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_archive/rhai_engine/rhaibook/ref/constants.md

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+Constants
+=========
+
+Constants can be defined using the `const` keyword and are immutable.
+
+```rust
+const X;            // 'X' is a constant '()'
+
+const X = 40 + 2;   // 'X' is a constant 42
+
+print(X * 2);       // prints 84
+
+X = 123;            // <- syntax error: constant modified
+```
+
+```admonish tip.small "Tip: Naming"
+
+Constants follow the same naming rules as [variables](variables.md),
+but as a convention are often named with all-capital letters.
+```
+
+
+Automatic Global Module
+-----------------------
+
+When a [constant](constants.md) is declared at global scope, it is added to a special
+[module](modules/index.md) called `global`.
+
+[Functions](functions.md) can access those [constants](constants.md) via the special `global`
+[module](modules/index.md).
+
+```rust
+const CONSTANT = 42;        // this constant is automatically added to 'global'
+
+{
+    const INNER = 0;        // this constant is not at global level
+}                           // <- it goes away here
+
+fn foo(x) {
+    x *= global::CONSTANT;  // ok! 'CONSTANT' exists in 'global'
+
+    x * global::INNER       // <- error: constant 'INNER' not found in 'global'
+}
+```