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Move Tutorial

Welcome to the Move Tutorial! In this tutorial, we are going to go through some steps of developing Move code including design, implementation, unit testing and formal verification of Move modules.

There are nine steps in total:

Each step is designed to be self-contained in the corresponding step_x folder. For example, if you would like to skip the contents in step 1 through 4, feel free to jump to step 5 since all the code we have written before step 5 will be in step_5 folder. At the end of some steps, we also include additional material on more advanced topics.

Now let's get started!

Step 0: Installation

If you haven't already, open your terminal and clone the Move repository:

git clone https://github.com/move-language/move.git

Go to the move directory and run the dev_setup.sh script:

cd move
./scripts/dev_setup.sh -ypt

Follow the script's prompts in order to install all of Move's dependencies.

The script adds environment variable definitions to your ~/.profile file. Include them by running this command:

source ~/.profile

Next, install Move's command-line tool by running this commands:

cargo install --path language/tools/move-cli

You can check that it is working by running the following command:

move --help

You should see something like this along with a list and description of a number of commands:

move-cli 0.1.0
Diem Association <[email protected]>
MoveCLI is the CLI that will be executed by the `move-cli` command The `cmd` argument is added here
rather than in `Move` to make it easier for other crates to extend `move-cli`

USAGE:
    move [OPTIONS] <SUBCOMMAND>

OPTIONS:
        --abi                          Generate ABIs for packages
...

If you want to find what commands are available and what they do, running a command or subcommand with the --help flag will print documentation.

Before running the next steps, cd to the tutorial directory:

cd <path_to_move>/language/documentation/tutorial
Visual Studio Code Move support There is official Move support for Visual Studio Code. You need to install the move analyzer first: 
cargo install --path language/move-analyzer

Now you can install the VS extension by opening VS Code, searching for the "move-analyzer" in the Extension Pane, and installing it. More detailed instructions can be found in the extension's README.

Step 1: Writing my first Move module

Change directory into the step_1/BasicCoin directory. You should see a directory called sources -- this is the place where all the Move code for this package lives. You should also see a Move.toml file as well. This file specifies dependencies and other information about the package; if you're familiar with Rust and Cargo, the Move.toml file is similar to the Cargo.toml file, and the sources directory similar to the src directory.

Let's take a look at some Move code! Open up sources/FirstModule.move in your editor of choice. The first thing you'll see is this:

// sources/FirstModule.move
module 0xCAFE::BasicCoin {
    ...
}

This is defining a Move module. Modules are the building blocks of Move code, and are defined with a specific address -- the address that the module can be published under. In this case, the BasicCoin module can only be published under 0xCAFE.

Let's now take a look at the next part of this file where we define a struct to represent a Coin with a given value:

module 0xCAFE::BasicCoin {
    struct Coin has key {
        value: u64,
    }
    ...
}

Looking at the rest of the file, we see a function definition that creates a Coin struct and stores it under an account:

module 0xCAFE::BasicCoin {
    struct Coin has key {
        value: u64,
    }

    public fun mint(account: signer, value: u64) {
        move_to(&account, Coin { value })
    }
}

Let's take a look at this function and what it's saying:

  • It takes a signer -- an unforgeable token that represents control over a particular address, and a value to mint.
  • It creates a Coin with the given value and stores it under the account using the move_to operator.

Let's make sure it builds! This can be done with the build command from within the package folder (step_1/BasicCoin):

move build
Advanced concepts and references

  • You can create an empty Move package by calling:

    move new <pkg_name>

  • Move code can also live a number of other places. More information on the Move package system can be found in the Move book

  • More information on the Move.toml file can be found in the package section of the Move book.

  • Move also supports the idea of named addresses, Named addresses are a way to parametrize Move source code so that you can compile the module using different values for NamedAddr to get different bytecode that you can deploy, depending on what address(es) you control. They are used quite frequently, and can be defined in the Move.toml file in the [addresses] section, e.g.,

    [addresses]
SomeNamedAddress = "0xC0FFEE"

  • Structures in Move can be given different abilities that describe what can be done with that type. There are four different abilities:

    • copy: Allows values of types with this ability to be copied.
    • drop: Allows values of types with this ability to be popped/dropped.
    • store: Allows values of types with this ability to exist inside a struct in global storage.
    • key: Allows the type to serve as a key for global storage operations.

