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_archive/rhai_engine/rhaibook/ref/variables.md

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+Variables
+=========
+
+
+Valid Names
+-----------
+
+Variables in Rhai follow normal C naming rules – must contain only ASCII letters, digits and underscores `_`.
+
+| Character set | Description              |
+| :-----------: | ------------------------ |
+|  `A` ... `Z`  | Upper-case ASCII letters |
+|  `a` ... `z`  | Lower-case ASCII letters |
+|  `0` ... `9`  | Digit 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.
+
+```admonish question.side.wide "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](keywords.md) (active or reserved).
+
+```admonish warning "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.
+
+~~~js
+// 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.
+
+```admonish tip.small "Tip: No initial value"
+
+Variables do not have to be given an initial value.
+If none is provided, it defaults to `()`.
+```
+
+```admonish warning.small "Variables are local"
+
+A variable defined within a [statements block](statements.md) is _local_ to that block.
+```
+
+~~~admonish tip.small "Tip: `is_def_var`"
+
+Use `is_def_var` to detect if a variable is defined.
+~~~
+
+```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
+
+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;
+```
+
+
+Shadowing
+---------
+
+New variables automatically _shadow_ existing ones of the same name.  There is no error.
+
+```rust
+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
+```