Tag: TypeLevel Programming

Tagged Type in Scala

Human always makes mistake. Also, software-engineer makes mistake. Guess there are some codes like this.

val userId: Long = 12
val deviceUuid: Long = 1234
val carSerialId: Long = 12345
def getHashCode(
userId: Long,
deviceUuid: Long,
carSerialId: Long
): String = {
val userIdPlus: Long = userId + 1
val deviceUuidPlus: Long = deviceUuid + 2
val carSerialIdPlus: Long = carSerialId + 3
s"$userIdPlus-$deviceUuidPlus-$carSerialIdPlus"
}
getHashCode(userId, deviceUuid, carSerialId) // Right Answer = res0: String = 13-1236-12348
getHashCode(deviceUuid, userId, carSerialId) // Wrong Answer = res1: String = 1235-14-12348
view raw TaggedTypeMistake.sc hosted with ❤ by GitHub

Two function calls


getHashCode(userId, deviceUuid, carSerialId) // Right Answer = res0: String = 13-1236-12348

getHashCode(deviceUuid, userId, carSerialId) // Wrong Answer = res1: String = 1235-14-12348

both compile. But the first one is a right answer, but secondly is wrong. Then What is a good way to help myself not make a mistake? At first thinking, It is very good way to make case class.

case class UserIdCaseClass(userId: Long)
case class DeviceUuidCaseClass(deviceUuid: Long)
case class CarSerialIdCaseClass(carSerialId: Long)
def getHashCodeCaseClass(
userIdCaseClass: UserIdCaseClass,
deviceUuidCaseClass: DeviceUuidCaseClass,
carSerialIdCaseClass: CarSerialIdCaseClass
): String = {
val userIdPlus: Long = userIdCaseClass.userId + 1
val deviceUuidPlus: Long = deviceUuidCaseClass.deviceUuid + 2
val carSerialIdPlus: Long = carSerialIdCaseClass.carSerialId + 3
s"$userIdPlus-$deviceUuidPlus-$carSerialIdPlus"
}
getHashCodeCaseClass(UserIdCaseClass(userId), DeviceUuidCaseClass(deviceUuid), CarSerialIdCaseClass(carSerialId)) // Right Answer = res2: String = 13-1236-12348
getHashCodeCaseClass(DeviceUuidCaseClass(deviceUuid), UserIdCaseClass(userId), CarSerialIdCaseClass(carSerialId)) // Not Even Compiled!
view raw SolveProblemByUsingCaseClass.sc hosted with ❤ by GitHub

This way, we can reduce mistakes. But we lost many things. First, we have to write many boilerplate. We have to make case class every Id and other Long types. And we lost that Id is the Long type. So, if we want to use userId or carSerialId or DeviceUuid, we have to call member. And cannot assign to Long type variable! To solve these problems, we can use Tagged Type.

Tagged Type

Tagged Type is tagging to some type A and Define as a subtype of A. For example, if we want to define userId as a subtype of Long, we can declare UserId as a Tagged Type of Long. Here is following an example.

import shapeless.tag
import shapeless.tag.@@
trait UserIdTag
trait DeviceUuidTag
trait CarSerialIdTag
type UserId = Long @@ UserIdTag
type DeviceUuid = Long @@ DeviceUuidTag
type CarSerialId = Long @@ CarSerialIdTag
def getHashCodeTagged(userId: UserId, deviceUuid: DeviceUuid, carSerialId: CarSerialId): String = {
val userIdPlus: Long = userId + 1
val deviceUuidPlus: Long = deviceUuid + 2
val carSerialIdPlus: Long = carSerialId + 3
s"$userIdPlus-$deviceUuidPlus-$carSerialIdPlus"
}
val taggedUserId: UserId = tag[UserIdTag][Long](userId)
val taggedDeviceUuid: DeviceUuid = tag[DeviceUuidTag][Long](deviceUuid)
val taggedCarSerialId: CarSerialId = tag[CarSerialIdTag][Long](carSerialId)
getHashCodeTagged(taggedUserId, taggedDeviceUuid, taggedCarSerialId) // Right Answer = res2: String = 13-1236-12348
getHashCodeTagged(taggedDeviceUuid, taggedCarSerialId, taggedUserId) // Not Even Compiled
view raw TaggedTypeAnswer.sc hosted with ❤ by GitHub

Now, UserId, DeviceUuid, CarSerialId is Subtype of Long. We can use this type as Long type. For example, add Number to UserId by just using ‘+’ operator. or can assign to Long. But, It is UserId Type, DeviceUuid cannot be passed as a parameter in UserId Position.

Tagged Type Eraser

Sometimes you want to override function by tagged type like this.

def getId(userId: UserId): Long = userId + 1
def getId(deviceUuid: DeviceUuid): Long = deviceUuid + 2 // Not Compiled because tagged type erased after compile
def getId(carSerialId: CarSerialId): Long = carSerialId + 3 // Not Compiled because tagged type erased after compile
view raw taggedTypeOverrideFail.sc hosted with ❤ by GitHub

But if you override like this, Compiler says that ‘Not Compiled because tagged type erased after compile’. Yes, Tagged Type is erased after compile time. Then how can we solve the problem? You can solve this by using Either.

def getUserIdOrDeviceIdOrCarSerialId(
id: Either[UserId, Either[DeviceUuid, CarSerialId]]
): Long = id match {
case Left(userId) => userId + 1
case Right(Left(deviceUuid)) => deviceUuid + 2
case Right(Right(carSerialId)) => carSerialId + 3
}
getUserIdOrDeviceIdOrCarSerialId(Left(taggedUserId))
getUserIdOrDeviceIdOrCarSerialId(Right(Left(taggedDeviceUuid)))
getUserIdOrDeviceIdOrCarSerialId(Right(Right(taggedCarSerialId)))
view raw TaggedTypeEither.sc hosted with ❤ by GitHub

