rhai/doc/src/language/variables.md

Variables

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Valid Names

Variables in Rhai follow normal C naming rules - must contain only ASCII letters, digits and underscores '_', and cannot start with a digit.

For example: '_c3po' and 'r2d2' are valid variable names, but '3abc' is not.

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

Therefore, some names acceptable to Rust, like '_', '_42foo', '_1' etc., are not valid in Rhai. This restriction is to reduce confusion because, for instance, '_1' can easily be misread (or mis-typed) as -1.

Variable names are case sensitive.

Variable names also cannot be the same as a [keyword].

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.

Declare a Variable

Variables are declared using the let keyword.

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

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

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
x == 42;
X == 123;

{
    let x = 999;    // local variable 'x' shadows the 'x' in parent block
    x == 999;       // access to local 'x'
}
x == 42;            // the parent block's 'x' is not changed