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POWorking with scala collections - CanBuildFrom trouble
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  1. POWorking with scala collections - CanBuildFrom trouble
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    copied!<p>I'm trying to write a method which accepts any type of collection <code>CC[_]</code> and maps it to a new collection (the same collection type but a different element type) and I am struggling royally. Basically I'm trying to implement <code>map</code> but <em>not on the collection itself</em>.</p> <h2>The Question</h2> <p>I'm trying to implement a method with a signature which looks a bit like:</p> <pre><code>def map[CC[_], T, U](cct: CC[T], f: T =&gt; U): CC[U] </code></pre> <p>It's usage would be:</p> <pre><code>map(List(1, 2, 3, 4), (_ : Int).toString) //would return List[String] </code></pre> <p>I'm interested in an answer which would also work where <code>CC</code> is <code>Array</code> and I'm interested in the reason my attempts (below) have ultimately not worked.</p> <hr/> <h2>My Attempts</h2> <p><em>(For the impatient, in what follows, I utterly fail to get this to work. To reiterate, <strong>the question</strong> is "how can I write such a method?")</em></p> <p>I start like this:</p> <pre><code>scala&gt; def map[T, U, CC[_]](cct: CC[T], f: T =&gt; U)(implicit cbf: CanBuildFrom[CC[T], U, CC[U]]): CC[U] = | cct map f ^ &lt;console&gt;:9: error: value map is not a member of type parameter CC[T] cct map f ^ </code></pre> <p>OK, that makes sense - I need to say that <code>CC</code> is traversable!</p> <pre><code>scala&gt; def map[T, U, X, CC[X] &lt;: Traversable[X]](cct: CC[T], f: T =&gt; U)(implicit cbf: CanBuildFrom[CC[T], U, CC[U]]): CC[U] = | cct map f &lt;console&gt;:10: error: type mismatch; found : Traversable[U] required: CC[U] cct map f ^ </code></pre> <p>Err, OK! Maybe if I actually specify that <code>cbf</code> instance. After all, it specifies the return type (<code>To</code>) as <code>CC[U]</code>:</p> <pre><code>scala&gt; def map[T, U, X, CC[X] &lt;: Traversable[X]](cct: CC[T], f: T =&gt; U)(implicit cbf: CanBuildFrom[CC[T], U, CC[U]]): CC[U] = | cct.map(t =&gt; f(t))(cbf) &lt;console&gt;:10: error: type mismatch; found : scala.collection.generic.CanBuildFrom[CC[T],U,CC[U]] required: scala.collection.generic.CanBuildFrom[Traversable[T],U,CC[U]] cct.map(t =&gt; f(t))(cbf) ^ </code></pre> <p>Err, OK! That's a more specific error. Looks like I can use that!</p> <pre><code>scala&gt; def map[T, U, X, CC[X] &lt;: Traversable[X]](cct: CC[T], f: T =&gt; U)(implicit cbf: CanBuildFrom[Traversable[T], U, CC[U]]): CC[U] = | cct.map(t =&gt; f(t))(cbf) map: [T, U, X, CC[X] &lt;: Traversable[X]](cct: CC[T], f: T =&gt; U)(implicit cbf: scala.collection.generic.CanBuildFrom[Traversable[T],U,CC[U]])CC[U] </code></pre> <p>Brilliant. I has me a <code>map</code>! Let's use this thing!</p> <pre><code>scala&gt; map(List(1, 2, 3, 4), (_ : Int).toString) &lt;console&gt;:11: error: Cannot construct a collection of type List[java.lang.String] with elements of type java.lang.String based on a collection of type Traversable[Int]. map(List(1, 2, 3, 4), (_ : Int).toString) ^ </code></pre> <p>Say, what?</p> <hr/> <h2>Observations</h2> <p><em>I really can't help but think that Tony Morris' observations about this at the time were absolutely spot on. What did he say? He said "<strong>Whatever that is, it is not map</strong>". Look at how easy this is in scalaz-style</em>:</p> <pre><code>scala&gt; trait Functor[F[_]] { def fmap[A, B](fa: F[A])(f: A =&gt; B): F[B] } defined trait Functor scala&gt; def map[F[_]: Functor, A, B](fa: F[A], f: A =&gt; B): F[B] = implicitly[Functor[F]].fmap(fa)(f) map: [F[_], A, B](fa: F[A], f: A =&gt; B)(implicit evidence$1: Functor[F])F[B] </code></pre> <p><em>Then</em></p> <pre><code>scala&gt; map(List(1, 2, 3, 4), (_ : Int).toString) &lt;console&gt;:12: error: could not find implicit value for evidence parameter of type Functor[List] map(List(1, 2, 3, 4), (_ : Int).toString) ^ </code></pre> <p><em>So that</em></p> <pre><code>scala&gt; implicit val ListFunctor = new Functor[List] { def fmap[A, B](fa: List[A])(f: A =&gt; B) = fa map f } ListFunctor: java.lang.Object with Functor[List] = $anon$1@4395cbcb scala&gt; map(List(1, 2, 3, 4), (_ : Int).toString) res5: List[java.lang.String] = List(1, 2, 3, 4) </code></pre> <p>Memo to self: <strong>listen to Tony!</strong></p>
 

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