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POComposing multiple Try objects in Scala/Play
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  1. POComposing multiple Try objects in Scala/Play
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    copied!<p>We have a Scala/Play application in which we have several implicit classes that create Try objects from the Request, e.g.</p> <pre><code>implicit class RequestUtils[+T](req: Request[T]) { def user: Try[User] = // pull the User from the Session, or throw an UnauthorizedException def paging: Try[Paging] = // create a Paging object, or throw an IllegalArgumentException } </code></pre> <p>We then access the wrapped objects via flatMaps</p> <pre><code>def route(pathParam: String) = BasicAction { request =&gt; request.user.flatMap(user =&gt; request.paging.flatMap(paging =&gt; Try{ ... } ))} </code></pre> <p>And finally, an ActionBuilder generates a SimpleResult from the Try</p> <pre><code>case class BasicRequest[A](request: Request[A]) extends WrappedRequest(request) class BasicActionBuilder extends ActionBuilder[BasicRequest] { def invokeBlock[A](request: Request[A], block: (BasicRequest[A]) =&gt; Future[SimpleResult]) = { block(BasicRequest(request)) } } def BasicAction[T](block: BasicRequest[AnyContent] =&gt; Try[T]) = { val f: BasicRequest[AnyContent] =&gt; SimpleResult = (req: BasicRequest[AnyContent]) =&gt; block(req) match { case Success(s) =&gt; Ok(convertToJson(s)) case Failure(e: UnauthorizedException) =&gt; Unauthorized(e.getMessage) case Failure(e: Exception) =&gt; BadRequest(e.getMessage) case Failure(t: Throwable) =&gt; InternalServerError(e.getMessage) } val ab = new BasicActionBuilder ab.apply(f) } </code></pre> <p>We're trying to find a way to essentially compose multiple Try objects together (or something along those lines - we're not wedded to using Trys) - the flatMaps are working fine for one or two Trys, but nesting them more than that hampers program readability. We can manually compose the objects together, e.g.</p> <pre><code>case class UserAndPaging(user: User, paging: Paging) implicit class UserAndPagingUtils[+T](req: Request[T]) { def userAndPaging: Try[UserAndPaging] = req.user.flatMap(user =&gt; req.paging.flatMap(paging =&gt; UserAndPaging(user, paging)) } </code></pre> <p>but that's going to result in an explosion of case class + implicit class def combinations. Ideally I'd like to be able to compose multiple Try objects together in an ad hoc fashion, e.g.</p> <pre><code>def route(pathParam: String) = BasicAction { request =&gt; compose(request.user, request.paging).flatMap(userWithPaging =&gt; ...) } </code></pre> <p>and have a Try[User with Paging] magically composed for me, but I have no idea how I'd go about doing this - I've been wrestling with the type system to try to assign a meaningful type to "compose" without any success.</p> <p>How can I compose multiple Try objects together, or something equivalent using another language construct?</p>
 

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