Modules and Scripts

Modules and Scripts

Move has two different types of programs: Modules and Scripts. Modules are libraries that define struct types along with functions that operate on these types. Struct types define the schema of Move's global storage, and module functions define the rules for updating storage. Modules themselves are also stored in global storage. A scripts is an executable entrypoint similar to a main function in a conventional language. A script typically calls functions of a published module that perform updates to global storage. Scripts are ephemeral code snippets that are not published in global storage.

A Move source file (or compilation unit) may contain multiple modules and scripts. However, publishing a module or executing a script are separate VM operations.

Syntax

Scripts

:::tip Tutorial To learn how to publish and execute a Move script, follow the Move Scripts example. :::

A script has the following structure:

script {
    <use>*
    <constants>*
    fun <identifier><[type parameters: constraint]*>([identifier: type]*) <function_body>
}

A script block must start with all of its use declarations, followed by any constants and (finally) the main function declaration. The main function can have any name (i.e., it need not be called main), is the only function in a script block, can have any number of arguments, and must not return a value. Here is an example with each of these components:

script {
    // Import the debug module published at the named account address std.
    use std::debug;

    const ONE: u64 = 1;

    fun main(x: u64) {
        let sum = x + ONE;
        debug::print(&sum)
    }
}

Scripts have very limited power—they cannot declare friends, struct types or access global storage. Their primary purpose is to invoke module functions.

Modules

A module has the following syntax:

module <address>::<identifier> {
    (<use> | <friend> | <type> | <function> | <constant>)*
}

where <address> is a valid named or literal address.

For example:

module 0x42::example {
    struct Example has copy, drop { i: u64 }

    use std::debug;
    friend 0x42::another_example;

    const ONE: u64 = 1;

    public fun print(x: u64) {
        let sum = x + ONE;
        let example = Example { i: sum };
        debug::print(&sum)
    }
}

The module 0x42::example part specifies that the module example will be published under the account address 0x42 in global storage.

Modules can also be declared using named addresses. For example:

module example_addr::example {
    struct Example has copy, drop { a: address }

    use std::debug;
    friend example_addr::another_example;

    public fun print() {
        let example = Example { a: @example_addr };
        debug::print(&example)
    }
}

Because named addresses only exist at the source language level and during compilation, named addresses will be fully substituted for their value at the bytecode level. For example if we had the following code:

script {
    fun example() {
        my_addr::m::foo(@my_addr);
    }
}

and we compiled it with my_addr set to 0xC0FFEE, then it would be equivalent to the following operationally:

script {
    fun example() {
        0xC0FFEE::m::foo(@0xC0FFEE);
    }
}

However, at the source level, these are not equivalent—the function m::foo must be accessed through the my_addr named address, and not through the numerical value assigned to that address.

Module names can start with letters a to z or letters A to Z. After the first character, module names can contain underscores _, letters a to z, letters A to Z, or digits 0 to 9.

module my_module {}
module foo_bar_42 {}

Typically, module names start with a lowercase letter. A module named my_module should be stored in a source file named my_module.move.

All elements inside a module block can appear in any order. Fundamentally, a module is a collection of types and functions. The use keyword is used to import types from other modules. The friend keyword specifies a list of trusted modules. The const keyword defines private constants that can be used in the functions of a module.