Of course, you can use Coproduct in shapeless

import shapeless.{Coproduct, CNil, :+:, Inl, Inr}
type Id = UserId :+: DeviceUuid :+: CarSerialId :+: CNil
def getId(id: Id): Long = id match {
case Inl(userId) => userId + 1
case Inr(Inl(deviceUuid)) => deviceUuid + 2
case Inr(Inr(Inl(carSerialId))) => carSerialId + 3
case Inr(Inr(Inr(cNil))) => 0
}
getId(Coproduct[Id](taggedUserId))
getId(Coproduct[Id](taggedDeviceUuid))
getId(Coproduct[Id](taggedCarSerialId))
view raw TaggedTypeShapeless.sc hosted with ❤ by GitHub

By this way, You can keep self from make mistake.

val userId: Long = 12
val deviceUuid: Long = 1234
val carSerialId: Long = 12345
def getHashCode(
userId: Long,
deviceUuid: Long,
carSerialId: Long
): String = {
val userIdPlus: Long = userId + 1
val deviceUuidPlus: Long = deviceUuid + 2
val carSerialIdPlus: Long = carSerialId + 3
s"$userIdPlus-$deviceUuidPlus-$carSerialIdPlus"
}
getHashCode(userId, deviceUuid, carSerialId) // Right Answer = res0: String = 13-1236-12348
getHashCode(deviceUuid, userId, carSerialId) // Wrong Answer = res1: String = 1235-14-12348
// Lets do it by case class
case class UserIdCaseClass(userId: Long)
case class DeviceUuidCaseClass(deviceUuid: Long)
case class CarSerialIdCaseClass(carSerialId: Long)
def getHashCodeCaseClass(
userIdCaseClass: UserIdCaseClass,
deviceUuidCaseClass: DeviceUuidCaseClass,
carSerialIdCaseClass: CarSerialIdCaseClass
): String = {
val userIdPlus: Long = userIdCaseClass.userId + 1
val deviceUuidPlus: Long = deviceUuidCaseClass.deviceUuid + 2
val carSerialIdPlus: Long = carSerialIdCaseClass.carSerialId + 3
s"$userIdPlus-$deviceUuidPlus-$carSerialIdPlus"
}
getHashCodeCaseClass(UserIdCaseClass(userId), DeviceUuidCaseClass(deviceUuid), CarSerialIdCaseClass(carSerialId)) // Right Answer = res2: String = 13-1236-12348
//getHashCodeCaseClass(DeviceUuidCaseClass(deviceUuid), UserIdCaseClass(userId), CarSerialIdCaseClass(carSerialId)) // Not Even Compiled!
import shapeless.tag
import shapeless.tag.@@
trait UserIdTag
trait DeviceUuidTag
trait CarSerialIdTag
type UserId = Long @@ UserIdTag
type DeviceUuid = Long @@ DeviceUuidTag
type CarSerialId = Long @@ CarSerialIdTag
def getHashCodeTagged(
userId: UserId,
deviceUuid: DeviceUuid,
carSerialId: CarSerialId
): String = {
val userIdPlus: Long = userId + 1
val deviceUuidPlus: Long = deviceUuid + 2
val carSerialIdPlus: Long = carSerialId + 3
s"$userIdPlus-$deviceUuidPlus-$carSerialIdPlus"
}
val taggedUserId: UserId = tag[UserIdTag][Long](userId)
val taggedDeviceUuid: DeviceUuid = tag[DeviceUuidTag][Long](deviceUuid)
val taggedCarSerialId: CarSerialId = tag[CarSerialIdTag][Long](carSerialId)
getHashCodeTagged(taggedUserId, taggedDeviceUuid, taggedCarSerialId) // Right Answer = res2: String = 13-1236-12348
//getHashCodeTagged(taggedDeviceUuid, taggedCarSerialId, taggedUserId) // Not Even Compiled
// Can't Override
def getId(userId: UserId): Long = userId + 1
//def getId(deviceUuid: DeviceUuid): Long = deviceUuid + 2 // Not Compiled because tagged type erased after compile
//def getId(carSerialId: CarSerialId): Long = carSerialId + 3 // Not Compiled because tagged type erased after compile
// Let's do by either
def getUserIdOrDeviceIdOrCarSerialId(
id: Either[UserId, Either[DeviceUuid, CarSerialId]]
): Long = id match {
case Left(userId) => userId + 1
case Right(Left(deviceUuid)) => deviceUuid + 2
case Right(Right(carSerialId)) => carSerialId + 3
}
getUserIdOrDeviceIdOrCarSerialId(Left(taggedUserId))
getUserIdOrDeviceIdOrCarSerialId(Right(Left(taggedDeviceUuid)))
getUserIdOrDeviceIdOrCarSerialId(Right(Right(taggedCarSerialId)))
// You can do by Coproduct by shapeless
import shapeless.{Coproduct, CNil, :+:, Inl, Inr}
type Id = UserId :+: DeviceUuid :+: CarSerialId :+: CNil
def getId(id: Id): Long = id match {
case Inl(userId) => userId + 1
case Inr(Inl(deviceUuid)) => deviceUuid + 2
case Inr(Inr(Inl(carSerialId))) => carSerialId + 3
case Inr(Inr(Inr(cNil))) => 0
}
getId(Coproduct[Id](taggedUserId))
getId(Coproduct[Id](taggedDeviceUuid))
getId(Coproduct[Id](taggedCarSerialId))
view raw TaggedType.sc hosted with ❤ by GitHub
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