Simple Dictionary extension to avoid the if let-dance when retrieving values

Autoclosure.swift

extension Dictionary {
    mutating func value(for key: Key, orAdd closure: @autoclosure () -> Value) -> Value {
        if let value = self[key] {
            return value
        }
        
        let value = closure()
        self[key] = value
        return value
    }
}

// Usage:

var colors = [
    "red" : UIColor.red
]

// Blue will be added, since it's missing from `colors`
let blue = colors.value(for: "blue", orAdd: UIColor.blue)