I am trying to create a type class in Scala and use it to make a simple polymorphic case class. This example doesn't compile and gives "could not find implicit value for parameter writer:A$A228.this.ValueWriter[T]". I can't really figure out what could be going wrong or where to start.
trait Keeper
case class StringKeeper(measure: String) extends Keeper
case class StringLengthKeeper(measure: Int) extends Keeper
trait ValueWriter[A] {
def write(value: String): A
}
object DefaultValueWriters {
implicit val stringWriter = new ValueWriter[StringKeeper] {
def write(value: String) = StringKeeper(value)
}
implicit val stringLengthWriter = new ValueWriter[StringLengthKeeper] {
def write(value: String) = StringLengthKeeper(value.length)
}
}
object Write {
def toWrite[A](value: String)(implicit writer: ValueWriter[A]) = {
writer.write(value)
}
}
case class WriterOfKeepers[T <: Keeper](value: String) {
def run: T = {
Write.toWrite[T](value)
}
}
import DefaultValueWriters._
val writerLengthKeeper = WriterOfKeepers[StringLengthKeeper]("TestString")
writerLengthKeeper.run
There is no implicit ValueWriter in scope when you call Write.toWrite.
You need to have the implicit parameter in the constructor
case class WriterOfKeepers[T <: Keeper : ValueWriter](value: String) {
def run: T = {
Write.toWrite[T](value)
}
}
or in the method
case class WriterOfKeepers[T <: Keeper](value: String) {
def run(implicit writer: ValueWriter[T]): T = {
Write.toWrite(value)
}
}
or find some other way compatible with your requirements (and I don't know those).
Related
I am trying to create a serializable trait that has a dependency on type class.
package dsl
import zio.schema._
sealed trait Random[A] {
def generate: A
}
object Random {
case object RandomDouble extends Random[Double] {
override def generate: Double = ???
implicit val RandomDoubleSchema: Schema[Random[Double]] = DeriveSchema.gen[Random[Double]]
}
}
sealed trait DummyExpr[A] {
def eval(value: DummyExpr[A]): A
}
object DummyExpr {
case object DummyTrue extends DummyExpr[Boolean] {
override def eval(value: DummyExpr[Boolean]): Boolean = ???
}
case object DummyFalse extends DummyExpr[Boolean] {
override def eval(value: DummyExpr[Boolean]): Boolean = ???
}
case class DummyOperator[A](random: Random[A], predicate: DummyExpr[Boolean]) extends DummyExpr[A] {
override def eval(value: DummyExpr[A]): A = ???
}
}
object main extends App {
val schemaRandom = DeriveSchema.gen[Random[Double]]
val schemaDummy = DeriveSchema.gen[DummyExpr[Double]]
}
Here is a reproducible link https://scastie.scala-lang.org/3PnmF52hSkuduzGP10wTdg
But type derivation for this fails with the error magnolia: could not find any direct subtypes of trait Random
I am using zio-schema which internally uses magnolia. I tried adding implicit Derivation of typeclass too but that didn't help too.
Following this answer shows how to bind an Enumeration to a form using a case class.
However in Play 2.7.3 this code fails with:
No Json serializer found for type jura.SearchRequest. Try to implement an implicit Writes or Format for this type.
When I implement the formatter:
object SearchRequest {
implicit val searchRequestFormat: OFormat[SearchRequest] = Json.format[SearchRequest]
}
I get
No instance of play.api.libs.json.Format is available for
scala.Enumeration.Value in the implicit scope
Should I be trying to write a formatter for the system scala.Enumeration type?
Or is there another way to implement a formatter when Enumerations are involved?
Test case here.
I use for any Enumeration this Library: enumeratum
With Dotty there will be great Enumerations, but until then I think moving to enumeratum is the best way handling Enumerations in Scala. See also Dotty - Enumerations.
As a bonus there is a play-json Extension, see Play JSON Extension.
