Functional String validation

String+IsValid.swift

func validate<T: Codable>(_ validator: @escaping (T) -> Bool) -> (T) -> Bool {
  return { input in
    return validator(input)
  }
}

enum StringValidator: CustomStringConvertible {
    case isNotEmpty
    case lengthIn(ClosedRange<Int>)
    
    var validator: (String) -> Bool {
        return validate { input in 
            switch self {
            case .isNotEmpty: 
                return !input.isEmpty
            case .lengthIn(let lengthRange):
                return lengthRange.contains(input.count)
            }
        }
    }
    
    var description: String {
        switch self {
        case .isNotEmpty:
            return "Input should not be empty"
        case .lengthIn(let lengthRange):
            return "Input should be between \(lengthRange.lowerBound) and \(lengthRange.upperBound) characters long"
        }
    }
}

extension String { 
    func failedValidators(in validators: StringValidator...) -> [StringValidator] {
        return validators.filter { !$0.validator(self) }
    }
}

/// Usage
let input = "Test"
print("Input: \"\(input)\"")
let failed = input.failedValidators(in: .isNotEmpty, .lengthIn(5...40))
/// Should be an error regarding the length
print("is valid:", failed.isEmpty)
if !failed.isEmpty {
    print(failed.map { $0.description }.joined(separator: "\n"))
}