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_archive/rhai_engine/rhaibook/language/modules/import.md

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+Import a Module
+===============
+
+{{#include ../../links.md}}
+
+```admonish info.side "See also"
+
+See [_Module Resolvers_][module resolver] for more details.
+```
+
+Before a [module] can be used (via an `import` statement) in a script, there must be a
+[module resolver] registered into the [`Engine`], the default being the `FileModuleResolver`.
+
+
+`import` Statement
+------------------
+
+```admonish tip.side.wide "Tip"
+
+A [module] that is only `import`-ed but not given any name is simply run.
+
+This is a very simple way to run another script file from within a script.
+```
+
+A [module] can be _imported_ via the `import` statement, and be given a name.
+
+Its members can be accessed via `::` similar to C++.
+
+```js
+import "crypto_banner";         // run the script file 'crypto_banner.rhai' without creating an imported module
+
+import "crypto" as lock;        // run the script file 'crypto.rhai' and import it as a module named 'lock'
+
+const SECRET_NUMBER = 42;
+
+let mod_file = `crypto_${SECRET_NUMBER}`;
+
+import mod_file as my_mod;      // load the script file "crypto_42.rhai" and import it as a module named 'my_mod'
+                                // notice that module path names can be dynamically constructed!
+                                // any expression that evaluates to a string is acceptable after the 'import' keyword
+
+lock::encrypt(secret);          // use functions defined under the module via '::'
+
+lock::hash::sha256(key);        // sub-modules are also supported
+
+print(lock::status);            // module variables are constants
+
+lock::status = "off";           // <- runtime error: cannot modify a constant
+```
+
+
+```admonish info "Imports are _scoped_"
+
+[Modules] imported via `import` statements are only accessible inside the relevant block scope.
+
+~~~js
+import "hacker" as h;           // import module - visible globally
+
+if secured {                    // <- new block scope
+    let mod = "crypt";
+
+    import mod + "o" as c;      // import module (the path needs not be a constant string)
+
+    let x = c::encrypt(key);    // use a function in the module
+
+    h::hack(x);                 // global module 'h' is visible here
+}                               // <- module 'c' disappears at the end of the block scope
+
+h::hack(something);             // this works as 'h' is visible
+
+c::encrypt(something);          // <- this causes a run-time error because
+                                //    module 'c' is no longer available!
+
+fn foo(something) {
+    h::hack(something);         // <- this also works as 'h' is visible
+}
+
+for x in 0..1000 {
+    import "crypto" as c;       // <- importing a module inside a loop is a Very Bad Idea™
+
+    c.encrypt(something);
+}
+~~~
+```
+
+~~~admonish note "Place `import` statements at the top"
+
+`import` statements can appear anywhere a normal statement can be, but in the vast majority of cases they are
+usually grouped at the top (beginning) of a script for manageability and visibility.
+
+It is not advised to deviate from this common practice unless there is a _Very Good Reason™_.
+
+Especially, do not place an `import` statement within a loop; doing so will repeatedly re-load the
+same [module] during every iteration of the loop!
+~~~
+
+~~~admonish danger "Recursive imports"
+
+Beware of _import cycles_ &ndash; i.e. recursively loading the same [module]. This is a sure-fire way to
+cause a stack overflow in the [`Engine`], unless stopped by setting a limit for [maximum number of modules].
+
+For instance, importing itself always causes an infinite recursion:
+
+```js
+┌────────────┐
+│ hello.rhai │
+└────────────┘
+
+import "hello" as foo;          // import itself - infinite recursion!
+
+foo::do_something();
+```
+
+[Modules] cross-referencing also cause infinite recursion:
+
+```js
+┌────────────┐
+│ hello.rhai │
+└────────────┘
+
+import "world" as foo;
+foo::do_something();
+
+
+┌────────────┐
+│ world.rhai │
+└────────────┘
+
+import "hello" as bar;
+bar::do_something_else();
+```
+~~~