Scala-Cats Validated: value mapN is not a member of ValidatedNel tuple - scala

Scala community.
Currently I'm trying to implement custom model/single parameter validation using cats Validated Monad. But, after removal of Cartesian product since 1.0 I'm unable to use (v1 |#| v2) map (f) and unable to compile my code:
import cats.Semigroupal
import cats.data.Validated.{Invalid, Valid}
import cats.data.{ValidatedNel, _}
import cats.implicits._
import cats.instances.all._
case class FieldErrorInfo(name: String, error: String)
type FieldName = String
type ValidationResult[A] = ValidatedNel[FieldErrorInfo, A]
trait SingleFieldValidationRule[U] extends ((U, FieldName) => ValidationResult[U])
trait ModelValidationRule[M] extends (M => ValidationResult[M])
object ValidateNameRule extends SingleFieldValidationRule[String] {
override def apply(v1: String, name: String): ValidationResult[String] = {
if (v1.contains("cats"))
v1.validNel
else
FieldErrorInfo(name, "Some Error").invalidNel
}
}
object ValidateQuantityRule extends SingleFieldValidationRule[Int] {
override def apply(v1: Int, name: String): ValidationResult[Int] =
if (v1 > 0)
v1.validNel
else FieldErrorInfo(name, "Some Error").invalidNel
}
case class SampleModel(name: String, quantity: Int)
object ValidateSampleModel extends ModelValidationRule[SampleModel] {
override def apply(v1: SampleModel): ValidationResult[SampleModel] = {
val stage1: ValidatedNel[FieldErrorInfo, String] = ValidateNameRule(v1.name, "name")
val stage2: ValidatedNel[FieldErrorInfo, Int] = ValidateQuantityRule(v1.quantity, "quantity")
implicit val sga: Semigroupal[NonEmptyList] = new Semigroupal[NonEmptyList] {
override def product[A, B](fa: NonEmptyList[A], fb: NonEmptyList[B]): NonEmptyList[(A, B)] = fa.flatMap(a => fb.map(b => a -> b))
}
(stage1, stage2).mapN(SampleModel)
}
}
Compiler says, that
Error:(43, 23) value mapN is not a member of (cats.data.ValidatedNel[FieldErrorInfo,String], cats.data.ValidatedNel[FieldErrorInfo,Int])
(stage1, stage2).mapN(SampleModel)
^
Point me please how to use new Applicative syntax or what I did wrong...(forgot to create/import some implicits)

You seem to be missing the following import:
import cats.syntax.apply._
for the mapN.
Please ensure that you have -Ypartial-unification compiler flag activated, otherwise the compiler will have hard time extracting ValidatedNel[FieldErrorInfo, ?] from the types of stage1 and stage2:
libraryDependencies += "org.typelevel" %% "cats-core" % "1.1.0"
scalaVersion := "2.12.5"
scalacOptions += "-Ypartial-unification"
With the above settings, the following works:
import cats.Semigroupal
import cats.data.Validated.{Invalid, Valid}
import cats.data.ValidatedNel
import cats.data.NonEmptyList
import cats.syntax.apply._ // for `mapN`
import cats.syntax.validated._ // for `validNel`
case class FieldErrorInfo(name: String, error: String)
type FieldName = String
type ValidationResult[A] = ValidatedNel[FieldErrorInfo, A]
trait SingleFieldValidationRule[U] extends ((U, FieldName) => ValidationResult[U])
trait ModelValidationRule[M] extends (M => ValidationResult[M])
object ValidateNameRule extends SingleFieldValidationRule[String] {
override def apply(v1: String, name: String): ValidationResult[String] = {
if (v1.contains("cats"))
v1.validNel
else
FieldErrorInfo(name, "Some Error").invalidNel
}
}
object ValidateQuantityRule extends SingleFieldValidationRule[Int] {
override def apply(v1: Int, name: String): ValidationResult[Int] =
if (v1 > 0)
v1.validNel
else FieldErrorInfo(name, "Some Error").invalidNel
}
case class SampleModel(name: String, quantity: Int)
object ValidateSampleModel extends ModelValidationRule[SampleModel] {
override def apply(v1: SampleModel): ValidationResult[SampleModel] = {
val stage1: ValidatedNel[FieldErrorInfo, String] = ValidateNameRule(v1.name, "name")
val stage2: ValidatedNel[FieldErrorInfo, Int] = ValidateQuantityRule(v1.quantity, "quantity")
implicit val sga: Semigroupal[NonEmptyList] = new Semigroupal[NonEmptyList] {
override def product[A, B](fa: NonEmptyList[A], fb: NonEmptyList[B]): NonEmptyList[(A, B)] = fa.flatMap(a => fb.map(b => a -> b))
}
(stage1, stage2).mapN(SampleModel)
}
}

