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HMap Key types are erased and allow for unsafe type cast #554

stewSquared opened this Issue Feb 29, 2016 · 0 comments


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stewSquared commented Feb 29, 2016

Here's a minimal example:

  case class Key[T](id: Int)
  case class Value[T](value: T)

  class MyMap[K,V]
  implicit object mapInts extends MyMap[Key[Int], Value[Int]]
  implicit object mapStrs extends MyMap[Key[String], Value[String]]

  val map = HMap[MyMap](Key[Int](0) -> Value(3))

  val three: String = map.get(Key[String](0)).get.value // java.lang.ClassCastException

Generally, this might be worked around by using an equivalent proper type rather than a type constructor. e.g. KeyInt extends Key[Int] (if Key were not a case class)

It should probably be documented somewhere that higher-kinded types are to be used as keys with caution in the current design of HMap.

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