How to check if some T is a case class at compile time in Scala? -


i have following macro:

package macros  import scala.reflect.macros.blackbox.context  object compiletimeassertions {   def mustbecaseclass[t]: unit =     macro compiletimeassertionsimpl.mustbecaseclass[t] }  object compiletimeassertionsimpl {   def mustbecaseclass[t: c.weaktypetag](c: context): c.expr[unit] = {     import c.universe._     val symbol = c.weaktypetag[t].tpe.typesymbol     if (!symbol.isclass || !symbol.asclass.iscaseclass) {       c.error(c.enclosingposition, s"${symbol.fullname} must case class")     }     reify(unit)   } }

it works when generics aren't involved, fails when are:

import macros.compiletimeassertions._ import org.scalatest.{matchers, wordspec}  case class acaseclass(foo: string, bar: string)  class notacaseclass(baz: string)  class macrospec extends wordspec matchers {   "the mustbecaseclass macro" should {     "compile when passed case class" in {       mustbecaseclass[acaseclass]     }      "not compile when passed vanilla class" in { //      mustbecaseclass[notacaseclass] // fails compile expected.     }      "compile when working generics" in { //      class caseclasscontainer[t] { mustbecaseclass[t] } // fails compile. //      new caseclasscontainer[acaseclass]     }   } }

the compiler error mine:

macrospec.caseclasscontainer.t must case class

i'd find out t when caseclasscontainer instantiated. possible? if can provide example?

thanks in advance.

thanks eugene , travis' advice able solve problem type classes. here's solution:

package macros  import scala.reflect.macros.blackbox.context  trait iscaseclass[t]  object iscaseclass {   implicit def iscaseclass[t]: iscaseclass[t] =     macro iscaseclassimpl.iscaseclass[t] }  object iscaseclassimpl {   def iscaseclass[t]       (c: context)       (implicit t: c.weaktypetag[t]): c.expr[iscaseclass[t]] = {     import c.universe._     val symbol = c.weaktypetag[t].tpe.typesymbol     if (!symbol.isclass || !symbol.asclass.iscaseclass) {       c.abort(c.enclosingposition, s"${symbol.fullname} must case class")     } else {       c.expr[iscaseclass[t]](q"_root_.macros.iscaseclassimpl[$t]()")     }   } }  case class iscaseclassimpl[t]() extends iscaseclass[t]

and here usage:

import macros.iscaseclass import org.scalatest.{matchers, wordspec}  case class acaseclass(foo: string, bar: string)  class notacaseclass(baz: string)  class caseclasscontainer[t: iscaseclass]  class macrospec extends wordspec matchers {   "the code" should {     "compile" in {       new caseclasscontainer[acaseclass]     }      "not compile" in { //      new caseclasscontainer[notacaseclass]     }   } }

worth noting use of abort instead of error. abort returns nothing whereas error returns unit. latter fine when macro wasn't returning anything.


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