Debugging with Forc Call

The forc call command includes interactive debugging capabilities through the --debug flag, enabling developers to debug contract function calls step-by-step after transaction execution.

Overview

When you add the --debug flag to any forc call command, it will:

Execute the contract function call as normal
Automatically launch an interactive debugging session
Allow you to step through the execution, inspect values, and set breakpoints
Provide full access to the forc-debug interface

This integration seamlessly combines contract interaction with debugging, making it easy to understand what happens during contract execution.

Basic Usage

Simply add the --debug flag to any existing forc call command:

forc call <CONTRACT_ID> \
    --abi <ABI_PATH> \
    <FUNCTION_NAME> [ARGS...] \
    --debug

Example: Debugging a Contract Call

Let's say you have a contract with a function that performs some calculations:

contract;

abi Calculator {
    fn factorial(n: u64) -> u64;
}

impl Calculator for Contract {
    fn factorial(n: u64) -> u64 {
        let mut result = 1;
        let mut counter = 0;
        while counter < n {
            counter = counter + 1;
            result = result * counter;
        }
        result
    }
}

Debugging the Factorial Function

forc call 0x1234567890abcdef1234567890abcdef12345678 \
    --abi ./out/debug/calculator-abi.json \
    factorial 5 \
    --debug

This command will:

Execute factorial(5) on the deployed contract
Show the transaction result (return value: 120)
Launch the interactive debugger with the transaction data pre-loaded

Interactive Debugging Session

Once the debugger launches, you'll see the pre-buffered debugger prompt to start the transaction:

Welcome to the Sway Debugger! Type "help" for a list of commands.
>> start_tx /var/folders/xz/5djvk4596k5c1fj2prcd0d880000gn/T/.tmpexrnFE.json /var/folders/xz/5djvk4596k5c1fj2prcd0d880000gn/T/.tmpuaPRtF.json