Values stage1 and stage2 must have type ValidationResult[_].
In this case implicit for mapN should work.
object ValidateSampleModel extends ModelValidationRule[SampleModel] {
override def apply(v1: SampleModel): ValidationResult[SampleModel] = {
val stage1: ValidationResult[String] = ValidateNameRule(v1.name, "name")
val stage2: ValidationResult[Int] = ValidateQuantityRule(v1.quantity, "quantity")
(stage1, stage2).mapN(SampleModel)
}
}

Related

Run Macro to generate code from parent class in Scala

I have a macro that I use to generate some code to call methods dynamically. The macro is more complex than this, but for simplicity let's say it works something like this
def myMacro[T]: Seq[MethodName]
so than when called on
class Hello {
def one(a: Int, b: UserId): String = a.toString + b.id
def two(c: Option[Int]): String = ""
def three(d: Seq[Int], f: Set[Int]): String = ""
}
println(myMacro[Hello]) // Seq("one", "two", "three")
I need this macro to generate code for an internal framework we use at Candu, but I need to be able to call it from the parent's class. So what I want to achieve is:
trait Superclass {
def aFakeMethod: String = ""
val methods = myMacro[Self] // FIXME! self is not defined here...
}
class Hello extends Superclass {
def one(a: Int, b: UserId): String = a.toString + b.id
def two(c: Option[Int]): String = ""
def three(d: Seq[Int], f: Set[Int]): String = ""
}
val hi = new Hello
println(hi.methods) // Seq("one", "two", "three")
Because the high number of classes in the framework, modifying the api between Hello and Superclass is very expansive. So I would need a way to do this without changing code in Hello
Any suggestions on how this could be achieved?
If myMacro worked outside Hello it should work inside Superclass as well
import scala.language.experimental.macros
import scala.reflect.macros.blackbox
def myMacro[T]: Seq[String] = macro impl[T]
def impl[T: c.WeakTypeTag](c: blackbox.Context): c.Tree = {
import c.universe._
val methodNames = weakTypeOf[T].decls
.filter(symb => symb.isMethod && !symb.isConstructor)
.map(_.name.toString).toList
val methodNamesTree = methodNames.foldRight[Tree](q"Nil")((name, names) => q"$name :: $names")
q"..$methodNamesTree"
}
Usage:
sealed trait Superclass {
def aFakeMethod: String = ""
val methods = myMacro[Hello]
}
val hi = new Hello
println(hi.methods) // List("one", "two", "three")
If for some reason you can't use the name of Hello you can try to make Superclass sealed and use knownDirectSubclasses
def myMacro1(): Seq[String] = macro impl1
def impl1(c: blackbox.Context)(): c.Tree = {
import c.universe._
val child = c.enclosingClass.symbol.asClass.knownDirectSubclasses.head
q"myMacro[$child]"
}
Usage:
sealed trait Superclass {
def aFakeMethod: String = ""
val methods = myMacro1()
}
val hi = new Hello
println(hi.methods) // List("one", "two", "three")
Or you can replace deprecated c.enclosingClass.symbol.asClass with c.internal.enclosingOwner.owner.asClass (now enclosingOwner is val methods, enclosingOwner.owner is trait Superclass).
If you can't make Superclass sealed try to traverse all classes and look for those extending Superclass
def myMacro2(): Seq[Seq[String]] = macro impl2
def impl2(c: blackbox.Context)(): c.Tree = {
import c.universe._
def treeSymbol(tree: Tree): Symbol = c.typecheck(tree, mode = c.TYPEmode).symbol
val enclosingClassSymbol = c.internal.enclosingOwner.owner
def isEnclosingClass(tree: Tree): Boolean = treeSymbol(tree) == enclosingClassSymbol
var methodss = Seq[Seq[String]]()
val traverser = new Traverser {
override def traverse(tree: Tree): Unit = {
tree match {
case q"$_ class $_[..$_] $_(...$_) extends { ..$_ } with ..$parents { $_ => ..$stats }"
if parents.exists(isEnclosingClass(_)) =>
val methods = stats.collect {
case q"$_ def $tname[..$_](...$_): $_ = $_" => tname.toString
}
methodss :+= methods
case _ => ()
}
super.traverse(tree)
}
}
c.enclosingRun.units.foreach(unit => traverser.traverse(unit.body))
def namesToTree[A: Liftable](names: Seq[A]): Tree =
names.foldRight[Tree](q"Seq()")((name, names) => q"$name +: $names")
namesToTree[Tree](methodss.map(namesToTree[String](_)))
}
Usage:
trait Superclass {
def aFakeMethod: String = ""
val methods = myMacro2()
}
class Hello1 extends Superclass {
def four = ???
def five = ???
}
class Hello extends Superclass {
def one(a: Int, b: UserId): String = a.toString + b.id
def two(c: Option[Int]): String = ""
def three(d: Seq[Int], f: Set[Int]): String = ""
}
val hi = new Hello
println(hi.methods) // List(List("four", "five"), List("one", "two", "three"))

