Is it possible to define partially parameterize generic implicit class ? For instance assume I have following class
implicit class IoExt[L, R](val io: IO[R]) {
def wrapped(errorCode: String): IO[Either[ProcessingResult[L], R]] = ???
}
How can I defined something like
type IoExtLocal[R] = IoExt[String, R]
And have IoExtLocal[R] be available as implicit class ?
The motivation is to free client code from specifying type parameter every time wrapped[](..) is called. It gets very verbose.
Just create another implicit class and import necessary one
object ioExt {
implicit class IoExt[L, R](val io: IO[R]) extends AnyVal {
def wrapped(errorCode: String): IO[Either[ProcessingResult[L], R]] = ???
}
}
object ioExtLocal {
implicit class IoExtLocal[R](val io: IO[R]) extends AnyVal {
def wrapped(errorCode: String): IO[Either[ProcessingResult[String], R]] =
(io: ioExt.IoExt[String, R]).wrapped(errorCode)
}
}
import ioExtLocal._
trait SomeR
val x: IO[SomeR] = ???
x.wrapped(???)
After trying multiple solutions I've found that following works without instantiating helper class on every call to wrapped
trait IoExtTrait[L, R] extends Any {
protected def io: IO[R]
def wrapped(errorCode: String): IO[Either[ProcessingResult[L], R]] =
io.attempt.map(_.leftMap(ex ⇒ FailureMsg[L](errorCode, Some(ex))))
def wrappedT(errorCode: String): EitherT[IO, ProcessingResult[L], R] =
EitherT(wrapped(errorCode))
}
implicit class IoExtLocalString[R](protected val io: IO[R]) extends AnyVal with IoExtTrait[String, R] {
override def wrapped(errorCode: String) = super.wrapped(errorCode)
}
on the other hand following instantiates helper class on every call
implicit class IoExtLocalString[R](protected val io: IO[R]) extends AnyVal with IoExtTrait[String, R] {}
If anyone know why it happens please let me know. I'm on Scala 2.12.8 (same behavior with 2.13-RC1).
Further conversation at https://github.com/scala/bug/issues/11526 confirmed that allocation happens on both cases. Too bad.
Related
I'm trying to solve a problem that may not be possible in Scala.
I want to have a Trait to solve default constructors
trait Builder[T <: Buildable] {
def build(code: String): T = new T(code)
def build: T = new T("bar")
}
So extending the Trait on the companion object automatically has access to functions that creates the class with specific constructors and parameters
class A(code: String) extends Buildable
object A extends Builder[A]
Extending the Trait, the companion object has the constructors
A.build("foo")
A.build
Is this possible in Scala?
Also tried abstract classes, but hadn't had any success
trait Builder[T <: BuildableClass] {
def build(code: String): T = new T(code)
def build: T = new T("bar")
}
abstract class BuildableClass(code: String)
class A(code: String) extends BuildableClass(code)
object A extends Builder[A]
Thanks in advance
Edit: currently locked on Scala 2.12
Because of the type erasure, in ordinary code new T is allowed only for class type T, not an abstract type/type parameter.
In Scala, is it possible to instantiate an object of generic type T?
How to create an instance of type T at runtime with TypeTags
Class type required but T found
An alternative to runtime reflection (see #StanislavKovalenko's answer) is macros. new T is possible there because during macro expansion T is not erased yet.
import scala.language.experimental.macros
import scala.reflect.macros.blackbox // libraryDependencies += scalaOrganization.value % "scala-reflect" % scalaVersion.value
abstract class BuildableClass(code: String)
trait Builder[T <: BuildableClass] {
def build(code: String): T = macro BuilderMacros.buildImpl[T]
def build: T = macro BuilderMacros.buildDefaultImpl[T]
}
class BuilderMacros(val c: blackbox.Context) {
import c.universe._
def buildImpl[T: WeakTypeTag](code: Tree): Tree = q"new ${weakTypeOf[T]}($code)"
def buildDefaultImpl[T: WeakTypeTag]: Tree = q"""new ${weakTypeOf[T]}("bar")"""
}
// in a different subproject
class A(code:String) extends BuildableClass(code)
object A extends Builder[A]
A.build("foo") // scalac: new A("foo")
A.build // scalac: new A("bar")
Alternative implementations:
trait Builder[T <: BuildableClass] {
def build(code: String): T = macro BuilderMacros.buildImpl
def build: T = macro BuilderMacros.buildDefaultImpl
}
class BuilderMacros(val c: blackbox.Context) {
import c.universe._
val tpe = c.prefix.tree.tpe.baseType(symbolOf[Builder[_]]).typeArgs.head
def buildImpl(code: Tree): Tree = q"new $tpe($code)"
def buildDefaultImpl: Tree = q"""new $tpe("bar")"""
}
class BuilderMacros(val c: blackbox.Context) {
import c.universe._
val symb = c.prefix.tree.symbol.companion
def buildImpl(code: Tree): Tree = q"new $symb($code)"
def buildDefaultImpl: Tree = q"""new $symb("bar")"""
}
class BuilderMacros(val c: blackbox.Context) {
import c.universe._
val tpe = c.prefix.tree.tpe.companion
def buildImpl(code: Tree): Tree = q"new $tpe($code)"
def buildDefaultImpl: Tree = q"""new $tpe("bar")"""
}
class BuilderMacros(val c: blackbox.Context) {
import c.universe._
val tpe = symbolOf[Builder[_]].typeParams.head.asType.toType
.asSeenFrom(c.prefix.tree.tpe, symbolOf[Builder[_]])
def buildImpl(code: Tree): Tree = q"new $tpe($code)"
def buildDefaultImpl: Tree = q"""new $tpe("bar")"""
}
One of the potential solution that uses a reflection looks a bit ugly, but it works.
