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. Scripts are executable entrypoints 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#

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 account address 0x1.
    // 0x1 is shorthand for the fully qualified address
    // 0x00000000000000000000000000000001.
    use 0x1::Debug;

    const ONE: u64 = 1;

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

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

Modules#

A Module has the following syntax:

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

For example:

address 0x42 {
module Test {
    resource struct Example { i: u64 }

    use 0x1::Debug;

    const ONE: u64 = 1;

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

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

Multiple modules can be declared in a single address block:

address 0x42 {
module M { ... }
module N { ... }
}

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 FooBar42 {}

Typically, module names start with an uppercase letter. A module named MyModule should be stored in a source file named MyModule.move.

All elements inside a module block can appear in any order. Fundamentally, a module is a collection of types and functions. Uses import types from other modules. Constants define private constants that can be used in the functions of a module.