Optionals

A value that may be missing has an optional type, T?. Only optionals can be None, and an optional must be checked before it’s used as a T. So a missing value can’t get as far as an AttributeError.

def find(names: list[str], prefix: str) -> str?:
    for name in names:
        if name.startswith(prefix):
            return name
    return None

let names = ["ada", "grace", "alan"]
let found = find(names, "gr")
print(found.upper())

main.tpn:9:7: error TP0203: str? must be narrowed before use
  help: narrow it with 'is some', or give a default with '??'

Checking an optional

There are three ways to get the T out:

def find(names: list[str], prefix: str) -> str?:
    for name in names:
        if name.startswith(prefix):
            return name
    return None

let names = ["ada", "grace", "alan"]

# is some binds the value, in that branch only.
if find(names, "gr") is some name:
    print(name.upper())

# is not None narrows a name that can't change.
let first = find(names, "a")
if first is not None:
    print(first.upper())

# ?? gives a default.
print(find(names, "z") ?? "nobody")

GRACE
ADA
nobody

Narrowing follows the code’s paths, as in mypy. After an early return or raise, the rest of the function sees the narrowed type:

def shout(name: str?) -> str:
    if name is None:
        return "..."
    return name.upper() + "!"

print(shout("hey"), shout(None))

HEY! ...

Narrowing variables and fields

A var stays narrowed until it’s assigned again, and a field path such as self.config.port stays narrowed until something could change it, like a call. When narrowing ends, the error says why:

@dataclass
class Config:
    port: int?

def reset(c: Config):
    c.port = None

def next_port(c: Config) -> int:
    if c.port is None:
        return 8080
    reset(c)
    return c.port + 1

main.tpn:12:12: error TP0203: int? must be narrowed before use
  help: the call on line 11 could have changed it; copy it to a 'let' first

Where optionals come from

• d.get(k) gives V?, None for a missing key.
• int(s) and float(s) give int? and float?, None when the text isn’t a number.
• next(it, None) gives T?.
• A function declared -> T?, and a parameter or field declared T?.

for text in ["42", "4x"]:
    if int(text) is some n:
        print(n * 2)
    else:
        print("not a number:", text)

84
not a number: 4x

T? never nests: a dict[str, int?]’s get gives int?, and a missing key and a stored None both give None.

Next: Classes.