    So in the BasicCoin module we are saying that the Coin struct can be used as a key in global storage and, because it has no other abilities, it cannot be copied, dropped, or stored as a non-key value in storage. So you can't copy coins, and you also can't lose coins by accident!

  • Functions are default private, and can also be public, public(friend), or public(script). The last of these states that this function can be called from a transaction script. public(script) functions can also be called by other public(script) functions.

  • move_to is one of the five different global storage operators.

Step 2: Adding unit tests to my first Move module

Now that we've taken a look at our first Move module, we'll take a look at a test to make sure minting works the way we expect it to by changing directory to step_2/BasicCoin. Unit tests in Move are similar to unit tests in Rust if you're familiar with them -- tests are annotated with #[test] and written like normal Move functions.

You can run the tests with the move test command:

move test

Let's now take a look at the contents of the FirstModule.move file. The first new thing you'll see is this test:

module 0xCAFE::BasicCoin {
    ...
    // Declare a unit test. It takes a signer called `account` with an
    // address value of `0xC0FFEE`.
    #[test(account = @0xC0FFEE)]
    fun test_mint_10(account: signer) acquires Coin {
        let addr = 0x1::signer::address_of(&account);
        mint(account, 10);
        // Make sure there is a `Coin` resource under `addr` with a value of `10`.
        // We can access this resource and its value since we are in the
        // same module that defined the `Coin` resource.
        assert!(borrow_global<Coin>(addr).value == 10, 0);
    }
}

This is declaring a unit test called test_mint_10 that mints a Coin struct under the account with a value of 10. It is then checking that the minted coin in storage has the value that is expected with the assert! call. If the assertion fails the unit test will fail.

Advanced concepts and exercises

  • There are a number of test-related annotations that are worth exploring, they can be found here. You'll see some of these used in Step 5.

  • Before running unit tests, you'll always need to add a dependency on the Move standard library. This can be done by adding an entry to the [dependencies] section of the Move.toml, e.g.,

    [dependencies]
MoveStdlib = { local = "../../../../move-stdlib/", addr_subst = { "std" = "0x1" } }

    Note that you may need to alter the path to point to the move-stdlib directory under <path_to_move>/language. You can also specify git dependencies. You can read more on Move package dependencies here.

Exercises

  • Change the assertion to 11 so that the test fails. Find a flag that you can pass to the move test command that will show you the global state when the test fails. It should look something like this: 
    ┌── test_mint_10 ──────
│ error[E11001]: test failure
│    ┌─ ./sources/FirstModule.move:24:9
│    │
│ 18 │     fun test_mint_10(account: signer) acquires Coin {
│    │         ------------ In this function in 0xcafe::BasicCoin
│    ·
│ 24 │         assert!(borrow_global<Coin>(addr).value == 11, 0);
│    │         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Test was not expected to abort but it aborted with 0 here
│
│
│ ────── Storage state at point of failure ──────
│ 0xc0ffee:
│       => key 0xcafe::BasicCoin::Coin {
│           value: 10
│       }
└──────────────────
  • Find a flag that allows you to gather test coverage information, and then play around with using the move coverage command to look at coverage statistics and source coverage.

Step 3: Designing my BasicCoin module

In this section, we are going to design a module implementing a basic coin and balance interface, where coins can be minted and transferred between balances held under different addresses.

The signatures of the public Move function are the following:

/// Publish an empty balance resource under `account`'s address. This function must be called before
/// minting or transferring to the account.
public fun publish_balance(account: &signer) { ... }

/// Mint `amount` tokens to `mint_addr`. Mint must be approved by the module owner.
public fun mint(module_owner: &signer, mint_addr: address, amount: u64) acquires Balance { ... }

/// Returns the balance of `owner`.
public fun balance_of(owner: address): u64 acquires Balance { ... }

/// Transfers `amount` of tokens from `from` to `to`.
public fun transfer(from: &signer, to: address, amount: u64) acquires Balance { ... }

Next we look at the data structs we need for this module.

A Move module doesn't have its own storage. Instead, Move "global storage" (what we call our blockchain state) is indexed by addresses. Under each address there are Move modules (code) and Move resources (values).

