swift - Add constraints to generic parameters in extension

Question

I have this function:

func flatten<Key: Hashable, Value>(dict: Dictionary<Key, Optional<Value>>) -> Dictionary<Key, Value> {
    var result = [Key: Value]()
    for (key, value) in dict {
        guard let value = value else { continue }
        result[key] = value
    }
    return result
}

As you can see, it transforms a [Key: Value?] dictionary into a [Key: Value] one (without the optional).

I wanted to extend the Dictionary class with a new method only for classes which value is an Optional of any type, but I am unable to add constraints to the generic parameters of the dictionary.

This is what I tried:

extension Dictionary where Value: Optional<Any> {
    func flatten() -> [Key: Any] {
        var result = [Key: Any]()
        for (key, value) in self {
            guard let value = value else { continue }
            result[key] = value
        }
        return result
    }
}

But fails with error:

Type 'Value' constrained to non-protocol type 'Optional<Any>'

Answer 1

Try this code in the Playground:

// make sure only `Optional` conforms to this protocol
protocol OptionalEquivalent {
  typealias WrappedValueType
  func toOptional() -> WrappedValueType?
}

extension Optional: OptionalEquivalent {
  typealias WrappedValueType = Wrapped

  // just to cast `Optional<Wrapped>` to `Wrapped?`
  func toOptional() -> WrappedValueType? {
    return self
  }
}

extension Dictionary where Value: OptionalEquivalent {
  func flatten() -> Dictionary<Key, Value.WrappedValueType> {
    var result = Dictionary<Key, Value.WrappedValueType>()
    for (key, value) in self {
      guard let value = value.toOptional() else { continue }
      result[key] = value
    }
    return result
  }
}

let a: [String: String?] = ["a": "a", "b": nil, "c": "c", "d": nil]
a.flatten() //["a": "a", "c": "c"]

Because you cannot specify an exact type in the where clause of a protocol extension, one way you may detect exactly the Optional type is to make Optional UNIQUELY conforms to a protocol (say OptionalEquivalent).

In order to get the wrapped value type of the Optional, I defined a typealias WrappedValueType in the custom protocol OptionalEquivalent and then made an extension of Optional, assgin the Wrapped to WrappedValueType, then you can get the type in the flatten method.

Note that the sugarCast method is just to cast the Optional<Wrapped> to Wrapped?(which is exactly the same thing), to enable the usage guard statement.

UPDATE

Thanks to Rob Napier 's comment I have simplified & renamed the sugarCast() method and renamed the protocol to make it more understandable.