10. Options

Option[T] is used when a value of type T may or may not be present.

10.1 Creating options

An option has two states:

Some{value} means a value is present
None means no value is present

Examples:

a := Some{3}
b: Option[int]
c: Option[String]

a has type Option[int]. Both b and c default to None.

When writing None directly, the payload type must be known from context:

x: Option[int]
x = None

10.2 Checking options

Use is Some and is None:

item := Some{"hello"}

if item is Some:
    println("present")
end

if item is None:
    println("missing")
end

These checks always produce bool.

10.3 Extracting the payload

If x has type Option[T], then x? has type T:

item := Some{7}
println(item?)

Applying ? to None is a runtime error, so it is usually used after an is Some check when absence is possible. Another way to extract the payload is to use the builtin function unwrap, so unwrap(x) is equivalent to x?.

10.4 Common uses

Options are the natural way to model:

dictionary lookup results
recursive links such as next: Option[Node]

Examples:

ages: Dict[int]
ages["Alice"] = 31

if ages["Alice"] is Some:
    println(ages["Alice"]?)
end

struct Node:
    next: Option[Node]
end

Recursive struct and variant cycles must pass through Option[...]. Direct cycles such as next: Node or items: Vec[Node] are compile errors.

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