The global storage looks roughly like this in Rust syntax:

struct GlobalStorage {
    resources: Map<address, Map<ResourceType, ResourceValue>>
    modules: Map<address, Map<ModuleName, ModuleBytecode>>
}

The Move resource storage under each address is a map from types to values. (An observant reader might observe that this means each address can only have one value of each type.) This conveniently provides us a native mapping indexed by addresses. In our BasicCoin module, we define the following Balance resource representing the number of coins each address holds:

/// Struct representing the balance of each address.
struct Balance has key {
    coin: Coin // same Coin from Step 1
}

Roughly the Move blockchain state should look like this:

Advanced topics:

public(script) functions

Only functions with public(script) visibility can be invoked directly in transactions. So if you would like to call the transfer method directly from a transaction, you'll want to change its signature to:

public(script) fun transfer(from: signer, to: address, amount: u64) acquires Balance { ... }

Read more on Move function visibilities here.

Comparison with Ethereum/Solidity

In most Ethereum ERC-20 contracts, the balance of each address is stored in a state variable of type mapping(address => uint256). This state variable is stored in the storage of a particular smart contract.

The Ethereum blockchain state might look like this:

Step 4: Implementing my BasicCoin module

We have created a Move package for you in folder step_4 called BasicCoin. The sources folder contains source code for all your Move modules in the package, including BasicCoin.move. In this section, we will take a closer look at the implementation of the methods inside BasicCoin.move.

Compiling our code

Let's first try building the code using Move package by running the following command in step_4/BasicCoin folder:

move build

Implementation of methods

Now let's take a closer look at the implementation of the methods inside BasicCoin.move.

Method publish_balance

This method publishes a Balance resource to a given address. Since this resource is needed to receive coins through minting or transferring, publish_balance method must be called by a user before they can receive money, including the module owner.

This method uses a move_to operation to publish the resource:

let empty_coin = Coin { value: 0 };
move_to(account, Balance { coin:  empty_coin });
Method mint

mint method mints coins to a given account. Here we require that mint must be approved by the module owner. We enforce this using the assert statement:

assert!(signer::address_of(&module_owner) == MODULE_OWNER, ENOT_MODULE_OWNER);

Assert statements in Move can be used in this way: assert!(<predicate>, <abort_code>);. This means that if the <predicate> is false, then abort the transaction with <abort_code>. Here MODULE_OWNER and ENOT_MODULE_OWNER are both constants defined at the beginning of the module. The standard library's error module also defines common error categories we can use. It is important to note that Move is transactional in its execution -- so if an abort is raised no unwinding of state needs to be performed, as no changes from that transaction will be persisted to the blockchain.

We then deposit a coin with value amount to the balance of mint_addr.

deposit(mint_addr, Coin { value: amount });
Method balance_of

We use borrow_global, one of the global storage operators, to read from the global storage.

borrow_global<Balance>(owner).coin.value
                 |       |       \    /
        resource type  address  field names
Method transfer

This function withdraws tokens from from's balance and deposits the tokens into tos balance. We take a closer look at withdraw helper function:

fun withdraw(addr: address, amount: u64) : Coin acquires Balance {
    let balance = balance_of(addr);
    assert!(balance >= amount, EINSUFFICIENT_BALANCE);
    let balance_ref = &mut borrow_global_mut<Balance>(addr).coin.value;
    *balance_ref = balance - amount;
    Coin { value: amount }
}

At the beginning of the method, we assert that the withdrawing account has enough balance. We then use borrow_global_mut to get a mutable reference to the global storage, and &mut is used to create a mutable reference to a field of a struct. We then modify the balance through this mutable reference and return a new coin with the withdrawn amount.

Exercises

There are two TODOs in our module, left as exercises for the reader:

  • Finish implementing the publish_balance method.
  • Implement the deposit method.

The solution to this exercise can be found in step_4_sol folder.

Bonus exercise

  • What would happen if we deposit too many tokens to a balance?

Step 5: Adding and using unit tests with the BasicCoin module

In this step we're going to take a look at all the different unit tests we've written to cover the code we wrote in step 4. We're also going to take a look at some tools we can use to help us write tests.

To get started, run the package test command in the step_5/BasicCoin folder

move test

You should see something like this:

INCLUDING DEPENDENCY MoveStdlib
BUILDING BasicCoin
Running Move unit tests
[ PASS    ] 0xcafe::BasicCoin::can_withdraw_amount
[ PASS    ] 0xcafe::BasicCoin::init_check_balance
[ PASS    ] 0xcafe::BasicCoin::init_non_owner
[ PASS    ] 0xcafe::BasicCoin::publish_balance_already_exists
[ PASS    ] 0xcafe::BasicCoin::publish_balance_has_zero
[ PASS    ] 0xcafe::BasicCoin::withdraw_dne
[ PASS    ] 0xcafe::BasicCoin::withdraw_too_much
Test result: OK. Total tests: 7; passed: 7; failed: 0

Taking a look at the tests in the BasicCoin module we've tried to keep each unit test to testing one particular behavior.