import scala.reflect._
trait Builder[T <: Buildable] {
def build(code: String)(implicit ct: ClassTag[T]): T =
ct.runtimeClass.getConstructors()(0).newInstance(code).asInstanceOf[T]
def build(implicit ct: ClassTag[T]): T =
ct.runtimeClass.getConstructors()(0).newInstance("bar").asInstanceOf[T]
}
trait Buildable
class A(code: String) extends Buildable {
def getCode = code
}
object A extends Builder[A]
val a: A = A.build
println(a.getCode)
The problem is that your Builder trait doesn't know anything about how to construct your instances. You can get this info from runtime with reflection or from compile time with macros.
Found a much cleaner solution to this problem without using macros / reflection / implicits
trait Buildable
trait Builder[T <: Buildable] {
self =>
def apply(code: String): T
def build: T = self.apply("foo")
def build(code: String): T = self.apply(code)
}
case class A(code: String) extends Buildable {
def getCode: String = code
}
object A extends Builder[A]
A.build("bar").getCode
A.build.getCode
I want to leverage Scala reflection to implicitly pass a ClassTag.
There are plenty solutions on StackOverflow on how to accomplish this.
import scala.reflect.ClassTag
// animal input
trait AnimalInput {}
case class DogInput() extends AnimalInput
case class CatInput() extends AnimalInput
// animals
trait Animal[T <: AnimalInput] {
def getInput(implicit ct: ClassTag[T]): Class[T] = {
ct.runtimeClass.asInstanceOf[Class[T]]
}
}
object Dog extends Animal[DogInput] {}
object Cat extends Animal[CatInput] {}
I can test that this is working well:
println(Dog.getInput) // Success: "DogInput"
println(Cat.getInput) // Success: "CatInput"
The problem is, the second I reference these objects in any collection, I run into trouble:
// Failure: "No ClassTag available for animal.T"
List(Dog, Cat).foreach(animal => println(animal.getInput))
I think I understand why this is happening but I'm not sure how to work around it.
Thank you in advance for your help!
Actually, I can't reproduce No ClassTag available for animal.T. Your code compiles and runs in Scala 2.13.9:
https://scastie.scala-lang.org/DmytroMitin/lonnNg0fR1qb4Lg7ChDBqg
Maybe by failure you meant that for collection you don't receive classes DogInput, CatInput.
Actually, I managed to reproduce No ClassTag available for animal.T for type member T <: AnimalInput rather than type parameter: https://scastie.scala-lang.org/v79Y37eDRXuoauWA9b8uhQ
This question is very close to recent question Trying to extract the TypeTag of a Sequence of classes that extend a trait with different generic type parameters
See the reasons there.
