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 to use custom operators first. Defining 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.

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.

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 or reserved keyword. In other words, any valid identifier that is not a keyword will work fine.

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.


fn main() {
exec $ident$ <- $expr$ : $block$

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

1execcustom keyword
21$ident$a variable name
3<-the left-arrow symbol (which is a reserved symbol in Rhai).
42$expr$an expression, which may be enclosed with { .. }, or not.
5:the colon symbol
63$block$a statement block, which must be enclosed with { .. }.

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

fn main() {
// Assuming the 'exec' custom syntax implementation declares the variable 'hello':
let x = exec hello <- 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 function signature of an implementation is:

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


context&mut EvalContextmutable reference to the current evaluation context
scope()&Scopereference to the current Scope
scope_mut()&mut &mut Scopemutable reference to the current Scope; variables can be added to/removed from it
engine()&Enginereference to the current Engine
source()Option<&str>reference to the current source, if any
iter_imports()impl Iterator<Item = (&str, &Module)>iterator of the current stack of modules imported via import statements
imports()&Importsreference to the current stack of modules imported via import statements; requires the internals feature
iter_namespaces()impl Iterator<Item = &Module>iterator of the namespaces (as modules) containing all script-defined functions
namespaces()&[&Module]reference to the namespaces (as modules) containing all script-defined functions; requires the internals feature
this_ptr()Option<&Dynamic>reference to the current bound [this] pointer, if any
call_level()usizethe current nesting level of function calls
inputs&[Expression]a list of input expression trees

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_variable_name().unwrap()&strname of a variable
$expr$inputs.get(n).unwrap()Expressionan expression tree
$block$inputs.get(n).unwrap()Expressionan expression tree

Evaluate an Expression Tree

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

fn main() {
let expression = inputs.get(0).unwrap();
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.

fn main() {
let var_name = inputs[0].get_variable_name().unwrap();
let expression = inputs.get(1).unwrap();

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.

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

    // Push new variable into the scope BEFORE 'context.eval_expression_tree'
    context.scope_mut().push(var_name.clone(), 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 stop = !context.eval_expression_tree(condition)?
                            .as_bool().map_err(|err| Box::new(

        if stop {


// 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:

fn main() {
// 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 ';'

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!

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:

fn main() {
// 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:

fn main() {
// 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

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

  • The symbols parsed so far; $ident$ is replaced with the actual identifier parsed, while $expr$ and $block$ stay as they were.

    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.

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


fn main() {
    // The custom parser implementation - always returns the next symbol expected
    // 'look_ahead' is the next symbol about to be read
    |symbols, look_ahead| match symbols.len() {
        // perform ...
        1 => Ok(Some("$ident$".to_string())),
        // 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(None),
            cmd => Err(ParseError(Box::new(ParseErrorType::BadInput(
                LexError::ImproperSymbol(format!("Improper command: {}", cmd))
            )), Position::NONE)),
        // perform command arg ...
        3 => match (symbols[1].as_str(), symbols[2].as_str()) {
            ("action", _) => Ok(None),
            ("hello", "world") => Ok(None),
            ("update", arg) if arg == "system" => Ok(None),
            ("update", arg) if arg == "client" => Ok(None),
            ("check", arg) => Ok(None),
            ("add", arg) => Ok(None),
            ("remove", arg) => Ok(None),
            (cmd, arg) => Err(ParseError(Box::new(ParseErrorType::BadInput(
                    format!("Invalid argument for command {}: {}", cmd, arg)
            )), Position::NONE)),
        _ => unreachable!(),
    // No variables declared/removed by this custom syntax
    // Implementation function

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 $block$; $ident$ is replaced by the actual identifier
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.

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(ParseError)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.