errors.baa

// Errors: throwing, catching, and what a caught runtime error looks like.

// `throw` sends any value up the stack.
fn admit(pen, name) {
    if pen.length() >= 3 {
        throw "the pen is full"
    }
    pen.push(name)
    return pen.length()
}

const pen = []
try {
    for name in ["Dolly", "Shaun", "Timmy", "Shirley"] {
        baa "admitted {admit(pen, name)}"
    }
} catch problem {
    baa "could not admit: {problem}"
}

// Thrown values keep their type, so structured errors work.
fn shear(sheep) {
    if sheep.get("shorn", false) {
        throw { code: "ALREADY_SHORN", sheep: sheep.name }
    }
    sheep.shorn = true
    return sheep
}

try {
    const dolly = { name: "Dolly", shorn: true }
    shear(dolly)
} catch problem {
    baa "{problem.code}: {problem.sheep}"
}

// Runtime errors are catchable too. They arrive as a map with a stable code.
try {
    const flock = ["Dolly"]
    baa flock[7]
} catch problem {
    baa "caught {problem.code} at line {problem.line}"
    baa problem.message
}

// `finally` always runs, whether or not anything went wrong.
fn guarded(shouldFail) {
    try {
        if shouldFail {
            throw "trouble"
        }
        return "fine"
    } catch problem {
        return "handled: {problem}"
    } finally {
        baa "gate closed"
    }
}

baa guarded(false)
baa guarded(true)

// `assert` and `assert_eq` fail loudly, which is what tests are built on.
assert(pen.length() == 3, "the pen should hold three sheep")
assert_eq(pen, ["Dolly", "Shaun", "Timmy"])

baa "all good"