Error handling

Langoost handles recoverable errors with try / catch / throw, and runs cleanup with defer.

try / catch

Wrap code that might fail in try, and handle the thrown value in catch. The catch clause binds the thrown value to a variable:

try {
    let data = io.read("config.json")
    process(data)
} catch err {
    println("failed: " + toString(err))
}

finally

An optional finally block runs on every exit path — normal completion, after a catch, and even when the catch itself re-throws (the value still unwinds after finally runs):

try {
    work()
} catch err {
    println("error: " + toString(err))
} finally {
    cleanup()          // always runs
}

Use finally for cleanup tied to a single block; use defer (below) for cleanup tied to the whole function.

throw

Raise an error with throw. You can throw any value; an object is a convenient way to carry structured detail:

fn parsePort(s: string): int {
    let n = toInt(s)
    if n <= 0 {
        throw {kind: "ParseError", message: "invalid port: " + s}
    }
    return n
}

An uncaught throw propagates up the call stack; if it reaches the top it aborts the program with the error.

defer

A defer statement schedules a function call to run when the enclosing function returns — whether it returns normally or unwinds from a throw. Deferred calls run in LIFO (last-in, first-out) order, and their arguments are evaluated immediately:

fn copyFile(src: string, dst: string) {
    let input = io.stream.open(src, "r")
    defer io.stream.close(input)       // always runs on return

    let output = io.stream.open(dst, "w")
    defer io.stream.close(output)

    io.stream.write(output, io.stream.readAll(input))
}

runtime helpers

The runtime module complements these keywords:

• runtime.error.new(msg) builds a structured error string, with runtime.error.message(e) and runtime.error.isError(v) to inspect it.
• runtime.panic.throw(msg) prints to stderr and exits with code 2 — for unrecoverable conditions rather than expected failures.

See the runtime module for details.