Valid Names

Tip: Unicode Standard Annex #31 identifiers

The unicode-xid-ident feature expands the allowed characters for variable names to the set defined by Unicode Standard Annex #31.

Variables in Rhai follow normal C naming rules – must contain only ASCII letters, digits and underscores _.

Character setDescription
AZUpper-case ASCII letters
azLower-case ASCII letters
09Digit characters
_Underscore character

However, unlike Rust, a variable name must also contain at least one ASCII letter, and an ASCII letter must come before any digits. In other words, the first character that is not an underscore _ must be an ASCII letter and not a digit.

Why this restriction?

To reduce confusion (and subtle bugs) because, for instance, _1 can easily be misread (or mistyped) as -1.

Rhai is dynamic without type checking, so there is no compiler to catch these typos.

Therefore, some names acceptable to Rust, like _, _42foo, _1 etc., are not valid in Rhai.

For example: c3po and _r2d2_ are valid variable names, but 3abc and ____49steps are not.

Variable names are case sensitive.

Variable names also cannot be the same as a keyword (active or reserved).

Avoid names longer than 11 letters on 32-Bit

Rhai uses SmartString which avoids allocations unless a string is over its internal limit (23 ASCII characters on 64-bit, but only 11 ASCII characters on 32-bit).

On 64-bit systems, most variable names are shorter than 23 letters, so this is unlikely to become an issue.

However, on 32-bit systems, take care to limit, where possible, variable names to within 11 letters. This is particularly true for local variables inside a hot loop, where they are created and destroyed in rapid succession.

// The following is SLOW on 32-bit
for my_super_loop_variable in array {
    print(`Super! ${my_super_loop_variable}`);

// Suggested revision:
for loop_var in array {
    print(`Super! ${loop_var}`);

Declare a Variable

Variables are declared using the let keyword.

Tip: No initial value

Variables do not have to be given an initial value. If none is provided, it defaults to ().

Variables are local

A variable defined within a statements block is local to that block.

Tip: is_def_var

Use is_def_var to detect if a variable is defined.

let x;              // ok - value is '()'
let x = 3;          // ok
let _x = 42;        // ok
let x_ = 42;        // also ok
let _x_ = 42;       // still ok

let _ = 123;        // <- syntax error: illegal variable name
let _9 = 9;         // <- syntax error: illegal variable name

let x = 42;         // variable is 'x', lower case
let X = 123;        // variable is 'X', upper case

print(x);           // prints 42
print(X);           // prints 123

    let x = 999;    // local variable 'x' shadows the 'x' in parent block

    print(x);       // prints 999

print(x);           // prints 42 - the parent block's 'x' is not changed

let x = 0;          // new variable 'x' shadows the old 'x'

print(x);           // prints 0

is_def_var("x") == true;

is_def_var("_x") == true;

is_def_var("y") == false;

Use Before Definition

By default, variables do not need to be defined before they are used.

If a variable accessed by a script is not defined previously within the same script, it is assumed to be provided via an external custom Scope passed to the Engine via the Engine::XXX_with_scope API.

let engine = Engine::new();"print(answer)")?;       // <- error: variable 'answer' not found

// Create custom scope
let mut scope = Scope::new();

// Add variable to custom scope
scope.push("answer", 42_i64);

// Run with custom scope
engine.run_with_scope(&mut scope,
    "print(answer)"                 // <- prints 42

No Scope

If no Scope is used to evaluate the script (e.g. when using Engine::run instead of Engine::run_with_scope), an undefined variable causes a runtime error when accessed.

Strict Variables Mode

With Engine::set_strict_variables, it is possible to turn on Strict Variables mode.

When strict variables mode is active, accessing a variable not previously defined within the same script directly causes a parse error when compiling the script.

let x = 42;

print(x);           // prints 42

print(foo);         // <- parse error under strict variables mode:
                    //    variable 'foo' is undefined


Turn on strict variables mode if no Scope is to be provided for script evaluation runs. This way, variable access errors are caught during compile time instead of runtime.