Logs

Whenever you log a value within a contract method, the resulting log entry is added to the log receipt and the variable type is recorded in the contract's ABI. The SDK lets you parse those values into Rust types.

Consider the following contract method:

    fn produce_logs_variables() {
        let f: u64 = 64;
        let u: b256 = 0xef86afa9696cf0dc6385e2c407a6e159a1103cefb7e2ae0636fb33d3cb2a9e4a;
        let e: str[4] = __to_str_array("Fuel");
        let l: [u8; 3] = [1u8, 2u8, 3u8];

        log(f);
        log(u);
        log(e);
        log(l);
    }

You can access the logged values in Rust by calling decode_logs_with_type::<T> from a FuelCallResponse, where T is the type of the logged variables you want to retrieve. The result will be a Vec<T>:

    let contract_methods = contract_instance.methods();
    let response = contract_methods.produce_logs_variables().call().await?;

    let log_u64 = response.decode_logs_with_type::<u64>()?;
    let log_bits256 = response.decode_logs_with_type::<Bits256>()?;
    let log_string = response.decode_logs_with_type::<SizedAsciiString<4>>()?;
    let log_array = response.decode_logs_with_type::<[u8; 3]>()?;

    let expected_bits256 = Bits256([
        239, 134, 175, 169, 105, 108, 240, 220, 99, 133, 226, 196, 7, 166, 225, 89, 161, 16, 60,
        239, 183, 226, 174, 6, 54, 251, 51, 211, 203, 42, 158, 74,
    ]);

    assert_eq!(log_u64, vec![64]);
    assert_eq!(log_bits256, vec![expected_bits256]);
    assert_eq!(log_string, vec!["Fuel"]);
    assert_eq!(log_array, vec![[1, 2, 3]]);

You can use the decode_logs() function to retrieve a LogResult struct containing a results field that is a vector of Result<String> values representing the success or failure of decoding each log.

    let contract_methods = contract_instance.methods();
    let response = contract_methods.produce_multiple_logs().call().await?;
    let logs = response.decode_logs();

Due to possible performance hits, it is not recommended to use decode_logs() outside of a debugging scenario.

Note: String slices cannot be logged directly. Use the __to_str_array() function to convert it to a str[N] first.