kms

Optional assignment in Swift

In Swift, given a variable x of type Optional<T>, how can guarantee it is not nil? You can force the unwrapping, but this may result in a runtime error. What if we could set x to some default value? We can manually, but this can be cumbersome to do often, so let’s make an operator! This is a good chance to learn about Swift’s optionals, generics, custom operators, and in-out parameters.¹

operator infix ~= {}

@assignment func ~=<T>(inout lhs: Optional<T>, rhs: T) {
    if lhs == nil {
        lhs = rhs
    }
}

var x: String? = "nil"
var y: String? = nil

x ~= "Test"
y ~= "Test"

x  // "nil"
y  // "Test" 

I’d prefer ?= but ? is not allowed in custom operators.