Optionals and None

None is the value Acton uses for "nothing here".

An optional type ?T means a value is either a T or None. Use an optional when absence is part of the normal result.

def lookup_name(users: dict[str, str], username: str) -> ?str:
    if username in users:
        return users[username]
    return None

actor main(env):
    users = {"alice": "Alice Andersson"}

    name = lookup_name(users, "bob")
    if name is None:
        print("No match")
    else:
        print("Found:", name)

    if name is not None:
        print("Upper:", name.upper())

    env.exit(0)

Use is None and is not None to check whether an optional is present. Those checks are the normal way to branch on the value.

Optional values are common in lookups, parsing, and APIs that may or may not find a result. None is not a general placeholder for every kind of empty value; use it when absence itself matters.

For nested access, see Optionals. It covers how None propagates through chains and when to force a value instead of carrying the optional further.