Debugging

Move does not currently have a built-in debugger. However, you can leverage the std::debug module to print various values directly to the console. This approach helps you observe and understand the behavior and logic of your modules. Start by importing the debug module into your source file:

use std::debug;

To print a variable v, irrespective of its type, use the following code:

debug::print(&v);

If v is already a reference, you can use:

debug::print(v);

Additionally, the debug module offers a function to display the current stack trace:

debug::print_stack_trace();

Moreover, if an abort or assertion fails, the stack trace at that failure point is automatically printed.

Implementing Debug in `first_package`

To demonstrate the use of the std::debug module, update your first_package by integrating debug statements. Specifically, modify the new_sword function to output the forge value both before and after incrementing swords_created. Incorporate a print_stack_trace as well, resulting in the following function:

/// Constructor for creating swords.
public fun new_sword(forge: &mut Forge, magic: u64, strength: u64, ctx: &mut TxContext): Sword {
    debug::print(forge);

    // Increment the `swords_created` counter.
    forge.swords_created = forge.swords_created + 1;

    debug::print(forge);
    debug::print_stack_trace();

    // Create a sword.
    Sword {
        id: object::new(ctx),
        magic: magic,
        strength: strength,
    }
}

To view the output, execute the module's tests:

$ iota move test test_sword_transactions

This will display the expected results as the test calls the new_sword function:

INCLUDING DEPENDENCY Iota
INCLUDING DEPENDENCY MoveStdlib
BUILDING my_first_package
Running Move unit tests
[debug] 0x0::first_package::Forge {
  id: 0x2::object::UID {
    id: 0x2::object::ID {
      bytes: @<OBJECT-ID>
    }
  },
  swords_created: 0
}
[debug] 0x0::first_package::Forge {
  id: 0x2::object::UID {
    id: 0x2::object::ID {
      bytes: @<OBJECT-ID>
    }
  },
  swords_created: 1
}
Call Stack:
    [0] 0000000000000000000000000000000000000000000000000000000000000000::first_package::test_sword_transactions

        Code:
            [24] LdU64(7)
            [25] CopyLoc(5)
            [26] Call(13)
          > [27] Call(6)
            [28] CopyLoc(4)
            [29] CallGeneric(2)
            [30] CopyLoc(5)

        Locals:
            [0] -
            [1] -
            [2] 000000000000000000000000000000000000000000000000000000000000face
            [3] { { { <OBJECT-ID-WITHOUT-0x> } }, 1 }
            [4] 000000000000000000000000000000000000000000000000000000000000cafe
            [5] (&) { 1, { 000000000000000000000000000000000000000000000000000000000000babe, [1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0], 0, 0, 0 } }
            [6] { 1, { 000000000000000000000000000000000000000000000000000000000000babe, [1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0], 0, 0, 0 } }
            [7] -


Operand Stack:

[ PASS    ] 0x0::first_package::test_sword_transactions
Test result: OK. Total tests: 1; passed: 1; failed: 0

How do you import the debug module in your Move source file?

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