With this your code would look like this:
import enumeratum.{ PlayJsonEnum, Enum, EnumEntry }
sealed trait SearchRequest extends EnumEntry
object SearchRequest extends Enum[SearchRequest] with PlayJsonEnum[SearchRequest] {
val values = findValues
case object SuperRequest extends SearchRequest
case object SimpleRequest extends SearchRequest
..
}
In essence PlayJsonEnum[SearchRequest] does all the work.
To write the enum as a string as cchantep says you can use Writes.enumNameWrites, we specifically use to read and write the ID. Therefore we have an EnumFormat in the package global for enums:
package object enums {
implicit def reads[E <: Enumeration](enum: E): Reads[E#Value] = new Reads[E#Value] {
def reads(json: JsValue): JsResult[E#Value] = json match {
case JsNumber(s) =>
try {
JsSuccess(enum.apply(s.toInt))
} catch {
case _: NoSuchElementException => JsError(s"Enumeration expected of type: '${enum.getClass}', but it does not appear to contain the value: '$s'")
}
case _ => JsError("Number value expected")
}
}
implicit def writes[E <: Enumeration]: Writes[E#Value] = new Writes[E#Value] {
def writes(v: E#Value): JsValue = JsNumber(v.id)
}
implicit def formatID[E <: Enumeration](enum: E): Format[E#Value] =
Format(reads(enum), writes)
def readsString[E <: Enumeration](enum: E): Reads[E#Value] = new Reads[E#Value] {
def reads(json: JsValue): JsResult[E#Value] = json match {
case JsString(s) => {
try {
JsSuccess(enum.withName(s))
} catch {
case _: NoSuchElementException => JsError(s"Enumeration expected of type: '${enum.getClass}', but it does not appear to contain the value: '$s'")
}
}
case _ => JsError("String value expected")
}
}
implicit def writesString[E <: Enumeration]: Writes[E#Value] = new Writes[E#Value] {
def writes(v: E#Value): JsValue = JsString(v.toString)
}
implicit def formatString[E <: Enumeration](enum: E): Format[E#Value] =
Format(readsString(enum), writesString)
}
And used:
object SearchRequest extends Enumeration(1) {
type SearchRequest = Value
val ONE /*1*/ , TWO /*2*/ , ETC /*n*/ = Value
implicit val searchRequestFormat: Format[SearchRequest] = formatID(SearchRequest)
}
Add the following to your Enumeration Object
implicit object MatchFilterTypeFormatter extends Formatter[MatchFilterType.Value] {
override val format = Some(("format.enum", Nil))
override def bind(key: String, data: Map[String, String]) = {
try {
Right(MatchFilterType.withName(data.get(key).head))
} catch {
case e:NoSuchElementException => Left(Seq(play.api.data.FormError(key, "Invalid MatchFilterType Enumeration")))
}
}
override def unbind(key: String, value: MatchFilterType.Value) = {
Map(key -> value.toString)
}
}
implicit val matchFilterTypeFormat = new Format[MatchFilterType.MatchFilterType] {
def reads(json: JsValue) = JsSuccess(MatchFilterType.withName(json.as[String]))
def writes(myEnum: MatchFilterType.MatchFilterType) = JsString(myEnum.toString)
}
And then the Formatter/Controller given in the question will work.
A working test case is here.
trait Encoder[From, To] {
def encode(x: From): To
}
object Encoder {
implicit val thingToString: Encoder[Thing, String] = new Encoder[Thing, String] {
def encode(x: Thing): String = x.toString
}
}
trait Config {
type Repr
}
class MyConfig extends Config { type Repr = String }
//class ConcreteConfig { type Repr = String }
class Api[T](val config: Config) {
def doSomething(value: T)(implicit encoder: Encoder[T, config.Repr]): Unit = {}
}
case class Thing(a: Int)
object Test extends App {
import Encoder._
val api = new Api[Thing](new MyConfig)
api.doSomething(Thing(42))
}
The call to api.doSomething fails to compile:
could not find implicit value for parameter encoder: Encoder[Thing,Test.api.config.Repr]
If I change the signature of class Api[T]'s constructor so that it takes a ConcreteConfig, then the compiler can tell that config.Repr == String and the implicit lookup succeeds. But this wouldn't really work for my use case.
