Recursive Swift Either Enum with Generic Associated Values

Question

I want to develop a recursive Either type as a Swift enum (at least a partially working version (;), and so have been playing around with generics (trying to work with the type system).

Here is what I have so far:

enum NumericEitherEnum<Left, Right> {
  case left(Left)
  case right(Right)

  var left: Left? {
    switch self {
      case .left(let leftie) where leftie is NumericEitherEnum<Int, Float>:
        return (leftie as? NumericEitherEnum<Int, Float>)?.left // This won't work (obviously)
      case .left(let left):
        return left
      case .right(_):
        return nil
    }
  }

  var right: Right? {
    switch self {
      case .left(_):
        return nil
      case .right(let right) where right is NumericEitherEnum:
        return (right as? NumericEitherEnum)?.right // This assumes the generic parameters of Left, Right in the current enum so won't work.
      case .right(let right):
        return right
    }
  }
}

I get cryptic diagnostic error messages in Xcode, which I haven't been able to get around:

1. 'Replace (leftie as? NumericEitherEnum<Int, Float>)? with NumericEitherEnum<Int, Float> '
2. 'Enum case 'left' cannot be used as an instance member'

What I am trying to achieve:

print(NumericEitherEnum<NumericEitherEnum<Int, Float>, NumericEitherEnum<Int, Float>>.left(.left(3)).left ?? "failed :(") // Parses the innermost value 3

The fix-it doesn't fix the error or advise how to actually address the underlying cause. I think this is probably an edge case for the compiler to parse (maybe even bug);). I also realise that in fact it doesn't make sense logically to return an Int for a Left? type but is there any way I can express this within the type system (I tried associated types but I still don't know how to make this dynamic). There's also a problem to handle nested enums that have a different generic type signature (not sure how to express this).

How can I resolve this issue in a better way? Ideally, I want to have a clean call site without too much indirection and extra data structures but would be open to trying out a different data structure if this isn't practically achievable.

Answer 1

So I discovered that Swift 2 introduced support for recursive enums ages ago - source , While this is great. I didn't manage to find any SO questions for this specific problem ie Generic Recursive Enums that had also been answered ( here ). I will summarise the final (partial solution) code now.

It turns out that I needed to describe what the problem is in terms of data structures. Essentially we want .left to contain either a value of type Left or a RecursiveEither<Left, Right> . With this simple idea, we can create two enums - one for the wrapper enum, and one for the nested enum which can take a Value or another wrapper.

enum RecursiveEither<Left, Right> {
  case left(ValueOrLeftOrRight<Left, Left, Right>)
  case right(ValueOrLeftOrRight<Right, Left, Right>)
  
  var left: Left? {
    guard case .left(let leftie) = self else { return nil }
    return leftie.left
  }

  var right: Right? {
    guard case .right(let rightie) = self else { return nil }
    return rightie.right
  }
}

enum ValueOrLeftOrRight<Value, Left, Right> {
  case value(Value)
  indirect case left(ValueOrLeftOrRight<Left, Left, Right>)
  indirect case right(ValueOrLeftOrRight<Right, Left, Right>)
  
  var left: Left? {
    switch self {
    case .value(let left): return left as? Left
    case .left(let content): return content.left
    default: return nil
    }
  }

  var right: Right? {
    switch self {
    case .value(let right): return right as? Right
    case .right(let content): return content.right
    default: return nil
    }
  }
}

The call site is nice with this design:

let e = RecursiveEither<Int, Int>.left(.left(.left(.left(.value(3)))))

That being said, there are still limitations to this answer - assuming Left and Right won't change even in nested Either . Also, it's possible to design this differently with only one indirect case to wrap the Either type in another associated value - that might be more memory efficient since fewer frames get pushed onto the call stack at runtime.

I have posted a gist with attempts on this problem.

If anyone has suggestions to improve this implementation, feel free to add to this.