Errors

A failure is a typed enum to match on, not a string.

Option and Result

The prelude defines Option(a) with Some and None, and Result(a, e) with Ok and Err. Option states a value may be absent. Result states an operation failed, and the failure is a typed enum to match on, not a string. A fallible operation with nothing to say returns Result(_, Nil).

Fallbacks

or supplies the fallback. Array indexing returns Option, never a crash.

println(users[9] or 'out of range')

The receiver form of or binds the error payload, so recovery reads why it failed.

score = parse_score('oops') or e -> {
	match e {
		Empty -> println('nothing to parse')
		NotANumber(t) -> println('${t} is not a number, using 0')
	}
	0
}

Chaining

<- chains fallible calls. a <- call(args) followed by the rest of its block becomes call(args, fn(a) { ... }), so with result.then each line binds the success value and the first error returns. The standard library is written this way.

pub fn read_text(path String) Result(String, IoError) {
	b <- result.then(read_file(path))
	result.replace_err(binary.to_string(b), InvalidData(path))
}