Errors

Errors are values. Use raise to stop normal execution with an error and try / catch to recover at a boundary that can make a useful decision.

func divide(a, b) {
    if b == 0 {
        raise ZeroDivisionError.wrap("divide")
    }
    return a / b
}

try {
    print(divide(10, 0))
} catch err {
    print(err.message)
    print(err.is(ZeroDivisionError))
}

The name after catch is local to its catch block. When no error is raised, the catch block is skipped. An unhandled error stops the program and prints a traceback.

Creating and matching errors

Error("message") creates a plain error. Use a sentinel error when callers need to distinguish a particular failure from unrelated errors.

var not_found = Error("not found")

func load_user(id) {
    if id == 0 {
        raise not_found.wrap("loading user")
    }
    return {"id": id}
}

try {
    load_user(0)
} catch err {
    if err.is(not_found) {
        print("choose another user")
    }
}

wrap("context") adds a message while preserving the original error. unwrap() returns the direct cause, and is() tests the complete error chain.

var base = Error("connection failed")
var err = base.wrap("loading profile")
print(err.message)
print(err.unwrap().message)
print(err.is(base))

Error kinds

The runtime uses named error kinds, which can be raised directly or wrapped.

Kind	Typical cause
TypeError	An operation receives an incompatible value
ValueError	A valid type has an invalid value
IndexError / KeyError	A missing list index or dictionary key
ZeroDivisionError	Division or modulo by zero
AttributeError / NameError	A missing member or identifier
ImportError	An unavailable module
ParseError	Invalid JSON or other parsed input
IOError	Generic file, network, or operating-system failure
NotExistError / ExistError / PermissionError	A missing, existing, or inaccessible filesystem path
TimeoutError / NetworkError	A timed-out or other network operation
NotImplementedError	An operation the runtime does not implement

Kinds are hierarchical. IndexError and KeyError are LookupErrors; ZeroDivisionError is an ArithmeticError; ParseError is a ValueError. Therefore err.is(LookupError) can handle more than one specific lookup failure.

Wrap errors where you can add useful context. Do not catch an error merely to discard it: either recover with a meaningful alternative, or use raise err to let an outer layer handle it.

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Errors

Creating and matching errors

Error kinds