std.optional

Applies f to a present value; absent stays absent.

• value: The optional to map.
• f: Applied to the present value.

println(some(2).map(|x| x * 10)) // some(20)

The value when present, otherwise other.

• value: The preferred optional.
• other: Used when value is absent.

let absent: Optional<int> = none
println(absent.or(some(9))) // some(9)

The value when present, otherwise whatever f produces.

• value: The preferred optional.
• f: Called only when value is absent.

The present value, otherwise the result of calling f.

• value: The optional to unwrap.
• f: Called only when value is absent.

let absent: Optional<int> = none
println(absent.unwrap_or_else(|| 7)) // 7

Turns a present value into ok, and absence into the given error.

• value: The optional to convert.
• failure: The error carried when value is absent.

let absent: Optional<int> = none
let missing: Result<int, Error> = err.NotFound("no such item")
println(absent.ok_or(missing.unwrap_err()).is_err()) // true

Keeps a present value only when predicate accepts it.

• value: The optional to test.
• predicate: Applied to the present value.

println(some(4).filter(|x| x > 3)) // some(4)
println(some(2).filter(|x| x > 3)) // none

Removes one level of nesting.

• value: An optional whose payload is itself optional.

let nested: Optional<Optional<int>> = some(some(1))
println(nested.flatten()) // some(1)