Extracting keys and values from a path in Swift

PathParams.swift

import Foundation

enum PathParamsError: Error {
    case unbalanced
    case nonExistentParam(key: String)
    case invalidConversion(value: String, type: String)

    var localizedDescription: String {
        switch self {
        case .unbalanced:
            return "The number of keys didn't correspond with the number of values"
        case .nonExistentParam(let key):
            return "There is no value for the key \(key)"
        case .invalidConversion(let value, let type):
            return "Couldn't convert the value \(value) to the specified type \(type)"
        }
    }
}

/**
 Used to extract path parameters from a path.
 */
struct PathParams {

    private let params: [String : String]

    init(path: String) throws {
        let parts = path.split(separator: "/")
        guard parts.count % 2 == 0 else { throw PathParamsError.unbalanced }
        let values = stride(from: 0, to: parts.count, by: 2).map { (index) -> (String, String) in
            (parts[index].lowercased(), "\(parts[index+1])")
        }
        params = Dictionary(uniqueKeysWithValues: values)
    }

    func optionalValue<T: LosslessStringConvertible>(forKey key: String, _ type: T.Type) throws -> T? {
        guard let stringValue = params[key.lowercased()] else { return nil }
        guard let value = T(stringValue) else {
            throw PathParamsError.invalidConversion(value: stringValue, type: String(describing: T.self)) }
        return value
    }

    func value<T: LosslessStringConvertible>(forKey key: String, _ type: T.Type) throws -> T {
        guard let value = try optionalValue(forKey: key, type) else {
            throw PathParamsError.nonExistentParam(key: key.lowercased()) }
        return value
    }
}

// Usage
do {
    let params = try PathParams(path: "/customerid/8945834/receiptid/36235432/category/seafood")

    let category = try params.value(forKey: "category", String.self)
    let customerId = try params.value(forKey: "customerid", Int.self)
    let receiptId = try params.value(forKey: "receiptid", Int.self)
    let sauceType = try params.optionalValue(forKey: "sauce", String.self)

    print("Category:", category)
    print("Customer ID:", customerId)
    print("Receipt ID:", receiptId)
    if let sauceType = sauceType {
        print("Sauce: \(sauceType)")
    }
} catch {
    if let error = error as? PathParamsError {
        print(error.localizedDescription)
    }
}