How to extend an array in Swift?

Question

I have this protocol

protocol JsonConvertable {
    init?(_ underlyingValue: UnderlyingValue)
}

UnderlyingValue is an enum:

enum UnderlyingValue {
    case string(String)
    case int(Int)
    case double(Double)
    case bool(Bool)
    case array(Array<Dictionary<String, AnyObject>>)
    case dictionary(Dictionary<String, AnyObject>)

    init?(value: JsonConvertable) {
        switch rawValue {
        case let value as [String: AnyObject]: self = .dictionary(value)
        case let value as Array<[String: AnyObject]>: self = .array(value)
        case let value as Double: self = .double(value)
        case let value as Int: self = .int(value)
        case let value as Bool: self = .bool(value)
        case let value as String: self = .string(value)
        default: return nil
        }
    }
}

I can extend most types as follows

extension String: JsonConvertable {
    init?(_ underlyingValue: UnderlyingValue) {
        switch underlyingValue {
        case .bool(let value): self = value
        default: return nil
    }
}

However the Array extension is giving me the error Cannot assign value of type 'Array<Dictionary<String, AnyObject>>' to type 'Array<_>'

extension Array: JsonConvertable {
    init?(_ underlyingValue: UnderlyingValue) {
        switch underlyingValue {
        case .array(let value): self = value
        default: return nil
        }
    }
}

I have tried everything I can think of but nothing is working. I tried to have JsonConvertable conform to ArrayLiteralConvertable, I have tried using generics in the init but I am just starting to understand generics and don't really know how it would be beneficial in this case, the same with AssociatedType. I have tried to constrict Element. I have been trying to get this to work for 2 whole days and everything I do seems to not make any headway. What am I missing?

jtbandes · Accepted Answer · 2016-06-09 05:36:54Z

4

In your extension method, you are extending Array which is a type of Array<_> and your _underlyingValue is type of Array<[String:AnyObject]>.

Have you try this way:

extension Array where Element:Dictionary<String, AnyObject>, Element:JsonConvertable  {
  init?(_underlyingValue : UnderlyingValue){
      switch _underlyingValue {
        case  .array(let value):
          self = value
        default:
          return nil
      }
  }
}

edited Jun 9, 2016 at 5:36

jtbandes

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answered Jun 9, 2016 at 4:03

Peter Peng

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2 Comments

Robert Masen Over a year ago

I will give this a chance once I can get back to my project. I have tried something similar, I remember getting an error message that you can only constrict Element with a protocol and not a concrete type.

Robert Masen Over a year ago

The error below shows up at the extension signature type 'Element' constrained to non-protocol type 'Dictionary<String, AnyObject>

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