Clamped+PropertyWrapper.swift

// Xcode 11b1

@propertyDelegate
struct Clamped<Value: Comparable> {
    private var storage: Value
    private var clamp: (Value) -> Value

    init(min: Value, max: Value, initialValue: Value) {
        let clampingFunction = { ($0...$0).clamped(to: min...max).lowerBound }
        self.storage = clampingFunction(initialValue)
        self.clamp = clampingFunction
    }

    var value: Value {
        get { storage }
        set {
            self.storage = clamp(newValue)
        }
    }
}

extension Clamped where Value: FixedWidthInteger {
    init(min: Value, initialValue: Value) {
        self.init(min: min, max: Value.max, initialValue: initialValue)
    }
    init(max: Value, initialValue: Value) {
        self.init(min: Value.min, max: max, initialValue: initialValue)
    }
}


struct Example: CustomStringConvertible {
    /// Soon possible after beta1:
    // @Clamped(min: 0)
    // var positive: Int = 0
    /// In the meantime:
    @Clamped(min: 0, initialValue: 0)
    var positive: Int

    @Clamped(min: 10, max: 20, initialValue: 0)
    var tenToTwenty: Int

    @Clamped(min: -1.0, max: +1.0, initialValue: 0)
    var adjustment: Double

    var description: String {
        """
        positive: \(positive)
        tenToTwenty: \(tenToTwenty)
        adjustment: \(adjustment)
        """
    }
}

var ex = Example()
print(ex)

ex.positive = -4
ex.tenToTwenty = 15
ex.adjustment = 0.5
print(ex)

ex.positive = +7
ex.tenToTwenty = 25
ex.adjustment = -2.5
print(ex)