Debug and Publish the Sui Move Package

Debugging a package

At the moment there isn't a yet debugger for Move. To help with debugging, however, you could use std::debug module to print out arbitrary value. To do so, first import the debug module:

use std::debug;

Then in places where you want to print out a value v, regardless of its type, simply do:


or the following if v is already a reference:


The debug module also provides a function to print out the current stacktrace:


Alternatively, any call to abort or assertion failure will also print the stacktrace at the point of failure.

Important: All calls to functions in the debug module must be removed from no-test code before the new module can be published (test code is marked with the #[test] annotation).

Publishing a package

For functions in a Move package to actually be callable from Sui (rather than for Sui execution scenario to be emulated), the package has to be published to Sui's distributed ledger where it is represented as a Sui object.

At this point, however, the sui move command does not support package publishing. In fact, it is not clear if it even makes sense to accommodate package publishing, which happens once per package creation, in the context of a unit testing framework. Instead, one can use a Sui CLI client to publish Move code and to call it. See the Sui CLI client documentation for a description of how to publish the package we have written as as part of this tutorial.

Module initializers

There is, however, an important aspect of publishing packages that affects Move code development in Sui - each module in a package can include a special initializer function that will be run at the publication time. The goal of an initializer function is to pre-initialize module-specific data (e.g., to create singleton objects). The initializer function must have the following properties in order to be executed at publication:

  • name init
  • single parameter of &mut TxContext type
  • no return values
  • private visibility

While the sui move command does not support publishing explicitly, we can still test module initializers using our testing framework - one can simply dedicate the first transaction to executing the initializer function. Let us use a concrete example to illustrate this.

Continuing our fantasy game example, let's introduce a concept of a forge that will be involved in the process of creating swords - for starters let it keep track of how many swords have been created. Let us define the Forge struct and a function returning the number of created swords as follows and put into the my_module.move file:

    struct Forge has key, store {
        id: UID,
        swords_created: u64,

    public fun swords_created(self: &Forge): u64 {

In order to keep track of the number of created swords we must initialize the forge object and set its sword_create counts to 0. And module initializer is the perfect place to do it:

    // module initializer to be executed when this module is published
    fun init(ctx: &mut TxContext) {
        use sui::transfer;
        use sui::tx_context;
        let admin = Forge {
            id: object::new(ctx),
            swords_created: 0,
        // transfer the forge object to the module/package publisher
        // (presumably the game admin)
        transfer::transfer(admin, tx_context::sender(ctx));

In order to use the forge, we need to modify the sword_create function to take the forge as a parameter and to update the number of created swords at the end of the function:

    public entry fun sword_create(forge: &mut Forge, magic: u64, strength: u64, recipient: address, ctx: &mut TxContext) {
        forge.swords_created = forge.swords_created + 1;

We can now create a function to test the module initialization:

    public fun test_module_init() {
        use sui::test_scenario;

        // create test address representing game admin
        let admin = @0xBABE;

        // first transaction to emulate module initialization
        let scenario_val = test_scenario::begin(admin);
        let scenario = &mut scenario_val;
        // second transaction to check if the forge has been created
        // and has initial value of zero swords created
        test_scenario::next_tx(scenario, admin);
            // extract the Forge object
            let forge = test_scenario::take_from_sender<Forge>(scenario);
            // verify number of created swords
            assert!(swords_created(&forge) == 0, 1);
            // return the Forge object to the object pool
            test_scenario::return_to_sender(scenario, forge);

As we can see in the test function defined above, in the first transaction we (explicitly) call the initializer, and in the next transaction we check if the forge object has been created and properly initialized.

If we try to run tests on the whole package at this point, we will encounter compilation errors in the existing tests due to the sword_create function signature change. We will leave the changes required for the tests to run again as an exercise for the reader. The entire source code for the package we have developed (with all the tests properly adjusted) can be found in my_module.move.

Last update 11/4/2022, 3:27:23 PM