Valid Names

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, 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, e.g. _, _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 inlines a string, which avoids allocations unless it 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 statement 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;


New variables automatically shadow existing ones of the same name. There is no error.

let x = 42;
let y = 123;

print(x);           // prints 42

let x = 88;         // <- 'x' is shadowed here

// At this point, it is no longer possible to access the
// original 'x' on the first line...

print(x);           // prints 88

let x = 0;          // <- 'x' is shadowed again

// At this point, it is no longer possible to access both
// previously-defined 'x'...

print(x);           // prints 0

    let x = 999;    // <- 'x' is shadowed in a block
    print(x);       // prints 999

print(x);           // prints 0 - shadowing within the block goes away

print(y);           // prints 123 - 'y' is not shadowed