`randomNumberPublisher` that fetches a random integer from random.org.

random_number_publisher.swift

import Combine
import Foundation

enum RandomDotOrgError: Error { /// (1) A sample error conformance we’ll lean on.
	case failedRequest
}

func randomNumberPublisher() -> Publishers.TryMap<URLSession.DataTaskPublisher, Int> {
	var components = URLComponents(string: "https://www.random.org/integers/")!
	components.queryItems = ["num", "min", "col"].map { URLQueryItem(name: $0, value: "1") } +
		[
			URLQueryItem(name: "max", value: "235866"),
			URLQueryItem(name: "base", value: "10"),
			URLQueryItem(name: "format", value: "plain"),
			URLQueryItem(name: "rnd", value: "new")
		]

	return URLSession.shared
		.dataTaskPublisher(for: components.url(relativeTo: nil)!)
		.tryMap { data, _ in
			let randomNumber = Int(
				String(
					decoding: data,
					as: UTF8.self
				)
				.trimmingCharacters(in: .newlines)
			)

			if let randomNumber = randomNumber,
				randomNumber.isMultiple(of: 2) {
				return randomNumber /// (2) Everything up to this point is scaffolding to make a request
				/// to random.org—from there, we make sure it parses as an `Int` and, for sake of example, is even.
			} else {
				throw RandomDotOrgError.failedRequest /// (3) To more frequently simulate an error,
				/// we `throw` if `randomNumber` is odd.
			}
		}
}