How to add another dictionary entry to an existing dictionary to form a new dictionary (i.e. not append)

Question

I want to add another dictionary entry to a dictionary in swift e.g.

let a: [String: Any] = ["Test": 1, "good":false]
let b = a + ["hello": "there"]
print(b)

(Sorry if + looks crazy here, as that's how Kotlin achieves this. I'm more familiar with Kotlin than Swift.)

But I get the error Binary operator '+' cannot be applied to two '[String : Any]' operands

I can't use updateValue too

let a: [String: Any] = ["Test": 1, "good":false]
let b = a.updateValue("hello": "there")
print(b)

It will error stating Cannot use mutating member on immutable value: 'a' is a 'let' constant

I can do it by an extension function as per proposed https://stackoverflow.com/a/26728685/3286489

but it looks overkill.

let b = a.merge(dict: ["hello": "there"])
print(b)


extension Dictionary {
    func merge(dict: Dictionary<Key,Value>) -> Dictionary<Key,Value> {
        var mutableCopy = self
        for (key, value) in dict {
            // If both dictionaries have a value for same key, the value of the other dictionary is used.
            mutableCopy[key] = value
        }
        return mutableCopy
    }
}

Is there a simple operator I can just add another entry to it to for a new dictionary?

Note: I'm not referring to append as per How to append elements into a dictionary in Swift?, as I receive an immutable dictionary let that I need to add another entry to it.

Answer 1

There is a built-in merging(_:uniquingKeysWith:) function on Dictionary that does exactly what you need.

let dictionary = [1:1]
let otherDictionary = [1:2, 2:3]
// This will take the values from `otherDictionary` if the same key exists in both
// You can use `$0` if you want to take the value from `dictionary` instead
let mergedDict = dictionary.merging(otherDictionary, uniquingKeysWith: { $1 })

If you want, you can easily define a + operator for Dictionary that uses the above function.

extension Dictionary {
    static func + (lhs: Self, rhs: Self) -> Self {
        lhs.merging(rhs, uniquingKeysWith: { $1 })
    }
}

let addedDict = dictionary + otherDictionary