Call response

You've probably noticed that you're often chaining .call().await.unwrap(). That's because:

1. You have to choose between .call() and .simulate() (more on this in the next section);
2. Contract calls are asynchronous, so you can choose to either .await it or perform concurrent tasks, making full use of Rust's async;
3. .unwrap() the Result<FuelCallResponse, Error> returned by the contract call.

Once you unwrap the FuelCallResponse, you have access to this struct:

pub struct FuelCallResponse<D> {
    pub value: D,
    pub receipts: Vec<Receipt>,
    pub gas_used: u64,
    pub log_decoder: LogDecoder,
}

Where value will hold the value returned by its respective contract method, represented by the exact type returned by the FuelVM, E.g., if your contract returns a FuelVM's u64, value's D will be a u64. If it's a FuelVM's tuple (u8,bool), then D will be a (u8,bool). If it's a custom type, for instance, a Sway struct MyStruct containing two components, a u64, and a b256, D will be a struct generated at compile-time, called MyStruct with u64 and a [u8; 32] (the equivalent of b256 in Rust-land).

• receipts will hold all receipts generated by that specific contract call.
• gas_used is the amount of gas it consumed by the contract call.

Error handling

You can use the is_ok and is_err methods to check if a contract call Result is ok or contains an error. These methods will return either true or false.

let is_ok = response.is_ok();
let is_error = response.is_err();

If is_err returns true, you can use the unwrap_err method to unwrap the error message.

if response.is_err() {
    let err = response.unwrap_err();
    println!("ERROR: {:?}", err);
};