Understand an API failure

1 min read

An operation that can fail usually returns Result<Value, ErrorType>. Check Ok and Err rather than assuming it worked. The error's message() is for people; its code() lets your program distinguish categories of failure.

import system;

fun main() {
    switch (int.parse("not a number")) {
        Ok(value) => System.println(value),
        Err(error) => System.println(error.code()),
    }
}

The parse operation takes the error branch. Read Option and Result for defaults, recovery, and forwarding a failure with ?.

Choose the error guide

Operation	Error type	Reference
Parse a value	ParseError	ParseError
Read or write a file	IoError	IoError
Run another process	ProcessError	ProcessError
Encrypt or decrypt	CryptoError	CryptoError

Encryption errors

CryptoError is returned by encryption and key operations. EINVAL_KEY indicates unsuitable key material, EINVAL_NONCE an invalid nonce, and EDECRYPT a decryption or authentication failure. Handle EDECRYPT as a failed operation; do not use unauthenticated output.

The public helpers invalid_key, invalid_nonce, and decrypt_failed create errors from a message. CryptoError(code, message) constructs one directly.

Cancellation and fatal errors

Async APIs with a cancellation token may report ECANCELLED; check the specific method's contract. A panic stops the program and is not a returned error you can recover from.