Missing values and errors

A search may find nothing, and a user may enter an invalid number. Represent these situations with result values. Option<T> and Result<T, E> are available without an import.

A possibly missing value: Option

Option.Some(value) contains a value; Option.None does not. The result of std.string.find is one example. Handle both cases with match. You can also choose a default with std.option.unwrap_or(value, fallback). The fallback is evaluated before the call even for Some; use match if calculating it is expensive or has side effects.

Success and failure: Result

Result.Ok(value) means success, and Result.Err(error) means failure. Save this as errors.xen:

use std.convert as convert;

fn double_number(text: String) -> Result<Int, convert.ParseIntError> {
    let number = convert.parse_int(text)?;
    return Result.Ok(number * 2);
}

fn main() {
    println(match double_number("21") {
        Result.Ok(number) => convert.format_int(number),
        Result.Err(_) => "Please enter a decimal integer.",
    });
    println(match double_number("hello") {
        Result.Ok(number) => convert.format_int(number),
        Result.Err(_) => "Please enter a decimal integer.",
    });
}

$ xen run errors.xen
42
Please enter a decimal integer.

Handling bad input with match lets you print an alternative result or retry. Parsing hello here does not force the program to terminate.

Propagate an error with ?

parse_int(text)? extracts a successful value or immediately returns the error from the current function. That function must return Result<U, E> with the same error type E as the call. ? cannot be used in a result-free function such as main(), or on Option.

For different error types, convert first with std.result.map_err(value, convert_error). The callback is a named function taking the old error and returning the new one. Then use ?. Xen has no exceptions or try/catch syntax.

When to stop the program

panic(message) prints an error and terminates unsuccessfully. assert(condition) and assert_msg(condition, message) terminate if the condition is false. Use Result for expected input and file failures, and choose termination where continuing is inappropriate.

Use xen check for errors detected before execution. Find the reported file, line, and column, then correct the code. Reference diagnostics include note locations showing where a borrow starts or is used later. See common problems for fixes.