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Debugging

Move does not currently have a native debugger. You can use the std::debug module, however, to print arbitrary values to the console. Monitoring variable values in this manner can provide insight into the logic of your modules. To do so, first declare an alias to the debug module in your source file for more concise access:

use std::debug;

Then in places where you want to print out a value v, regardless of its type, add the following code:

debug::print(&v);

or the following if v is already a reference:

debug::print(v);

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

debug::print_stack_trace();

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

Using debug in my_module​

To see the module in action, update your my_module code to include debug calls. Specifically, update the new_sword function so that you print the value of forge before and after updating swords_created. Also, include a print_stack_trace so that the function looks like the following:

public fun new_sword(
    forge: &mut Forge,
    magic: u64,
    strength: u64,
    ctx: &mut TxContext,
): Sword {
    debug::print(forge);
    forge.swords_created = forge.swords_created + 1;
    debug::print(forge);
    debug::print_stack_trace();
    Sword {
        id: object::new(ctx),
        magic: magic,
        strength: strength,
    }
}

To see the results, run the module's tests.

$ sui move test

The response prints out the expected results as the test calls the new_sword function.

INCLUDING DEPENDENCY Sui
INCLUDING DEPENDENCY MoveStdlib
BUILDING my_first_package
Running Move unit tests
[ PASS    ] 0x0::my_module::test_module_init
[debug] 0x0::my_module::Forge {
  id: 0x2::object::UID {
    id: 0x2::object::ID {
      bytes: @0x34401905bebdf8c04f3cd5f04f442a39372c8dc321c29edfb4f9cb30b23ab96
    }
  },
  swords_created: 0
}
[debug] 0x0::my_module::Forge {
  id: 0x2::object::UID {
    id: 0x2::object::ID {
      bytes: @0x34401905bebdf8c04f3cd5f04f442a39372c8dc321c29edfb4f9cb30b23ab96
    }
  },
  swords_created: 1
}
Call Stack:
    [0] 0000000000000000000000000000000000000000000000000000000000000000::my_module::test_module_init

        Code:
            [35] LdU64(7)
            [36] MutBorrowLoc(3)
            [37] Call(15)
          > [38] Call(5)
            [39] LdConst(0)
            [40] CallGeneric(2)
            [41] ImmBorrowLoc(3)

        Locals:
            [0] -
            [1] { { { <OBJECT-ID-WITHOUT-0x> } }, 1 }
            [2] -
            [3] { 2, { 00000000000000000000000000000000000000000000000000000000000000ad, [2, 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 } }


Operand Stack:

[ PASS    ] 0x0::my_module::test_sword_transactions
Test result: OK. Total tests: 2; passed: 2; failed: 0

The output shows the value of the swords_created field of the Forge change after the increment. The stack trace shows the bytecode instructions that have been executed so far, and the next few instructions to execute.

info

The specific bytecode offsets and the indices of the local variables might vary depending on the version of the Sui toolchain.