Exercise

After taking a look at the tests, try and write a unit test called balance_of_dne in the BasicCoin module that tests the case where a Balance resource doesn't exist under the address that balance_of is being called on. It should only be a couple lines!

The solution to this exercise can be found in step_5_sol

Step 6: Making my BasicCoin module generic

In Move, we can use generics to define functions and structs over different input data types. Generics are a great building block for library code. In this section, we are going to make our simple BasicCoin module generic so that it can serve as a library module that can be used by other user modules.

First, we add type parameters to our data structs:

struct Coin<phantom CoinType> has store {
    value: u64
}

struct Balance<phantom CoinType> has key {
    coin: Coin<CoinType>
}

We also add type parameters to our methods in the same manner. For example, withdraw becomes the following:

fun withdraw<CoinType>(addr: address, amount: u64) : Coin<CoinType> acquires Balance {
    let balance = balance_of<CoinType>(addr);
    assert!(balance >= amount, EINSUFFICIENT_BALANCE);
    let balance_ref = &mut borrow_global_mut<Balance<CoinType>>(addr).coin.value;
    *balance_ref = balance - amount;
    Coin<CoinType> { value: amount }
}

Take a look at step_6/BasicCoin/sources/BasicCoin.move to see the full implementation.

At this point, readers who are familiar with Ethereum might notice that this module serves a similar purpose as the ERC20 token standard, which provides an interface for implementing fungible tokens in smart contracts. One key advantage of using generics is the ability to reuse code since the generic library module already provides a standard implementation and the instantiating module can provide customizations by wrapping the standard implementation.

We provide a little module called MyOddCoin that instantiates the Coin type and customizes its transfer policy: only odd number of coins can be transferred. We also include two tests to test this behavior. You can use the commands you learned in step 2 and step 5 to run the tests.

Advanced topics:

phantom type parameters

In definitions of both Coin and Balance, we declare the type parameter CoinType to be phantom because CoinType is not used in the struct definition or is only used as a phantom type parameter.

Read more about phantom type parameters here.

Advanced steps

Before moving on to the next steps, let's make sure you have all the prover dependencies installed.

Try running boogie /version . If an error message shows up saying "command not found: boogie", you will have to run the setup script and source your profile:

# run the following in move repo root directory
./scripts/dev_setup.sh -yp
source ~/.profile

Step 7: Use the Move prover

Smart contracts deployed on the blockchain may manipulate high-value assets. As a technique that uses strict mathematical methods to describe behavior and reason correctness of computer systems, formal verification has been used in blockchains to prevent bugs in smart contracts. The Move prover is an evolving formal verification tool for smart contracts written in the Move language. The user can specify functional properties of smart contracts using the Move Specification Language (MSL) and then use the prover to automatically check them statically. To illustrate how the prover is used, we have added the following code snippet to the BasicCoin.move:

    spec balance_of {
        pragma aborts_if_is_strict;
    }

Informally speaking, the block spec balance_of {...} contains the property specification of the method balance_of.

Let's first run the prover using the following command inside BasicCoin directory:

move prove

which outputs the following error information:

error: abort not covered by any of the `aborts_if` clauses
   ┌─ ./sources/BasicCoin.move:38:5
   │
35 │           borrow_global<Balance<CoinType>>(owner).coin.value
   │           ------------- abort happened here with execution failure
   ·
38 │ ╭     spec balance_of {
39 │ │         pragma aborts_if_is_strict;
40 │ │     }
   │ ╰─────^
   │
   =     at ./sources/BasicCoin.move:34: balance_of
   =         owner = 0x29
   =     at ./sources/BasicCoin.move:35: balance_of
   =         ABORTED

Error: exiting with verification errors

The prover basically tells us that we need to explicitly specify the condition under which the function balance_of will abort, which is caused by calling the function borrow_global when owner does not own the resource Balance<CoinType>. To remove this error information, we add an aborts_if condition as follows:

    spec balance_of {
        pragma aborts_if_is_strict;
        aborts_if !exists<Balance<CoinType>>(owner);
    }

After adding this condition, try running the prove command again to confirm that there are no verification errors:

move prove

Apart from the abort condition, we also want to define the functional properties. In Step 8, we will give more detailed introduction to the prover by specifying properties for the methods defined the BasicCoin module.

