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](http://www.unicode.org/reports/tr31/).


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.

```rust
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
```