Need to mutate a value type but don't want to create a var in your scope?

Mutated.swift

protocol Mutatable {}

extension Mutatable {
    func mutated(f: (inout Self) -> Void) -> Self {
        var copy = self
        f(&copy)
        return copy
    }
}

/*
Example usage:

    extension CGPoint: Mutatable {}

    let p = CGPoint.zero.mutated { $0.x = 32 }

*/

This technique can also be seen here: https://github.com/devxoul/Then/blob/master/Sources/Then.swift#L25-L42