functs list

type List<A> = Nil | Cons<A>

interface Nil {
    _tag: 'Nil'
}
interface Cons<A> {
    _tag: 'Cons',
    head: A,
    tail: List<A>,
}

const nil: List<never> = { _tag: 'Nil' }
const cons = <A>(head: A, tail: List<A>):List<A> => ({
    _tag: 'Cons',
    head, tail,
})

const isNil = <A>(xs: List<A>): xs is Nil => xs._tag === 'Nil'

const myList = cons(1, cons(2, cons(3, nil)))
console.log(JSON.stringify(myList, null, 2))

type ShowList = <A>(xs: List<A>) => string
const showList: ShowList = xs => isNil(xs) ? '' : `${xs.head}, ${showList(xs.tail)}`

console.log(showList(myList))