Is there any other way to guide the implicit lookup? Am I losing type info because I'm missing a type refinement or something?
This can't be achieved since config.Repr is not a stable path. The compiler can't determine that config.Repr = String, even though the runtime value of config is of type MyConfig.
This is going to work only if you can provide concrete config instance as a type parameter in Api, which will tell compiler what is the exact type of Repr:
class Api[T, C <: Config](val config: C) {
def doSomething(value: T)(implicit encoder: Encoder[T, config.Repr]): Unit = {}
}
object Test extends App {
import Encoder._
val api = new Api[Thing, MyConfig](new MyConfig)
api.doSomething(Thing(42))
}
Infer types one step at a time. Also, I don't see any reason for using value-dependent types here (or, for that matter, anywhere; that's a different question though).
trait Encoder[From, To] {
def encode(x: From): To
}
object Encoder {
implicit val thingToString: Encoder[Thing, String] = new Encoder[Thing, String] {
def encode(x: Thing): String = x.toString
}
}
trait Config {
type Repr
}
class MyConfig extends Config { type Repr = String }
// class ConcreteConfig { type Repr = String }
case class Api[T, C <: Config](val config: C) {
def doSomething(value: T)(implicit encoder: Encoder[T, C#Repr]): Unit = ()
}
case class ApiFor[T]() {
def withConfig[C <: Config](config: C): Api[T,C] = Api(config)
}
case class Thing(a: Int)
object Test extends App {
import Encoder._
val api = ApiFor[Thing] withConfig new MyConfig
// compiles!
api.doSomething(Thing(42))
}
How is it possible that type parameter in call to get[A] is Nothing in this snippet? What can I do to force a compiler to produce an error when get is called without explicit type parameter?
case class User(email: String)
object Hello {
def main(args: Array[String]): Unit = {
val store = new ObjectStore
store.get
}
}
class ObjectStore {
def get[A: Manifest]: Option[A] = {
println(manifest[A].toString())
None
}
}
Based on this blog post, the following should work:
#implicitNotFound("Nothing was inferred")
sealed trait NotNothing[-T]
object NotNothing {
implicit object notNothing extends NotNothing[Any]
implicit object `\n The error is because the type parameter was resolved to Nothing` extends NotNothing[Nothing]
}
class ObjectStore {
def get[T](implicit evManifest: Manifest[T], evNotNothing: NotNothing[T]): Option[T] = {
println(manifest[T].toString())
None
}
}
object X {
val oo = new ObjectStore().get[Any]
//fails to compile
//val o = new ObjectStore().get
}
I have a trait and a class that extends the trait. I can use the methods from the trait as follows:
trait A {
def a = ""
}
class B(s: String) extends A {
def b = a
}
However, when I use the trait's method in the constructor like this:
trait A {
def a = ""
}
class B(s: String) extends A {
def this() = this(a)
}
then the following error appears:
error: not found: value a
Is there some way to define default parameters for the construction of classes in the trait?
EDIT: To clarify the purpose: There is the akka-testkit:
class TestKit(_system: ActorSystem) extends { implicit val system = _system }
And each test looks like this:
class B(_system: ActorSystem) extends TestKit(_system) with A with ... {
def this() = this(actorSystem)
...
}
because I want to create common creation of the ActorSystem in A:
trait A {
val conf = ...
def actorSystem = ActorSystem("MySpec", conf)
...
}
It's a little bit tricky because of Scala initialization order. The simplest solution I found is to define a companion object for your class B with apply as factory method:
trait A {
def a = "aaaa"
}
class B(s: String) {
println(s)
}
object B extends A {
def apply() = new B(a)
def apply(s: String) = new B(s)
}