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POMap on HList fails with subtypes of generic type in Scala & Shapeless
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copied!<p>Say we have the following classes and some values (in Scala):</p> <pre><code>class A[T](val x: T) class B[T](x: T, val y: T) extends A[T](x) val x1 = new A("test") val x2 = new B(1,2) val x3 = new B("foo","bar") val x4 = new A(1) </code></pre> <p>Further, we define the following polymorphic function value (using shapeless):</p> <pre><code>object f extends (A ~> Option) { def apply[T](s: A[T]) = Some(s.x) } </code></pre> <p>Now we can call:</p> <pre><code>f(x1); f(x2); f(x3); f(x4) </code></pre> <p>Which all succeed (and should IMHO). However:</p> <pre><code>val list = x1 :: x2 :: x3 :: x4 :: HNil list.map(f) // could not find implicit value for parameter mapper: // shapeless.Mapper[f.type,shapeless.::[A[String],shapeless.::[ // B[Int],shapeless.::[B[String],shapeless.::[A[Int],shapeless.HNil]]]]] </code></pre> <p>Where I was expecting:</p> <pre><code>Some("test") :: Some(1) :: Some("foo") :: Some(1) :: HNil </code></pre> <p>Note that this works:</p> <pre><code>val list2 = x1 :: x4 :: HNil // only instances of A list2.map(f) </code></pre> <p><strong>UPDATE</strong></p> <p>It seems that if we specify each case separately, it's fine:</p> <pre><code>object f extends Poly1 { implicit def caseA[T] = at[A[T]]{s => Some(s.x)} implicit def caseB[T] = at[B[T]]{s => Some(s.x)} } </code></pre> <p>However, trying to express this a bit smarter, does not work (not even for simple applications):</p> <pre><code>object f extends Poly1 { implicit def caseA[T, S <: A[T]] = at[S]{s => Some(s.x)} } </code></pre>

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