index expressions for collections

IndexExpression.swift

enum IndexType<C: Collection> {
  case keypath(KeyPath<C, C.Index>)
  case index(C.Index)
}

struct IndexExpression<C: Collection> {
  let base: IndexType<C>
  let distance: C.IndexDistance

  func resolve(in collection: C) -> C.Index {
    let baseIdx: C.Index
    switch base {
    case .index(let idx):  baseIdx = idx
    case .keypath(let kp): baseIdx = collection[keyPath: kp]
    }
    return collection.index(baseIdx, offsetBy: distance)
  }
}

func + <C>(lhs: IndexExpression<C>, distance: C.IndexDistance) -> IndexExpression<C> {
  return IndexExpression(base: lhs.base, distance: lhs.distance + distance)
}
func - <C>(lhs: IndexExpression<C>, distance: C.IndexDistance) -> IndexExpression<C> {
  return IndexExpression(base: lhs.base, distance: lhs.distance - distance)
}

func + <C: Collection>(kp: KeyPath<C, C.Index>, distance: C.IndexDistance) -> IndexExpression<C> {
  return IndexExpression(base: .keypath(kp), distance: distance)
}

func - <C: Collection>(kp: KeyPath<C, C.Index>, distance: C.IndexDistance) -> IndexExpression<C> {
  return IndexExpression(base: .keypath(kp), distance: -1 * distance)
}

func + <C: Collection>(idx: C.Index, distance: C.IndexDistance) -> IndexExpression<C> {
  return IndexExpression(base: .index(idx), distance: distance)
}

func - <C: Collection>(idx: C.Index, distance: C.IndexDistance) -> IndexExpression<C> {
  return IndexExpression(base: .index(idx), distance: -1 * distance)
}

extension Collection {
  subscript(expr: IndexExpression<Self>) -> Element {
    return self[expr.resolve(in: self)]
  }
}

let string = "hello everybody!"
let myIdx = string.startIndex
string[myIdx + 2]
string[string.startIndex + 7]
string[\.endIndex - 3]

Moving the IndexType enum outside the IndexExpression struct works (Xcode 9.2, Swift 4.0.3):

enum IndexType<C: Collection> {
    case keypath(KeyPath<C, C.Index>)
    case index(C.Index)
}

struct IndexExpression<C: Collection> {
    let base: IndexType<C>
    let distance: C.IndexDistance

    func resolve(in collection: C) -> C.Index {
        let baseIdx: C.Index
        switch base {
        case .index(let idx):  baseIdx = idx
        case .keypath(let kp): baseIdx = collection[keyPath: kp]
        }
        return collection.index(baseIdx, offsetBy: distance)
    }
}

Oh great, thanks for the tip!