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POHow to describe and fix this Scala type mismatch error?
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copied!<p>Below is a specific instance of a kind of situation that I sometimes encounter with parameterized types. Basically, there are type parameters that I know are compatible, but I don't know how to prove that to some parts of the code.</p> <p>I'm writing a request router that maps urls to handler functions. Below is some simplified code. I create a <code>List[Route]</code> where a <code>Route</code> is basically a <code>UrlMatcher, Function</code> pair.</p> <pre><code>class Route[A](matcher: UrlMatcher[A], handler: HandlerFunction[A]) abstract class UrlMatcher[A] { def match(url: String): Option[A] // None if no match </code></pre> <p>The type parameter <code>A</code> is for the "arguments" that the matcher might extract from the URL. They would be passed to the handler function. For example, a <code>UrlMatcher[Int]</code> that sees a URL path like "/users/123" could pass 123 to a <code>getUser(id: Int)</code> function. The router might look like:</p> <pre><code>val routes = ArrayBuffer[Route[_]] def callHandler(url: String) { for (r <- routes) { val args = r.matcher.matchUrl(url) if (args.isDefined) r.handler(args.get) // <--- error here } </code></pre> <p>The problem is that I get type mismatch errors because I don't know how to tell it the two types are the same. </p> <pre><code>type mismatch; found: args.type (with underlying type Option[Any]) required: _$1 </code></pre> <p>I know I can redesign it so that <code>Route</code> has a method like <code>matchAndCall</code>, but I'd like to keep this logical flow if possible. </p> <p><strong>Update/Edit</strong></p> <p>I don't fully understand existential types, but I tried this...</p> <pre><code>val routes = ArrayBuffer[T forSome { type T }]() </code></pre> <p>And it removed the mismatch error above. However, I have another one where I was inserting into the <code>ArrayBuffer</code>.</p> <pre><code>def route[P](matcher: UrlMatcher[P], handler: Handler[P]): AbstractRoute = { val route = new Route(matcher, handler) otherRoutes.append(route) // error here route } </code></pre> <p>Now the error is...</p> <pre><code>type mismatch; found : Route[P] required: Route[T forSome { type T }] Note: P <: T forSome { type T }, but class Route is invariant in type P. You may wish to define P as +P instead. (SLS 4.5) </code></pre> <p>Why is <code>P</code> incompatible with <code>T</code>, since their are no constraints on <code>T</code>?</p>

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