Step 8: Write formal specifications for the BasicCoin module

Method withdraw

The signature of the method withdraw is given below:

fun withdraw<CoinType>(addr: address, amount: u64) : Coin<CoinType> acquires Balance

The method withdraws tokens with value amount from the address addr and returns a created Coin of value amount. The method withdraw aborts when 1) addr does not have the resource Balance<CoinType> or 2) the number of tokens in addr is smaller than amount. We can define conditions like this:

    spec withdraw {
        let balance = global<Balance<CoinType>>(addr).coin.value;
        aborts_if !exists<Balance<CoinType>>(addr);
        aborts_if balance < amount;
    }

As we can see here, a spec block can contain let bindings which introduce names for expressions. global<T>(address): T is a built-in function that returns the resource value at addr. balance is the number of tokens owned by addr. exists<T>(address): bool is a built-in function that returns true if the resource T exists at address. Two aborts_if clauses correspond to the two conditions mentioned above. In general, if a function has more than one aborts_if condition, those conditions are or-ed with each other. By default, if a user wants to specify aborts conditions, all possible conditions need to be listed. Otherwise, the prover will generate a verification error. However, if pragma aborts_if_is_partial is defined in the spec block, the combined aborts condition (the or-ed individual conditions) only imply that the function aborts. The reader can refer to the MSL document for more information.

The next step is to define functional properties, which are described in the two ensures clauses below. First, by using the let post binding, balance_post represents the balance of addr after the execution, which should be equal to balance - amount. Then, the return value (denoted as result) should be a coin with value amount.

    spec withdraw {
        let balance = global<Balance<CoinType>>(addr).coin.value;
        aborts_if !exists<Balance<CoinType>>(addr);
        aborts_if balance < amount;

        let post balance_post = global<Balance<CoinType>>(addr).coin.value;
        ensures balance_post == balance - amount;
        ensures result == Coin<CoinType> { value: amount };
    }
Method deposit

The signature of the method deposit is given below:

fun deposit<CoinType>(addr: address, check: Coin<CoinType>) acquires Balance

The method deposits the check into addr. The specification is defined below:

    spec deposit {
        let balance = global<Balance<CoinType>>(addr).coin.value;
        let check_value = check.value;

        aborts_if !exists<Balance<CoinType>>(addr);
        aborts_if balance + check_value > MAX_U64;

        let post balance_post = global<Balance<CoinType>>(addr).coin.value;
        ensures balance_post == balance + check_value;
    }

balance represents the number of tokens in addr before execution and check_value represents the number of tokens to be deposited. The method would abort if 1) addr does not have the resource Balance<CoinType> or 2) the sum of balance and check_value is greater than the maxium value of the type u64. The functional property checks that the balance is correctly updated after the execution.

Method transfer

The signature of the method transfer is given below:

public fun transfer<CoinType: drop>(from: &signer, to: address, amount: u64, _witness: CoinType) acquires Balance

The method transfers the amount of coin from the account of from to the address to. The specification is given below:

    spec transfer {
        let addr_from = signer::address_of(from);

        let balance_from = global<Balance<CoinType>>(addr_from).coin.value;
        let balance_to = global<Balance<CoinType>>(to).coin.value;
        let post balance_from_post = global<Balance<CoinType>>(addr_from).coin.value;
        let post balance_to_post = global<Balance<CoinType>>(to).coin.value;

        ensures balance_from_post == balance_from - amount;
        ensures balance_to_post == balance_to + amount;
    }

addr_from is the address of from. Then the balances of addr_from and to before and after the execution are obtained. The ensures clauses specify that the amount number of tokens is deducted from addr_from and added to to. However, the prover will generate the error information as below:

error: post-condition does not hold
   ┌─ ./sources/BasicCoin.move:57:9
   │
62 │         ensures balance_from_post == balance_from - amount;
   │         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   │
   ...

The property is not held when addr_from is equal to to. As a result, we could add an assertion assert!(from_addr != to) in the method to make sure that addr_from is not equal to to.

Exercises
  • Implement the aborts_if conditions for the transfer method.
  • Implement the specification for the mint and publish_balance method.

The solution to this exercise can be found in step_8_sol.