Type parameters and inheritance in Scala

Is there a simple way to return a concrete type in an override method? And what about creating an instance of a concrete implementation? And calling chained methods implemented in the concrete class, so they return a correct type, too? I have a solution (based on https://stackoverflow.com/a/14905650) but I feel these things should be simpler.
There are many similar questions, but everyone's case is a little different, so here is another example (shortened from https://github.com/valdanylchuk/saiml/tree/master/src/main/scala/saiml/ga). When replying, if possible, please check if the whole code block compiles with your suggested change, because there are subtle cascading effects. I could not make this work with the "curiously recurring template pattern", for example (not that I find it nicer).
import scala.reflect.ClassTag
import scala.util.Random
abstract class Individual(val genome: String) {
type Self
def this() = this("") // please override with a random constructor
def crossover(that: Individual): Self
}
class HelloGenetic(override val genome: String) extends Individual {
type Self = HelloGenetic
def this() = this(Random.alphanumeric.take("Hello, World!".length).mkString)
override def crossover(that: Individual): HelloGenetic = {
val newGenome = this.genome.substring(0, 6) + that.genome.substring(6)
new HelloGenetic(newGenome)
}
}
class Population[A <: Individual {type Self = A} :ClassTag]( val size: Int,
tournamentSize: Int, givenIndividuals: Option[Vector[A]] = None) {
val individuals: Vector[A] = givenIndividuals getOrElse
Vector.tabulate(size)(_ => implicitly[ClassTag[A]].runtimeClass.newInstance.asInstanceOf[A])
def tournamentSelect(): A = individuals.head // not really, skipped
def evolve: Population[A] = {
val nextGen = (0 until size).map { _ =>
val parent1: A = tournamentSelect()
val parent2: A = tournamentSelect()
val child: A = parent1.crossover(parent2)
child
}.toVector
new Population(size, tournamentSize, Some(nextGen))
}
}
class Genetic[A <: Individual {type Self = A} :ClassTag](populationSize: Int, tournamentSize: Int) {
def optimize(maxGen: Int, maxMillis: Long): Individual = {
val first = new Population[A](populationSize, tournamentSize)
val optPop = (0 until maxGen).foldLeft(first) { (pop, _) => pop.evolve }
optPop.individuals.head
}
}
The CRTP version is
abstract class Individual[A <: Individual[A]](val genome: String) {
def this() = this("") // please override with a random constructor
def crossover(that: A): A
}
class HelloGenetic(override val genome: String) extends Individual[HelloGenetic] {
def this() = this(Random.alphanumeric.take("Hello, World!".length).mkString)
override def crossover(that: HelloGenetic): HelloGenetic = {
val newGenome = this.genome.substring(0, 6) + that.genome.substring(6)
new HelloGenetic(newGenome)
}
}
class Population[A <: Individual[A] :ClassTag]( val size: Int,
tournamentSize: Int, givenIndividuals: Option[Vector[A]] = None) {
val individuals: Vector[A] = givenIndividuals getOrElse
Vector.tabulate(size)(_ => implicitly[ClassTag[A]].runtimeClass.newInstance.asInstanceOf[A])
def tournamentSelect(): A = individuals.head // not really, skipped
def evolve: Population[A] = {
val nextGen = (0 until size).map { _ =>
val parent1: A = tournamentSelect()
val parent2: A = tournamentSelect()
val child: A = parent1.crossover(parent2)
child
}.toVector
new Population(size, tournamentSize, Some(nextGen))
}
}
class Genetic[A <: Individual[A] :ClassTag](populationSize: Int, tournamentSize: Int) {
