@Argument(help: "The highest value to pick.")
    var highValue: Int
    
    func validate() throws {
        guard highValue >= 1 else {
            throw ValidationError("'<high-value>' must be at least 1.")
        }
    }

Not saying there's anything wrong with this. But I wrote a generic command line parsing library for C# some years back, and simple validators like min / max values were handled with attributes. The elegance was that limits were defined in exactly one place in the code, and error messages like that manually constructed above were synthesized automatically. (Of course you could override with your own validation function for bespoke cases)