'context.eval_expression_tree'!

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 statement 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 – 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())

Really Advanced – Custom Parsers

Sometimes it is desirable to have multiple custom syntax starting with the same symbol.

This is especially common for command-style syntax where the second symbol calls a particular command:

// The following simulates a command-style syntax, all starting with 'perform'.
perform hello world;        // A fixed sequence of symbols
perform action 42;          // Perform a system action with a parameter
perform update system;      // Update the system
perform check all;          // Check all system settings
perform cleanup;            // Clean up the system
perform add something;      // Add something to the system
perform remove something;   // Delete something from the system

Alternatively, a custom syntax may have variable length, with a termination symbol:

// The following is a variable-length list terminated by '>'  
tags < "foo", "bar", 123, ... , x+y, true >

For even more flexibility in order to handle these advanced use cases, there is a low level API for custom syntax that allows the registration of an entire mini-parser.

Use Engine::register_custom_syntax_raw to register a custom syntax parser together with the implementation function.

How Custom Parsers Work

Leading Symbol

Under this API, the leading symbol for a custom parser is no longer restricted to be valid identifiers.

It can either be:

Function Signature

The custom syntax parser has the following signature.

Fn(symbols: &[ImmutableString], look_ahead: &str) -> Result<Option<ImmutableString>, ParseError>


symbols&[ImmutableString]a slice of symbols that have been parsed so far, possibly containing $expr$ and/or $block$; $ident$ and other literal markers are replaced by the actual text
look_ahead&stra string slice containing the next symbol that is about to be read

Most strings are ImmutableString’s so it is usually more efficient to just clone the appropriate one (if any matches, or keep an internal cache for commonly-used symbols) as the return value.


A custom parser takes as input parameters two pieces of information:

  • The symbols (as ImmutableStrings) parsed so far:

    Argument typeValue
    text stringtext value
    $ident$identifier name
    $symbol$symbol literal
    $bool$true or false
    $int$value of number
    $float$value of number
    $string$string text

    The custom parser can inspect this symbols stream to determine the next symbol to parse.

  • The look-ahead symbol, which is the symbol that will be parsed next.

    If the look-ahead is an expected symbol, the customer parser just returns it to continue parsing, or it can return $ident$ to parse it as an identifier, or even $expr$ to start parsing an expression.

    If the look-ahead is {, then the custom parser may also return $block$ to start parsing a statements block.

    If the look-ahead is unexpected, the custom parser should then return the symbol expected and Rhai will fail with a parse error containing information about the expected symbol.

Return value

The return value is Result<Option<ImmutableString>, ParseError> where:

Ok(None)parsing complete and there are no more symbols to match
Ok(Some(symbol))the next symbol to match, which can also be $expr$, $ident$ or $block$
Err(error)error that is reflected back to the Engine – normally ParseError(ParseErrorType::BadInput(LexError::ImproperSymbol(message)), Position::NONE) to indicate that there is a syntax error, but it can be any ParseError.

A custom parser always returns Some with the next symbol expected (which can be $ident$, $expr$, $block$ etc.) or None if parsing should terminate (without reading the look-ahead symbol).

Tip: $$ return symbol

A return symbol starting with $$ is treated specially.

Like None, it also terminates parsing, but at the same time it adds this symbol as text into the inputs stream at the end.

This is typically used to inform the implementation function which custom syntax variant was actually parsed.


    // The leading symbol - which needs not be an identifier.
    // The custom parser implementation - always returns the next symbol expected
    // 'look_ahead' is the next symbol about to be read
    // Return symbols starting with '$$' terminate parsing but also allows us
    // to determine which syntax variant was actually parsed so we can perform the
    // appropriate action.
    // The return type is 'Option<ImmutableString>' to allow common text strings
    // to be interned and shared easily, reducing allocations during parsing.
    |symbols, look_ahead| match symbols.len() {
        // perform ...
        1 => Ok(Some("$ident$".into())),
        // perform command ...
        2 => match symbols[1].as_str() {
            "action" => Ok(Some("$expr$".into())),
            "hello" => Ok(Some("world".into())),
            "update" | "check" | "add" | "remove" => Ok(Some("$ident$".into())),
            "cleanup" => Ok(Some("$$cleanup".into())),
            cmd => Err(LexError::ImproperSymbol(format!("Improper command: {}", cmd))
        // perform command arg ...
        3 => match (symbols[1].as_str(), symbols[2].as_str()) {
            ("action", _) => Ok(Some("$$action".into())),
            ("hello", "world") => Ok(Some("$$hello-world".into())),
            ("update", arg) => match arg {
                "system" => Ok(Some("$$update-system".into())),
                "client" => Ok(Some("$$update-client".into())),
                _ => Err(LexError::ImproperSymbol(format!("Cannot update {}", arg))
            ("check", arg) => Ok(Some("$$check".into())),
            ("add", arg) => Ok(Some("$$add".into())),
            ("remove", arg) => Ok(Some("$$remove".into())),
            (cmd, arg) => Err(LexError::ImproperSymbol(
                format!("Invalid argument for command {}: {}", cmd, arg)
        _ => unreachable!(),
    // No variables declared/removed by this custom syntax
    // Implementation function