def optimize(maxGen: Int, maxMillis: Long): Individual[A] = {
val first = new Population[A](populationSize, tournamentSize)
val optPop = (0 until maxGen).foldLeft(first) { (pop, _) => pop.evolve }
optPop.individuals.head
}
}
which compiles. For creating the instances, I'd suggest just passing functions:
class Population[A <: Individual[A]](val size: Int,
tournamentSize: Int, makeIndividual: () => A, givenIndividuals: Option[Vector[A]] = None) {
val individuals: Vector[A] = givenIndividuals getOrElse
Vector.fill(size)(makeIndividual())
...
}
If you want to pass them implicitly, you can easily do so:
trait IndividualFactory[A] {
def apply(): A
}
class HelloGenetic ... // remove def this()
object HelloGenetic {
implicit val factory: IndividualFactory[HelloGenetic] = new IndividualFactory[HelloGenetic] {
def apply() = new HelloGenetic(Random.alphanumeric.take("Hello, World!".length).mkString)
}
}
class Population[A <: Individual[A]](val size: Int,
tournamentSize: Int, givenIndividuals: Option[Vector[A]] = None)
(implicit factory: IndividualFactory[A]) {
val individuals: Vector[A] = givenIndividuals getOrElse
Vector.fill(size)(factory())
...
}

Apply a function to an object with generic type in Scala

I have this code
import scala.reflect.ClassTag
case class Data[T: ClassTag](list: List[T]) {
}
trait Transformation {
type T
type U
def transform(data: Data[T]) : Data[U]
}
class FromInt2String extends Transformation {
override type T = Int
override type U = String
override def transform(data: Data[T]) = new Data(List("1", "2", "3"))
}
class FromString2Int extends Transformation {
override type T = String
override type U = Int
override def transform(data: Data[T]) = new Data(List(1, 2, 3))
}
object Test extends App {
override def main(args: Array[String]) {
val data = new Data(List(1, 2, 3))
val int2String = new FromInt2String()
val data2 = int2String.transform(data)
val string2Int = new FromString2Int()
val data3 = string2Int.transform(data2)
val transformations = List(int2String, string2Int)
val data4 = transformations.foldLeft(data)((data, transformation) => {
transformation.transform(data)
})
}
}
The problem is in the foldLeft method. I can't do it because the type isn't compatible but I need to apply all the transforms in my initial object data
Any ideas how to do it?
Thanks
I've solved it using shapeless and this post
import scala.reflect.ClassTag
import shapeless._
object andThen extends Poly2 {
implicit def functions[A, B, C] = at[A => B, B => C](_ andThen _)
}
case class Data[T: ClassTag](list: List[T]) {
}
trait Transformation {
type T
type U
def transform(data: Data[T]) : Data[U]
}
class FromInt2String extends Transformation {
override type T = Int
override type U = String
override def transform(data: Data[T]) = new Data(List("1s", "2s", "3s"))
}
class FromString2Int extends Transformation {
override type T = String
override type U = Int
override def transform(data: Data[T]) = new Data(List(4, 5, 6))
}
object Test extends App {
override def main(args: Array[String]) {
val data = new Data(List(1, 2, 3))
println(data)
val int2String = new FromInt2String()
val data2 = int2String.transform(data)
println(data2)
val string2Int = new FromString2Int()
val data3 = string2Int.transform(data2)
println(data3)
val transformations = int2String.transform _ :: string2Int.transform _ :: HNil
val functions = transformations.reduceLeft(andThen)
val data4 = functions(data)
println(data4)
}
}
Thanks to all of you that help me

