How to marshal array of arrays?

[jlyonsmith]

I'm trying to marshal a property like:

var arrayOfArrayOfThings: [[Thing]]? 
// ... later on ...
arrayOfArray = try object.value(for: "arrayOfArrayOfThings") 

Where Thing is a Unmarshaling. But I get a No 'value' candidates produce the expected contextual result type '[[Thing]]?'. I tried defining extensions to MarshableObject.value that return [[A]] but got stuck. It seems like for Array<Array<A>> Swift doesn't recognize the inner Array<A> type as a ValueType?

[jlyonsmith]

Here's a more complete example of the problem. Paste this into a playground with the Marshal sources:

public struct Tableau: Marshaling, Unmarshaling {
    public var matrix: [[Int]]?

    public init(object: MarshaledObject) throws {
        self.matrix = try object.value(for: "matrix")
    }

    public init(matrix: [[Int]]?) {
        self.matrix = matrix
    }

    public func marshaled() -> [String: Any] {
        return [
            "matrix": matrix ?? [],
        ]
    }
}

[jarsen]

Sorry. I read this the other day and then got pulled away while trying to figure out a good response and then the weekend happened.

This is a really good question: I'm not sure what the answer is here. It's not a case I've dealt with yet.

We have support for value(for:) -> [A] on MarshaledObject, and [ValueType] conforms to Marshal.ValueType. So in theory, I would expect an array of arrays of ValueType to work, but apparently it doesn't. I'm going to file this as a bug.

We may need to add explicit support for [[A]] on MarshaledObject.

[brianmullen]

@jarsen An array doesn't conform to ValueType which is why an array of arrays don't work. I think we could add support for it by extending MarshaledObject with a version of value(for:) -> [A] where A doesn't conform to ValueType and it would call a new value(from:) method on Array where Element doesn't conform to ValueType.

[jarsen]

AH. Yes you're right. I misread the extension Array where Element: ValueType 😝

That could work.

[alexsmith84]

Hi, I'm trying to deserialize like so:

let object = try! JSONParser.JSONObjectWithData(jsonData)
let array: [[String: Any]] = try object.value(for: "data.arrayOfDictionaries")

I'd really love to be able to do this! I feel like this basically the same issue as described above.

[KingOfBrian]

I was looking into this to see how #86 impacted this, it now won't compile instead of not working.

let names: [[String]] = try! json.value(for: "names")

I think this is better behavior, and should just need more methods to support all of the possibilities. I want to check in here before I add all of these possibilities though. It will almost double the amount of methods in here to fully support nested arrays, which is a bit painful. Array and Dictionary are ballooning the number of value(for:) functions a lot. I believe this will clean up nicely when Swift 4 lands with constrained protocol conformances. It may make more sense to not support nested arrays until Swift gets constrained protocol conformances.

[jarsen]

I'm of the mind to not add explicit support, and leave that as an exercise to those who need it. I'd rather keep things simple, especially if they're more edge case.

[jarsen]

Given our discussion up to this point, I'm going to suggest that Marshal itself will not add the functionality necessary for arrays of arrays. I am willing to revisit the topic if someone comes with a compelling proposal/PR. Thanks!

[jarsen]

@Shmaff I believe with some of the latest changes that unmarshaling [MarshaledObject] should work. I think it is a different issue though from the nested arrays. If you would like to verify, that would be great! If not feel free to open up a new issue :)

[amarcadet]

I'm facing almost the same issue of having to parse an array of arrays of arrays \o/.

"boundaries": [
  [
    [
      0.148271,
      51.6723432
    ],
    [
      0.148271,
      51.3849401
    ],
    [
      -0.3514683,
      51.3849401
    ],
    [
      -0.3514683,
      51.6723432
    ],
    [
      0.148271,
      51.6723432
    ]
  ]
]

Since the last nested array level is coordinates, I've figured out that I could make CLLocationCoordinate2D conforms to ValueType.

extension CLLocationCoordinate2D: ValueType {

    public static func value(from object: Any) throws -> CLLocationCoordinate2D {
        guard let array = object as? [Double] else {
            throw MarshalError.typeMismatch(expected: [Double].self, actual: type(of: object))
        }

        guard array.count == 2 else {
            throw MarshalError.typeMismatch(expected: "Array expected to contains 2 values", actual: "Array contains \(array.count) value(s)")
        }

        return CLLocationCoordinate2D(latitude: array[0], longitude: array[1])
    }

}

However I couldn't figure out how to parse the array of arrays. This trigger a compile error as expected by reading this comment:

let coordinates: [[CLLocationCoordinate2D]] = try object.value(for: "boundaries") 

I'm considering switching to Marshal but this is a key feature for me.

Of course I can't change the JSON format, that would be too easy...

[gereons]

+1 for adding multi-dimensional array support to Marshal.

@amarcadet: I currently need to parse similar data (simple polygons from a GEOJson-compatible format), and use this:

let coords = try object.any(for: "coordinates")
if let coordinates = coords as? [[[Double]]] {
    self.coordinates = coordinates[0].map { CLLocationCoordinate2D(latitude: $0[0], longitude: $0[1]) }     } 
else {
    throw MarshalError.typeMismatchWithKey(key: "coordinates", expected: "[[[Double]]]", actual: coords)
}

[jslew]

Do the Swift 3.1 generic constraint additions make it any more possible to support this? I had a quick look, but couldn't make it work.

[yanbraslavsky-home24]

So is there any ugly workaround at least ? I am using Marshal and I love it , but how should I parse 2 dimensional arrays ?

[yurii-voievodin]

I do it like this:

let anyObject: Any = try object.any(for: "json_key")
let matrix = handle(anyObject)

func handle(_ object: Any) -> [[CustomStruct]] {
    var map: [[CustomStruct]] = []

    if let array = object as? [Any] {
      for subArrayItem in array {
        if let subArray = subArrayItem as? [Any] {

          var row: [CustomStruct] = []
          for item in subArray {
            if let json = item as? [String: Any] {

              if let brick = try? CustomStruct(object: json) {
                row.append(brick)
              }
            }
          } 
          map.append(row)
        }
      }
    }
    return map
  }