maybe.ts

interface MaybeBase {
    readonly loaded: boolean
}

interface Some<X> extends MaybeBase {
    readonly loaded: true
    readonly value: X
}

interface None extends MaybeBase {
    readonly loaded: false
}

type Maybe<X> = Some<X> | None

const none = (): None => ({ loaded: false })
const some = <X> (x: X): Some<X> => ({ loaded: true, value: x })

function maybe(x: null | undefined): None
function maybe<X>(x: X): Some<X>
function maybe<X>(x: X): Maybe<X> {
    return x === undefined || x === null
        ? none()
        : some(x)
}

const loaded = <X>(x: Maybe<X>): x is Some<X> => x.loaded
const isSome = loaded
const isNone = (x: Maybe<any>): x is None => !x.loaded

// --------------

const foo = maybe(null) // None
const bar = maybe(undefined) // None
const baz = maybe(123) // Some<number>

maybe.ts.md

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