Optional Values and Narrowing

none is a real value representing a missing alternative. It is not void or an uninitialized name.

An optional type is a union with none:

let answer:int64 | none = lookup_answer()

Checking an Optional

Use is? or isnt? to establish which alternative is present:

if answer isnt? none
    printl"the next answer is {answer + 1}"

Inside the body, answer is known to be int64. An early exit can establish the same fact afterward:

if answer is? none
    return

printl"answer is {answer}"

value is? Type tests membership in a type. Literal alternatives can be tested directly as well.

Equality against a value of one alternative asks both questions at once: answer =? 3 is true when answer holds an int64 equal to 3, and false when it is none (so answer not=? 3 is true then). answer =? none is the same test as answer is? none. Equality narrows like a test: inside if answer =? 3, answer is an int64 whose value is known to be 3 (so it even fits an int8), and the else branch of answer not=? 3 knows the same.

let answer:int64|none = 3
if answer =? 3 { let small:int8 = answer  printl"{small}" }
let words:array<string|none> = ["a" none]
if words[1] =? none { printl"missing" }

Producing Optional Values

A function can return an optional explicitly:

let choose = (enabled:bool):>int64 | none =>
    if enabled 42 else none

Any expression whose alternatives include a value and none can produce an optional.

Absence Is an Exception, Not an Error

none says that a value is absent. An error says that an operation failed and carries its own error type. Both descend from Dewy's exception type family, so navigation forwards either one without trying to access a member on it. They nevertheless remain distinct contracts:

User | none       # a user may simply be absent

User | NotFoundError   # looking up the user may fail

Exception values automatically forward when they are encountered as the receiver of a navigation route:

let user:User | none = findUser(id)
let city = user.profile.address.city

# city has type string | none

If users want a non-forwarding sentinel, they can define an ordinary type that does not descend from exception. Every ordinary alternative must support a requested member or be narrowed away first.

This is ordinary typed value flow, not a hidden throw or stack unwind. The next chapter develops the exception family and propagation in full.

General unions with several unrelated runtime layouts use the same type-theoretic model, though their complete representation and narrowing support is still a design and implementation frontier.