Either use a heterogeneous collection
object classPoly extends Poly1 {
implicit def cse[T <: AnimalInput : ClassTag, A](implicit
ev: A <:< Animal[T]
): Case.Aux[A, Class[T]] =
at(_ => classTag[T].runtimeClass.asInstanceOf[Class[T]])
}
(Dog :: Cat :: HNil).map(classPoly).toList.foreach(println)
// class DogInput
// class CatInput
or use runtime reflection
trait Animal[T <: AnimalInput] {
def getInput: Class[T] = {
val classSymbol = runtimeMirror.classSymbol(this.getClass)
val animalSymbol = typeOf[Animal[_]].typeSymbol
val extendeeType = classSymbol.typeSignature.baseType(animalSymbol)
val extenderSymbol = extendeeType.typeArgs.head.typeSymbol.asClass
runtimeMirror.runtimeClass(extenderSymbol).asInstanceOf[Class[T]]
}
}
List(Dog, Cat).foreach(animal => println(animal.getInput))
// class DogInput
// class CatInput
The easiest is
trait Animal[T <: AnimalInput] {
def getInput: Class[T]
}
object Dog extends Animal[DogInput] {
val getInput = classOf[DogInput]
}
object Cat extends Animal[CatInput] {
val getInput = classOf[CatInput]
}
One more option is magnet pattern (1 2 3 4 5 6)
trait AnimalMagnet[T <: AnimalInput] {
def getInput: Class[T]
}
import scala.language.implicitConversions
implicit def animalToMagnet[A, T <: AnimalInput : ClassTag](a: A)(implicit
ev: A <:< Animal[T]
): AnimalMagnet[T] = new AnimalMagnet[T] {
override def getInput: Class[T] = classTag[T].runtimeClass.asInstanceOf[Class[T]]
}
List[AnimalMagnet[_]](Dog, Cat).foreach(animal => println(animal.getInput))
//class DogInput
//class CatInput
Also you can move ClassTag implicit from the method to the trait (and make the trait an abstract class)
abstract class Animal[T <: AnimalInput](implicit ct: ClassTag[T]) {
def getInput: Class[T] = ct.runtimeClass.asInstanceOf[Class[T]]
}
List(Dog, Cat).foreach(animal => println(animal.getInput))
// class DogInput
// class CatInput
In Scala, how can we determine if a class is a subclass of a parent class or a trait? For example:
trait MyTrait
class MyParentClass()
class MyOtherParentClass()
case class MySubClass() extends MyParentClass with MyTrait
case class MyOtherSubClass() extends MyOtherParentClass
Is it possible to identify if class such as MySubClass extends from MyParentClass or MyTrait without instantiating an object and through reflection APIs? Given an unknown generic type T, I am interested in having it match a case if T extends a particular parent class or a trait:
def example[T](): Unit = {
T match {
case if T extends MyParentClass => ...
case if T extends MyOtherParentClass => ...
case if T extends MyOtherTrait => ...
case _ => default case ...
}
If you write
case class MySubClass() extends MyParentClass with MyTrait
then obviously MySubClass extends MyParentClass and MyTrait so you can check that for a generic type T with
def test[T](implicit ev: T <:< MyParentClass, ev1: T <:< MyTrait) = ???
test[MySubClass] // compiles
at compile time.
If the thing is you want to check that with OR instead of AND, then you can use shapeless.OrElse or implicitbox.Priority
Scala method that needs either one of two implicit parameters
I updated the question with an example of the desired usage
It seems you want a type class
trait Example[T] {
def example(): Unit
}
object Example {
implicit def subtypeOfMyParentClass[T <: MyParentClass] = new Example[T] {
override def example(): Unit = ???
}
implicit def subtypeOfMyOtherParentClass[T <: MyOtherParentClass] = new Example[T] {
override def example(): Unit = ???
}
implicit def subtypeOfMyOtherTrait[T <: MyOtherTrait] = new Example[T] {
override def example(): Unit = ???
}
implicit def default[T] = new Example[T] {
override def example(): Unit = ???
}
}
def example[T]()(implicit e: Example[T]): Unit = e.example()
A type class is a compile-time (i.e. type-level) replacement for pattern matching.
If there is ambiguity among implicits you can prioritize them.
Just curious, do you know if there is a way to do this in a simple one line conditional such as if (T extends from MyParentClass) then ... through reflection APIs (is it possible through classOf[] or typeOf[]?)
You can do that at runtime
import scala.reflect.runtime.universe._
def example[T: TypeTag](): Unit =
if (typeOf[T] <:< typeOf[MyParentClass]) ???
else if (typeOf[T] <:< typeOf[MyOtherParentClass]) ???
else if (typeOf[T] <:< typeOf[MyOtherTrait]) ???
else ???
or
import scala.reflect.ClassTag
def example[T: ClassTag](): Unit =
if (classOf[MyParentClass] isAssignableFrom classOf[T]) ???
else if (classOf[MyOtherParentClass] isAssignableFrom classOf[T]) ???
else if (classOf[MyOtherTrait] isAssignableFrom classOf[T]) ???
else ???
or at compile time
import scala.language.experimental.macros
import scala.reflect.macros.blackbox
def example[T](): Unit = macro exampleImpl[T]
def exampleImpl[T: c.WeakTypeTag](c: blackbox.Context)(): c.Tree = {
import c.universe._
if (weakTypeOf[T] <:< typeOf[MyParentClass]) ???
else if (weakTypeOf[T] <:< typeOf[MyOtherParentClass]) ???
else if (weakTypeOf[T] <:< typeOf[MyOtherTrait]) ???
else ???
}
But implicits and types is a preferable way rather than (compile-time or especially runtime) reflection. It's not clear why you need reflection at all.
https://users.scala-lang.org/t/how-to-access-the-method/6281
I try to write general apply method to serve as a factory method for child classes. What I came up with is following:
trait A
object A {
def apply[T <: A](someParam: String): T {
new T()
}
}
class B private extends A
class C private extends A
So one can create new B or C instance:
def main(args: Array[String]): Unit = {
A[B]("test")
}
Is it acceptable that trait has an accompanying object?Is there a better way to implement this?
