Example of monadic error handling posted to Twitter

1-sugared.scala

/**
 * Fetch credentials from git's credential interface.
 *
 * @param scheme The URL scheme (eg, http, https).
 * @param host The destination host.
 * @param realm The authentication realm (eg, HTTP Basic realm).
 * @param user The username if known; otherwise, the git credential interface will attempt to fill in this information.
 * @return Returns credentials, or a string explaining why credentials could not be fetched.
 */
private def getGitCredentials (scheme: String, host: String, realm: String, user: Option[String]): Result[KeychainError, Credentials] = {
  import scala.sys.process._
  
  for (
    /* Fetch the configured git credential helper */
    helper <- executeCommand(Seq("git", "config", "credential.helper"));

    /* Generate the request we'll submit to the helper */
    request <- new ByteArrayInputStream(Seq(
      Some(s"protocol=$scheme"),
      Some(s"host=$host"),
      Some(s"realm=$realm"),
      user.map(s => s"username=$s")
    ).flatten.mkString("\n").getBytes(StandardCharsets.UTF_8)).success;

    /* Execute the git-credential-$helper and parse the results */
    lines <- executeCommand(Seq(s"git-credential-$helper", "fill") #< request);
    parsed <- KeychainGitResultParser.parseInput(lines);

    /* Extract the credentials */
    username <- parsed.get("username").toSuccess(AccountNotFound("No username returned in git credential output"));
    password <- parsed.get("password").toSuccess(AccountNotFound("No password returned in git credential output"))
  ) yield Credentials(realm, host, username, password)
}

2-desugared.scala

private def getGitCredentials (scheme: String, host: String, realm: String, user: Option[String]): Result[KeychainError, Credentials] = {
  import scala.sys.process._

  /* Fetch the configured git credential helper */
  executeCommand(Seq("git", "config", "credential.helper")).flatMap { helper =>
    /* Generate the request we'll submit to the helper */
    val request = new ByteArrayInputStream(Seq(
      Some(s"protocol=$scheme"),
      Some(s"host=$host"),
      Some(s"realm=$realm"),
      user.map(s => s"username=$s")
    ).flatten.mkString("\n").getBytes(StandardCharsets.UTF_8))

    /* Execute the git-credential-$helper and read the output */
    executeCommand(Seq(s"git-credential-$helper", "fill") #< request).flatMap { lines =>
      KeychainGitResultParser.parseInput(lines).flatMap { parsed =>
        /* Extract the credentials */
        parsed.get("username").toSuccess(AccountNotFound("No username returned in git credential output")).flatMap { username =>
          parsed.get("password").toSuccess(AccountNotFound("No password returned in git credential output")).map { password
            Credentials(realm, host, username, password)
          }
        }
      }
    }
  }
}