wrapper.swift

import Cocoa

@propertyWrapper
struct Clapping<Value: Comparable> {

    var value: Value
    let range: ClosedRange<Value>

    init(wrappedValue: Value, _ range: ClosedRange<Value>) {
        precondition(range.contains(wrappedValue))
        self.value = wrappedValue
        self.range = range
    }

    var wrappedValue: Value {
        get { value }
        set { value = min(max(range.lowerBound, newValue), range.upperBound)}
    }
}

@propertyWrapper
struct StringValidator<Value: StringProtocol> {

    var value: Value
    let regex: String

    init(wrappedValue: Value, _ regex: String) {
        let emailPred = NSPredicate(format:"SELF MATCHES %@", regex)
        precondition(emailPred.evaluate(with: wrappedValue) || wrappedValue == "")
        self.value = wrappedValue
        self.regex = regex
    }

    var wrappedValue: Value {
        get { value }
        set {
            let emailPred = NSPredicate(format:"SELF MATCHES %@", regex)
            if emailPred.evaluate(with: newValue) {
                value = newValue
            } else {
                value = ""
            }
        }
    }
}

struct Solution {

    @Clapping(0...14) var pH: Double = 7.0

    @StringValidator("[A-Z0-9a-z._%+-]+@[A-Za-z0-9.-]+\\.[A-Za-z]{2,64}") var email: String = ""
}

var solution = Solution(pH: 7.0, email: "")
solution.pH = 20
solution.email = "digo.reis@gmail.com"
print(solution.email)
solution.email = "digo.reis.com"
print(solution.email)
solution.email = "digo.reis@gmail.com"
print(solution.email)
solution.email = "rr@test.com"
print(solution.email)