Packages

Packages allow Move programmers to more easily re-use code and share it across projects. The Move package system allows programmers to easily:

Define a package containing Move code;
Parameterize a package by named addresses;
Import and use packages in other Move code and instantiate named addresses;
Build packages and generate associated compilation artifacts from packages; and
Work with a common interface around compiled Move artifacts.

Package Layout and Manifest Syntax

A Move package source directory contains a Move.toml package manifest file along with a set of subdirectories:

a_move_package
├── Move.toml      (required)
├── sources        (required)
├── examples       (optional, test & dev mode)
├── scripts        (optional)
├── doc_templates  (optional)
└── tests          (optional, test mode)

The directories marked required must be present in order for the directory to be considered a Move package and to be compiled. Optional directories can be present, and if so will be included in the compilation process. Depending on the mode that the package is built with (test or dev), the tests and examples directories will be included as well.

The sources directory can contain both Move modules and Move scripts (both transaction scripts and modules containing script functions). The examples directory can hold additional code to be used only for development and/or tutorial purposes that will not be included when compiled outside test or dev mode.

A scripts directory is supported so transaction scripts can be separated from modules if that is desired by the package author. The scripts directory will always be included for compilation if it is present. Documentation will be built using any documentation templates present in the doc_templates directory.

Move.toml

The Move package manifest is defined within the Move.toml file and has the following syntax. Optional fields are marked with *, + denotes one or more elements:

[package]
name = <string>                  # e.g., "MoveStdlib"
version = "<uint>.<uint>.<uint>" # e.g., "0.1.1"
license* = <string>              # e.g., "MIT", "GPL", "Apache 2.0"
authors* = [<string>]            # e.g., ["Joe Smith (joesmith@noemail.com)", "Jane Smith (janesmith@noemail.com)"]

[addresses]  # (Optional section) Declares named addresses in this package and instantiates named addresses in the package graph
# One or more lines declaring named addresses in the following format
<addr_name> = "_" | "<hex_address>" # e.g., std = "_" or my_addr = "0xC0FFEECAFE"

[dependencies] # (Optional section) Paths to dependencies and instantiations or renamings of named addresses from each dependency
# One or more lines declaring dependencies in the following format
<string> = { local = <string>, addr_subst* = { (<string> = (<string> | "<hex_address>"))+ } } # local dependencies
<string> = { git = <URL ending in .git>, subdir=<path to dir containing Move.toml inside git repo>, rev=<git commit hash>, addr_subst* = { (<string> = (<string> | "<hex_address>"))+ } } # git dependencies

[dev-addresses] # (Optional section) Same as [addresses] section, but only included in "dev" and "test" modes
# One or more lines declaring dev named addresses in the following format
<addr_name> = "_" | "<hex_address>" # e.g., std = "_" or my_addr = "0xC0FFEECAFE"

[dev-dependencies] # (Optional section) Same as [dependencies] section, but only included in "dev" and "test" modes
# One or more lines declaring dev dependencies in the following format
<string> = { local = <string>, addr_subst* = { (<string> = (<string> | <address>))+ } }

An example of a minimal package manifest with one local dependency and one git dependency:

[package]
name = "AName"
version = "0.0.0"

An example of a more standard package manifest that also includes the Move standard library and instantiates the named address Std from it with the address value 0x1:

[package]
name = "AName"
version = "0.0.0"
license = "Apache 2.0"

[addresses]
address_to_be_filled_in = "_"
specified_address = "0xB0B"

[dependencies]
# Local dependency
LocalDep = { local = "projects/move-awesomeness", addr_subst = { "std" = "0x1" } }
# Git dependency
MoveStdlib = { git = "https://github.com/diem/diem.git", subdir="language/move-stdlib", rev = "56ab033cc403b489e891424a629e76f643d4fb6b" }

[dev-addresses] # For use when developing this module
address_to_be_filled_in = "0x101010101"

Most of the sections in the package manifest are self explanatory, but named addresses can be a bit difficult to understand so it's worth examining them in a bit more detail.

