Comments and Logging


Comments in Sway start with two slashes and continue until the end of the line. For comments that extend beyond a single line, you'll need to include // on each line.

// hello world
// let's make a couple of lines
// commented.

You can also place comments at the ends of lines containing code.

fn main() {
    let baz = 8; // Eight is a lucky number

You can also do block comments

fn main() {
    You can write on multiple lines
    like this if you want
    let baz = 8;


The logging library provides a generic log function that can be imported using use std::logging::log and used to log variables of any type. Each call to log appends a receipt to the list of receipts. There are two types of receipts that a log can generate: Log and LogData.

fn log_values(){
  // Generates a Log receipt

  // Generates a LogData receipt
  let string = "sway";

Log Receipt

The Log receipt is generated for non-reference types, namely bool, u8, u16, u32, and u64.

For example, logging an integer variable x that holds the value 42 using log(x) may generate the following receipt:

"Log": {
  "id": "0000000000000000000000000000000000000000000000000000000000000000",
  "is": 10352,
  "pc": 10404,
  "ra": 42,
  "rb": 1018205,
  "rc": 0,
  "rd": 0

Note that ra will include the value being logged. The additional registers rc and rd will be zero when using log while rb may include a non-zero value representing a unique ID for the log instance. The unique ID is not meaningful on its own but allows the Rust and the TS SDKs to know the type of the data being logged, by looking up the log ID in the JSON ABI file.

LogData Receipt

LogData is generated for reference types which include all types except for non_reference types; and for non-reference types bigger than 64-bit integers, for example, u256;

For example, logging a b256 variable b that holds the value 0x1111111111111111111111111111111111111111111111111111111111111111 using log(b) may generate the following receipt:

"LogData": {
  "data": "1111111111111111111111111111111111111111111111111111111111111111",
  "digest": "02d449a31fbb267c8f352e9968a79e3e5fc95c1bbeaa502fd6454ebde5a4bedc",
  "id": "0000000000000000000000000000000000000000000000000000000000000000",
  "is": 10352,
  "len": 32,
  "pc": 10444,
  "ptr": 10468,
  "ra": 0,
  "rb": 1018194

Note that data in the receipt above will include the value being logged as a hexadecimal. Similarly to the Log receipt, additional registers are written: ra will always be zero when using log, while rb will contain a unique ID for the log instance.

Note The Rust SDK exposes APIs that allow you to retrieve the logged values and display them nicely based on their types as indicated in the JSON ABI file.