Assertions in design by contract

Assertions in design by contract is a draft programming task. It is not yet considered ready to be promoted as a complete task, for reasons that should be found in its talk page.

According to Wikipedia; Assertions can function as a form of documentation: they can describe the state the code expects to find before it runs (its preconditions), and the state the code expects to result in when it is finished running (postconditions); they can also specify invariants of a class.

Show in the program language of your choice an example of the expecting results as a form of documentation.

Scala

Library: Scala

The last line is only executed if all assertions are met.<lang scala>import java.net.{URLDecoder, URLEncoder}

import scala.compat.Platform.currentTime

object UrlCoded extends App {

 val original = """http://foo bar/"""
 val encoded: String = URLEncoder.encode(original, "UTF-8")

 assert(encoded == "http%3A%2F%2Ffoo+bar%2F", s"Original: $original not properly encoded: $encoded")

 val percentEncoding = encoded.replace("+", "%20")
 assert(percentEncoding == "http%3A%2F%2Ffoo%20bar%2F", s"Original: $original not properly percent-encoded: $percentEncoding")

 assert(URLDecoder.decode(encoded, "UTF-8") == URLDecoder.decode(percentEncoding, "UTF-8"))

 println(s"Successfully completed without errors. [total ${currentTime - executionStart} ms]")

}</lang>