You could:
trait A
object A {
def apply[T <: A ](someParam: String) (implicit ev: ClassTag[T]): T ={
ev.runtimeClass.newInstance().asInstanceOf[T]
}
}
But its not very useful with varying constructor's for each sub class
sealed trait A
object A {
def apply[T <: A](someParam: String)(implicit ev: T): T = {
ev
}
}
class B(n: Int) extends A
class C(n: String) extends A
object B {
implicit def defaultCons: B = new B(1)
}
object C {
implicit def defaultCons: C = new C("hi")
}
Or you could also go via combination of ClassTag approach with pattern match based on the class instead of using reflection for instantiation.
There is the following API for Slick CRUD (Slick-2.1.0, Scala-2.11.4):
trait HasId {
type Id
def id: Option[Id]
}
trait HasIdColumn[E <: HasId] {
def id: scala.slick.lifted.Column[E#Id]
}
trait SlickExtensions {
val driver: scala.slick.driver.JdbcProfile
import driver.simple._
trait BaseDAO[T <: Table[E], E] {
val query: TableQuery[T]
}
trait HasIdActions[T <: Table[E] with HasIdColumn[E], E <: HasId]
extends BaseDAO[T, E] {
//line L1: this implicit val is needed to execute query.filter(_.id === id)
// what can be done in order to save the user from the necessity
// to override this value?
implicit val mappingId: BaseColumnType[E#Id]
def findById(id: E#Id)(implicit session: Session): Option[E] =
query.filter(_.id === id).firstOption
...
}
}
I apply this SlickExtensions as follows:
case class Entity(id: Option[Long] = None, value: String) extends HasId {
type Id = Long }
trait EntityComponent extends SlickExtensions {
import driver.simple._
class EntitiesTable(tag: Tag) extends Table[Entity](tag, "entities")
with HasIdColumn[Entity] {
def id = column[Long]("id", O.PrimaryKey, O.AutoInc)
def value = column[String]("value", O.NotNull)
def * = (id.?, value) <>(Entity.tupled, Entity.unapply)
}
object Entities extends HasIdActions[EntitiesTable, Entity] {
val query = TableQuery[EntitiesTable]
/* line L2: from slick library: ImplicitColumnTypes */
override implicit val mappingId = driver.simple.longColumnType
}
}
End point to execute queries:
val c = new EntityComponent {
lazy val driver = play.api.db.slick.Config.driver
}
db.withSession { implicit session =>
c.Entities.findById(1) foreach println
}
The main question is how to get rid of "implicit val mappingId" overriding in line L2?
I tried to create a class:
abstract class IdImplicits[E<:HasId](implicit val mappingId:BaseColumnType[E#Id])
and inherited it as follows:
object Entities extends IdImplicits[EntitiesTable, Entity]
with HasIdActions[EntitiesTable, Entity] {
val query = TableQuery[EntitiesTable]
//override implicit val mappingId: driver.simple.longColumnType
}
However it seems to me that such approach is redundant.
It would be great if I could hide "implicit val mappingId" inside SlickExtensions.
Here is the link to the same question
UPD:
In my project, I'd like to add HasName, HasValue[V] and some other mixins to construct the following DAOs:
object EntitiesDAO extends HasIdActions
with HasNameActions
with HasValueActions with NameToIdActions with ValueToIdActions {
...
override def nameToId(name:String):Option[E#Id]
override def valueToId(value:E#ValueType):Option[E#Id]
...
}
It leads to the following problems:
1) implicits for BaseColumnTypes, mentioned in my topic, should be taken into consideration for HasId, HasValue mixins
2) If implicits BaseColumnTypes are used as parameters of constructor of abstract classes then these classes can't be mixed in one EntityDAO object (the problem is described here).
3) If one abstract class is used for each variant of EntityDAO, then we get ugly-looking combinations, for example:
abstract class IdValueNameImplicits[E <: HasId with HasValue with HasName]
(implicit val idMapper:BaseColumnType[E#Id],
implicit val valueMapper:BaseColumnType[E#ValueType])
You can't do that because you are inside a trait and E#Id is only defined when you have a concrete implementation of it.
As you already discovered, you have to define your BaseColumnType when your trait is implemented because only then you have a defined type for E#Id.
Another option is not to have a trait but an abstract class where you can have a implicit BaseColumnType passed to the constructor.
I have a small project that does exactly what you are looking for. You can find it here: https://github.com/strongtyped/active-slick
There is also an Activator template.
http://typesafe.com/activator/template/slick-active-record
You can use it as is or as inspiration for your own.
Have fun!