NEHL.swift

NonEmptyHeteroList.swift

protocol NonEmptyHList {
    associatedtype Head
    associatedtype Tail
}

struct NEHLCons<H, T: NonEmptyHList>: NonEmptyHList {
    typealias Head = H
    typealias Tail = T
    
    let head: H
    let tail: T
    
    init(_ head: H, _ tail: T) {
        self.head = head
        self.tail = tail
    }
}

struct NEHLLast<H>: NonEmptyHList {
    typealias Head = H
    typealias Tail = Never
    
    let head: H
    
    init(_ head: H) {
        self.head = head
    }
}

typealias NEHL1<A> = NEHLLast<A>
typealias NEHL2<A, B> = NEHLCons<A, NEHLLast<B>>
typealias NEHL3<A, B, C> = NEHLCons<A, NEHLCons<B, NEHLLast<C>>>
typealias NEHL4<A, B, C, D> = NEHLCons<A, NEHLCons<B, NEHLCons<C, NEHLLast<D>>>>

struct ErrorA: Error {}
struct ErrorB: Error {}

let hnel: NEHL2<ErrorA, ErrorB> = NEHLCons(ErrorA(), NEHLLast(ErrorB()))