Organize contract errors with an error enum type

A convenient way to manage and meaningfully communicate contract errors is to collect them into an enum struct. These errors are a special type of enum integer type that are stored on the ledger as Error values containing a u32 code. First, create the Error struct in your smart contract.

#[contracterror]
#[derive(Copy, Clone, Debug, Eq, PartialEq, PartialOrd, Ord)]
#[repr(u32)]
pub enum Error {
    FirstError = 1,
    AnotherError = 2,
    YetAnotherError = 3,
    GenericError = 4
}

Smart contracts can fail with error enums in two different ways. They can either return a Result with their intended return value and the #[contracterror] struct as the error, or just invoke panic_with_error! with the appropriate Error enum value whenever an error condition is reached. By default, most ecosystem standards assume that contract functions do not return a Result, so using panic_with_error! is recommended. However, both styles behave in the same way. If an error is returned or panic_with_error! is invoked, the transaction will fail. Contracts making cross contract calls have the ability to catch and handle these failures with try_ functions.

#[contractimpl]
impl Contract {
    /// Call `panic_with_error!` to fail with custom errors
    /// This is the default, recommended approach adopted by most SEP standards
    pub fn cause_error(env: Env, error_code: u32) -> u32 {
        let error_type = match error_code {
            0 => return 0,
            1 => Error::FirstError,
            2 => Error::AnotherError,
            3 => Error::YetAnotherError,
            _ => Error::GenericError,
        };
        panic_with_error!(env, error_type);
    }

    /// Return `Err` to fail with custom errors
    pub fn cause_error_result(env: Env, error_code: u32) -> Result<u32, Error> {
        let error_type = match error_code {
            0 => return Ok(0),
            1 => Error::FirstError,
            2 => Error::AnotherError,
            3 => Error::YetAnotherError,
            _ => Error::GenericError,
        };
        return Err(error_type);
    }
}

When converted to XDR, the value becomes an ScVal, containing a ScError, containing the integer value of the error as contract error.

{ "error": { "contractError": 1 } }