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POCombining Futures, Eithers and Options in for comprehensions
 

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    <p>I have a collection of methods that return different types:</p> <pre><code>Either[ErrorResponse, X] Future[Either[ErrorResponse, X]] Option[ErrorResponse] </code></pre> <p>These methods need the result from a previous method to perform their computation. The methods:</p> <pre><code>type Parameters = Map[String, String] // allows me to flatmap on an either implicit def toRightProjection[Failure, Success](e: Either[Failure, Success]) = e.right // converts anything to a future implicit def toFuture[T](t: T) = Future.successful(t) // retrieves the request paramters from the given request def requestParameters(request: RequestHeader): Either[ErrorResponse, Parameters] = ??? // retrieves the response type from the given parameters def responseType(p: Parameters): Either[ErrorResponse, String] = ??? // retrieves the client id from the given parameters def clientId(p: Parameters): Either[ErrorResponse, String] = ??? // retrieves the client using the given client id def client(clientId: String): Future[Either[ErrorResponse, Client]] = ??? // validates the response type of the client def validateResponseType(client: Client, responseType: String): Option[ErrorResponse] = ??? </code></pre> <p>I can the wire them together with the following for comprehension (note that I wrote down some types to clarify the contents of specific parts of the computation).</p> <pre><code>val result: Either[ErrorResponse, Future[Either[ErrorResponse, Client]]] = for { parameters &lt;- requestParameters(request) clientId &lt;- clientId(parameters) responseType &lt;- responseType(parameters) } yield { val result: Future[Either[ErrorResponse, Either[ErrorResponse, Client]]] = for { errorOrClient &lt;- client(clientId) client &lt;- errorOrClient } yield validateResponseType(client, responseType).toLeft(client) result.map(_.joinRight) } val wantedResult: Future[Either[ErrorResponse, Client]] = result.left.map(Future successful Left(_)).merge </code></pre> <p>The above code is quite messy and I feel this can be done differently. I read about monads and monad transformers. The concept of those is very new to me and I can not get my head around it.</p> <p>Most of the examples only deal with two types of results: <code>Either[X, Y]</code> and <code>Future[Either[X, Y]]</code>. I still find it very hard to bend my mind around it.</p> <p><strong>How can I write a nice and clean for comprehension that replaces the above one?</strong></p> <p>Something like this would be awesome (I am not sure if that is even possible):</p> <pre><code>val result: Future[Either[ErrorResponse, Client]] = for { parameters &lt;- requestParameters(request) clientId &lt;- clientId(parameters) responseType &lt;- responseType(parameters) client &lt;- client(clientId) _ &lt;- validateResponseType(client, responseType) } </code></pre>
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