Swift implementation of Haskell `Bounded` type

Bounded.swift

/// Sometimes we need to limit values
/// For instance, you are making application for the Tour De France, and 
/// You have a variable depicting the Climb category (1..5)
/// Here is the place, where `Bounded` can be used
struct Bounded<T: Comparable> {
    var minimum: T
    var maximum: T
    
    var value: T? {
        willSet {
            if let val = newValue {
                assert(val >= minimum && val <= maximum, "Error! `value` is out of bounds!")
                value = val
            }
        }
    }
    
    init(min: T, max: T, val: T? = nil) {
        assert(min <= max, "Error! `min > max` or `max < min`")
        minimum = min
        maximum = max
        value = val
    }
}

/// Usage
/// let a = Bounded(min: 10, max: 20, val: 12)
/// var b = Bounded(min: 0, max: 3)
/// b.value = 3
/// b.value = 2
/// b.value