type parameter mismatch with WeakTypeTag reflection + quasiquoting (I think!)

Inspired by travisbrown, I'm trying to use a macro to create some "smart constructors".
Given
package mypkg
sealed trait Hello[A]
case class Ohayo[A,B](a: (A,B)) extends Hello[A]
and
val smartConstructors = FreeMacros.liftConstructors[Hello]
The macro should find all the subclasses of Hello, look at their constructors, and extract a few elements to populate this tree for the "smart constructor":
q"""
def $methodName[..$typeParams](...$paramLists): $baseType =
$companionSymbol[..$typeArgs](...$argLists)
"""
I hoped to get:
val smartConstructors = new {
def ohayo[A, B](a: (A, B)): Hello[A] = Ohayo[A, B](a)
}
but instead get:
error: type mismatch;
found : (A(in class Ohayo), B(in class Ohayo))
required: ((some other)A(in class Ohayo), (some other)B(in class Ohayo))
val liftedConstructors = FreeMacros.liftConstructors[Hello]
At a glance, the tree looks ok to me:
scala> q" new { ..$wellTyped }"
res1: u.Tree =
{
final class $anon extends scala.AnyRef {
def <init>() = {
super.<init>();
()
};
def ohayo[A, B](a: (A, B)): net.arya.constructors.Hello[A] = Ohayo[A, B](a)
};
new $anon()
}
but I guess it invisibly isn't. If I naively try to freshen up the typeParams with info.typeParams.map(p => TypeName(p.name.toString)), I get "can't splice A as type parameter" when I do the quasiquoting.
Where am I going wrong? Thanks for taking a look.
-Arya
import scala.language.experimental.macros
import scala.reflect.api.Universe
import scala.reflect.macros.whitebox
class FreeMacros(val c: whitebox.Context) {
import c.universe._
import FreeMacros._
def liftedImpl[F[_]](implicit t: c.WeakTypeTag[F[_]]): Tree = {
val atc = t.tpe
val childSymbols: Set[ClassSymbol] = subCaseClassSymbols(c.universe)(atc.typeSymbol.asClass)
val wellTyped = childSymbols.map(ctorsForSymbol(c.universe)(atc)).unzip
q"new { ..${wellTyped} }"
}
}
object FreeMacros {
def liftConstructors[F[_]]: Any = macro FreeMacros.liftedImpl[F]
def smartName(name: String): String = (
name.toList match {
case h :: t => h.toLower :: t
case Nil => Nil
}
).mkString
def subCaseClassSymbols(u: Universe)(root: u.ClassSymbol): Set[u.ClassSymbol] = {
val subclasses = root.knownDirectSubclasses
val cast = subclasses.map(_.asInstanceOf[u.ClassSymbol])
val partitioned = mapped.partition(_.isCaseClass)
partitioned match {
case (caseClasses, regularClasses) => caseClasses ++ regularClasses.flatMap(r => subCaseClassSymbols(u)(r))
}
}
def ctorsForSymbol(u: Universe)(atc: u.Type)(caseClass: u.ClassSymbol): (u.DefDef, u.DefDef) = {
import u._
import internal._
// these didn't help
// def clearTypeSymbol(s: Symbol): TypeSymbol = internal.newTypeSymbol(NoSymbol, s.name.toTypeName, s.pos, if(s.isImplicit)Flag.IMPLICIT else NoFlags)
// def clearTypeSymbol2(s: Symbol): TypeSymbol = internal.newTypeSymbol(NoSymbol, s.name.toTypeName, NoPosition, if(s.isImplicit)Flag.IMPLICIT else NoFlags)
