Pitch: Key-Path Member Lookup

Evolution Pitches
21

This introduces some interesting use cases with generics and tuples. This could allow, for instance, a generic type to wrap a poorly-typed dictionary with its expected key-value mapping expressed as a tuple. Combined with StoredPropertyIterable and the ability to extract the names of key paths, you could write a wrapper that checks the contents of a dictionary on construction, and provides strongly-typed access to its contents:

struct WellTypedDictionary<Schema> {
  var dictionary: [String: Any]

  init?(_ dictionary: [String: Any]) {
    for key in keys(Schema.self) {
      guard
        let value = dictionary[key.name],
        type(of: value) == key.valueType
      else {
        return nil
      }
    }
    self.dictionary = dictionary
  }

  subscript<U>(keyPathMember: WritableKeyPath<Schema, U>) -> U {
    get {
      return dictionary[keyPathMember.name] as! U
    }
    set {
      dictionary[keyPathMember.name] = newValue
    }
  }
}

let record = ["name": "Julius", "job": "Caesar", age: 55, "leastFavoriteDish": "et-tu-ffée"]

guard let checkedRecord = WellTypedDictionary<(name: String, age: Int)>(record) else {
  print("invalid record")
  return
}

print("\(checkedRecord.name) is \(checkedRecord.age)")
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