Debugging

Forc provides tools for debugging both live transactions as well as Sway unit tests. Debugging can be done via CLI or using the VSCode IDE.

Unit testing refers to "in-language" test functions annotated with #[test]. Line-by-line debugging is available within the VSCode IDE.

Live transaction refers to the testing sending a transaction to a running Fuel Client node to exercise your Sway code. Instruction-by-instruction debugging is available in the forc debug CLI.

• Debugging with CLI
• Debugging with IDE

__dbg intrinsic function

Sway also offers the __dbg intrinsic function to help debug all applications types: scripts, contracts and predicates. When called, this intrinsic function will print the current file, line and column, together with a customizable print of the specified value.

script;
fn main() -> u64 {
    __dbg(1u64)
}

The application above will print:

[src/main.sw:3:5] = 1

Structs can be customized by implementing the Debug trait.

script;
struct S { }
impl Debug for S {
    fn fmt(self, ref mut f: Formatter) {
        f.debug_struct("S2")
            .field("field1", 1)
            .field("field2", "Hello")
            .finish();
    }
}
fn main() -> u64 {
    let _ = __dbg(S {});
    __dbg(1u64)
}

This code is very similar to what the Sway compiler generates by default for all declared types. And this is what is printed:

[src/main.sw:12:13] = S2 { field1: 1, field2: "Hello" }
[src/main.sw:13:5] = 1