// def clearTypeDef(d: TypeDef): TypeDef = internal.typeDef(clearTypeSymbol(d.symbol))
val companionSymbol: Symbol = caseClass.companion
val info: Type = caseClass.info
val primaryCtor: Symbol = caseClass.primaryConstructor
val method = primaryCtor.asMethod
val typeParams = info.typeParams.map(internal.typeDef(_))
// val typeParams = info.typeParams.map(s => typeDef(newTypeSymbol(NoSymbol, s.name.toTypeName, NoPosition, NoFlags)))
// val typeParams = info.typeParams.map(s => internal.typeDef(clearTypeSymbol2(s)))
val typeArgs = info.typeParams.map(_.name)
val paramLists = method.paramLists.map(_.map(internal.valDef(_)))
val argLists = method.paramLists.map(_.map(_.asTerm.name))
val baseType = info.baseType(atc.typeSymbol)
val List(returnType) = baseType.typeArgs
val methodName = TermName(smartName(caseClass.name.toString))
val wellTyped =
q"""
def $methodName[..$typeParams](...$paramLists): $baseType =
$companionSymbol[..$typeArgs](...$argLists)
"""
wellTyped
}
}
P.S. I have been experimenting with toolbox.untypecheck / typecheck per this article but haven't found a working combination.
you need using
clas.typeArgs.map(_.toString).map(name => {
TypeDef(Modifiers(Flag.PARAM),TypeName(name), List(),TypeBoundsTree(EmptyTree, EmptyTree))
}
replace
info.typeParams.map(p => TypeName(p.name.toString))
it si my code
object GetSealedSubClass {
def ol3[T]: Any = macro GetSealedSubClassImpl.ol3[T]
}
class GetSealedSubClassImpl(val c: Context) {
import c.universe._
def showInfo(s: String) =
c.info(c.enclosingPosition, s.split("\n").mkString("\n |---macro info---\n |", "\n |", ""), true)
def ol3[T: c.WeakTypeTag]: c.universe.Tree = {
//get all sub class
val subClass = c.weakTypeOf[T]
.typeSymbol.asClass.knownDirectSubclasses
.map(e => e.asClass.toType)
//check type params must ia s sealed class
if (subClass.size < 1)
c.abort(c.enclosingPosition, s"${c.weakTypeOf[T]} is not a sealed class")
// get sub class constructor params
val subConstructorParams = subClass.map { e =>
//get constructor
e.members.filter(_.isConstructor)
//if the class has many Constructor then you need filter the main Constructor
.head.map(s => s.asMethod)
//get function param list
}.map(_.asMethod.paramLists.head)
.map(_.map(e => q"""${e.name.toTermName}:${e.info} """))
val outfunc = subClass zip subConstructorParams map {
case (clas, parm) =>
q"def smartConstructors[..${
clas.typeArgs.map(_.toString).map(name => {
TypeDef(Modifiers(Flag.PARAM), TypeName(name), List(), TypeBoundsTree(EmptyTree, EmptyTree))
})
}](..${parm})=${clas.typeSymbol.name.toTermName} (..${parm})"
}
val outClass =
q"""
object Term{
..${outfunc}
}
"""
showInfo(show(outClass))
q"""{
$outClass
Term
}
"""
}
}
using like this
sealed trait Hello[A]
case class Ohayo[A, B](a: (A, B)) extends Hello[A]
object GetSealed extends App {
val a = GetSealedSubClass.ol3[Hello[_]]
val b=a.asInstanceOf[ {def smartConstructors[A, B](a: (A, B)): Ohayo[A, B]}].smartConstructors(1, 2).a
println(b)
}

