Higher Kinded Type-Polymorphic Collections

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Type Bound Maps

A TypeBoundMap is a map where the key and value types both take a type parameter. For any key-value pair in the map, the key and value will have the same type argument for their type parameter. Let’s take a look at an example and see how this works.

Suppose we are building a system called “UserComment”, where users can post comments about whatever is on their mind. We start out with two entity classes to represent our domain model: User and Comment. We choose to label our entity classes with an Entity trait, and we also provide a type class for every entity type, in which we store information such as the natural keys for that type:

trait Entity

trait EntityType[E <: Entity] {
  val naturalKeys: Seq[NaturalKey] = Seq()

We provide a default empty set of natural keys, but implementing classes want to override it where appropriate. Our convention is to have the companion object to the entity class implement the EntityType, like so:

case class User(userId: String, firstName: String, lastName: String)
extends Entity

object User extends EntityType[User] {
  override val naturalKeys = Seq(NaturalKey("User.userId"))

We also have repositories for each of our entity classes, like so:

trait Repository[E <: Entity]

class UserRepo extends Repository[User]
class CommentRepo extends Repository[Comment]

Now let’s say we want to maintain a map from the entity type to the repository. We could try something like this:

val empty: Map[EntityType[_ <: Entity],
               Repository[_ <: Entity]] = Map()

val repositories = empty + (User -> userRepo) + (Comment -> commentRepo)

But the typing of this collection does not reflect the fact that for any given pair in the map, the key and value both have same kind of entity as its type argument. This means we have to cast when retrieving a repo. For instance:

val repositories: Map[EntityType[_ <: Entity],
                      Repository[_ <: Entity]] = initialize()

val userRepo: Repository[User] =

There is no guarantee that the type cast will succeed. (Even worse, the type cast probably will succeed due to type erasure, and you’ll get some other ClassCastException down the line.)

To solve this problem, we use a TypeBoundMap instead of a Map. The TypeBoundMap has three type parameters: the key type, the value type, and a bounds type that provides an upper bound for the type parameters to the key and value types. In our example, the key type is EntityType, the value type is Repository and the bounds type is Entity. We can build our TypeBoundMap like so:

import emblem.typeBound.TypeBoundMap

val repositories =
  TypeBoundMap[Entity, EntityType, Repository]() +
  (User -> userRepo) +
  (Comment -> commentRepo)

It’s a compiler error to try to add a key-value pair to a TypeBoundMap when the type parameter for the key and value don’t match, as in both of the following expressions:

repositories + (User -> commentRepo) // does not compile!
repositories + (Comment -> userRepo) // does not compile!

Pulling a repository out of the map is type-safe, and does not require a cast:

val userRepo: Repository[User] = repositories(User)

The TypeKeyMap that we looked at in the last chapter is really just a special case of a TypeBoundMap where the key type is fixed as a TypeKey. It’s included as a separate class so that we can make use of implicit TypeKey values for a more succinct API. For instance, consider the ComputerPart example from the previous chapter. We could have chosen to use a TypeBoundMap[ComputerPart, TypeKey, List] instead of a TypeKeyMap[ComputerPart, List]:

import emblem.TypeKeyMap
import emblem.typeKey

val inventories: TypeKeyMap[ComputerPart, List] = initializeInventories()
val inventories2: TypeBoundMap[ComputerPart, TypeKey, List] = initializeInventories2()

// longhand for adding a key-value pair to the map:
inventories + typeKey[Memory] -> memoryList
inventories2 + typeKey[Memory] -> memoryList

// shorthand only available for TypeKeyMap:
inventories + memoryList

// longhand for checking for containment:

// shorthand only available for TypeKeyMap:

// longhand for retrieving a value:
val memList = inventories(typeKey[Memory])
val memList2 = inventories2(typeKey[Memory])

// shorthand only available for TypeKeyMap:
val memList3 = inventories[Memory]

Because TypeKeyMap and TypeBoundMap share a lot of underlying code, they both implement a similar subset of the scala.collections.immutable.Map API:

I expect to continue to grow the API to mirror the Scala library map API as much as possible. If you are using TypeBoundMaps, and there is a method missing from the API that you would like to have, please let me know, and I will do my best to put it in.

For more examples of using TypeBoundMaps, take a look at the TypeBoundMap unit tests.