Named Addresses During Compilation

Recall that Move has named addresses and that named addresses cannot be declared in Move. Because of this, until now named addresses and their values needed to be passed to the compiler on the command line. With the Move package system this is no longer needed, and you can declare named addresses in the package, instantiate other named addresses in scope, and rename named addresses from other packages within the Move package system manifest file. Let's go through each of these individually:

Declaration

Let's say we have a Move module in example_pkg/sources/A.move as follows:

module named_addr::A {
    public fun x(): address { @named_addr }
}

We could in example_pkg/Move.toml declare the named address named_addr in two different ways. The first:

[package]
name = "ExamplePkg"
...
[addresses]
named_addr = "_"

Declares named_addr as a named address in the package ExamplePkg and that this address can be any valid address value. Therefore an importing package can pick the value of the named address named_addr to be any address it wishes. Intuitively you can think of this as parameterizing the package ExamplePkg by the named address named_addr, and the package can then be instantiated later on by an importing package.

named_addr can also be declared as:

[package]
name = "ExamplePkg"
...
[addresses]
named_addr = "0xCAFE"

which states that the named address named_addr is exactly 0xCAFE and cannot be changed. This is useful so other importing packages can use this named address without needing to worry about the exact value assigned to it.

With these two different declaration methods, there are two ways that information about named addresses can flow in the package graph:

The former ("unassigned named addresses") allows named address values to flow from the importation site to the declaration site.
The latter ("assigned named addresses") allows named address values to flow from the declaration site upwards in the package graph to usage sites.

With these two methods for flowing named address information throughout the package graph the rules around scoping and renaming become important to understand.

Scoping and Renaming of Named Addresses

A named address N in a package P is in scope if:

It declares a named address N; or
A package in one of P's transitive dependencies declares the named address N and there is a dependency path in the package graph between between P and the declaring package of N with no renaming of N.

Additionally, every named address in a package is exported. Because of this and the above scoping rules each package can be viewed as coming with a set of named addresses that will be brought into scope when the package is imported, e.g., if the ExamplePkg package was imported, that importation would bring into scope the named_addr named address. Because of this, if P imports two packages P1 and P2 both of which declare a named address N an issue arises in P: which "N" is meant when N is referred to in P? The one from P1 or P2? To prevent this ambiguity around which package a named address is coming from, we enforce that the sets of scopes introduced by all dependencies in a package are disjoint, and provide a way to rename named addresses when the package that brings them into scope is imported.

Renaming a named address when importing can be done as follows in our P, P1, and P2 example above:

[package]
name = "P"
...
[dependencies]
P1 = { local = "some_path_to_P1", addr_subst = { "P1N" = "N" } }
P2 = { local = "some_path_to_P2"  }

With this renaming N refers to the N from P2 and P1N will refer to N coming from P1:

module N::A {
    public fun x(): address { @P1N }
}

It is important to note that renaming is not local: once a named address N has been renamed to N2 in a package P all packages that import P will not see N but only N2 unless N is reintroduced from outside of P. This is why rule (2) in the scoping rules at the start of this section specifies a "dependency path in the package graph between between P and the declaring package of N with no renaming of N."

Instantiation

Named addresses can be instantiated multiple times across the package graph as long as it is always with the same value. It is an error if the same named address (regardless of renaming) is instantiated with differing values across the package graph.

A Move package can only be compiled if all named addresses resolve to a value. This presents issues if the package wishes to expose an uninstantiated named address. This is what the [dev-addresses] section solves. This section can set values for named addresses, but cannot introduce any named addresses. Additionally, only the [dev-addresses] in the root package are included in dev mode. For example a root package with the following manifest would not compile outside of dev mode since named_addr would be uninstantiated:

[package]
name = "ExamplePkg"
...
[addresses]
named_addr = "_"

[dev-addresses]
named_addr = "0xC0FFEE"

Usage, Artifacts, and Data Structures

The Move package system comes with a command line option as part of the Move CLI move <flags> <command> <command_flags>. Unless a particular path is provided, all package commands will run in the current working directory. The full list of commands and flags for the Move CLI can be found by running move --help.

Usage

A package can be compiled either through the Move CLI commands, or as a library command in Rust with the function compile_package. This will create a CompiledPackage that holds the compiled bytecode along with other compilation artifacts (source maps, documentation, ABIs) in memory. This CompiledPackage can be converted to an OnDiskPackage and vice versa -- the latter being the data of the CompiledPackage laid out in the file system in the following format:

a_move_package
├── Move.toml
...
└── build
    ├── <dep_pkg_name>
    │   ├── BuildInfo.yaml
    │   ├── bytecode_modules
    │   │   └── *.mv
    │   ├── source_maps
    │   │   └── *.mvsm
    │   ├── bytecode_scripts
    │   │   └── *.mv
    │   ├── abis
    │   │   ├── *.abi
    │   │   └── <module_name>/*.abi
    │   └── sources
    │       └── *.move
    ...
    └── <dep_pkg_name>
        ├── BuildInfo.yaml
        ...
        └── sources

See the move-package crate for more information on these data structures and how to use the Move package system as a Rust library.

The Move Book