Scala macro and type erasure

I'm having some problems with a macro I've written to help me log metrics represented as case class instances to to InfluxDB. I presume I'm having a type erasure problem and that the tyep parameter T is getting lost, but I'm not entirely sure what's going on. (This is also my first exposure to Scala macros.)
import scala.language.experimental.macros
import play.api.libs.json.{JsNumber, JsString, JsObject, JsArray}
abstract class Metric[T] {
def series: String
def jsFields: JsArray = macro MetricsMacros.jsFields[T]
def jsValues: JsArray = macro MetricsMacros.jsValues[T]
}
object Metrics {
case class LoggedMetric(timestamp: Long, series: String, fields: JsArray, values: JsArray)
case object Kick
def log[T](metric: Metric[T]): Unit = {
println(LoggedMetric(
System.currentTimeMillis,
metric.series,
metric.jsFields,
metric.jsValues
))
}
}
And here's an example metric case class:
case class SessionCountMetric(a: Int, b: String) extends Metric[SessionCountMetric] {
val series = "sessioncount"
}
Here's what happens when I try to log it:
scala> val m = SessionCountMetric(1, "a")
m: com.confabulous.deva.SessionCountMetric = SessionCountMetric(1,a)
scala> Metrics.log(m)
LoggedMetric(1411450638296,sessioncount,[],[])
Even though the macro itself seems to work fine:
scala> m.jsFields
res1: play.api.libs.json.JsArray = ["a","b"]
scala> m.jsValues
res2: play.api.libs.json.JsArray = [1,"a"]
Here's the actual macro itself:
import scala.language.experimental.macros
import scala.reflect.macros.blackbox.Context
object MetricsMacros {
private def fieldNames[T: c.WeakTypeTag](c: Context)= {
val tpe = c.weakTypeOf[T]
tpe.decls.collect {
case field if field.isMethod && field.asMethod.isCaseAccessor => field.asTerm.name
}
}
def jsFields[T: c.WeakTypeTag](c: Context) = {
import c.universe._
val names = fieldNames[T](c)
Apply(
q"play.api.libs.json.Json.arr",
names.map(name => Literal(Constant(name.toString))).toList
)
}
def jsValues[T: c.WeakTypeTag](c: Context) = {
import c.universe._
val names = fieldNames[T](c)
Apply(
q"play.api.libs.json.Json.arr",
names.map(name => q"${c.prefix.tree}.$name").toList
)
}
}
Update
I tried Eugene's second suggestion like this:
abstract class Metric[T] {
def series: String
}
trait MetricSerializer[T] {
def fields: Seq[String]
def values(metric: T): Seq[Any]
}
object MetricSerializer {
implicit def materializeSerializer[T]: MetricSerializer[T] = macro MetricsMacros.materializeSerializer[T]
}
object Metrics {
def log[T: MetricSerializer](metric: T): Unit = {
val serializer = implicitly[MetricSerializer[T]]
println(serializer.fields)
println(serializer.values(metric))
}
}
with the macro now looking like this:
object MetricsMacros {
def materializeSerializer[T: c.WeakTypeTag](c: Context) = {
import c.universe._
val tpe = c.weakTypeOf[T]
val names = tpe.decls.collect {
case field if field.isMethod && field.asMethod.isCaseAccessor => field.asTerm.name
}
val fields = Apply(
q"Seq",
names.map(name => Literal(Constant(name.toString))).toList
)
val values = Apply(
q"Seq",
names.map(name => q"metric.$name").toList
)
q"""
new MetricSerializer[$tpe] {
def fields = $fields
def values(metric: Metric[$tpe]) = $values
}
"""
}
}
However, when I call Metrics.log -- specifically when it calls implicitly[MetricSerializer[T]] I get the following error:
error: value a is not a member of com.confabulous.deva.Metric[com.confabulous.deva.SessionCountMetric]
Why is it trying to use Metric[com.confabulous.deva.SessionCountMetric] instead of SessionCountMetric?
Conclusion
Fixed it.
def values(metric: Metric[$tpe]) = $values
should have been
def values(metric: $tpe) = $values
You're in a situation that's very close to one described in a recent question: scala macros: defer type inference.
As things stand right now, you'll have to turn log into a macro. An alternative would also to turn Metric.jsFields and Metric.jsValues into JsFieldable and JsValuable type classes materialized by implicit macros at callsites of log (http://docs.scala-lang.org/overviews/macros/implicits.html).