I intend to create the simple polymorphic function expression below using the Quotes.reflect
API:
new PolyFunction {
def apply[X](a: X): X = a
}
What I have attempted is shown below with parts that I could not implement replaced by ???
:
val name: String = "$anon"
val parents = List(TypeTree.of[Object], TypeTree.of[PolyFunction])
def decls(cls: Symbol): List[Symbol] =
List(
Symbol.newMethod(
cls,
"apply",
MethodType(List("a"))(
{ mt =>
???
},
mt => ???
)
)
)
val cls = Symbol.newClass(Symbol.spliceOwner, name, parents = parents.map(_.tpe), decls, selfType = None)
val applySym = cls.declaredMethod("apply").head
val applyDef = DefDef(applySym, argss => Some(???))
val clsDef = ClassDef(cls, parents, body = List(applyDef))
val closure = Block(List(clsDef),Apply(Select(New(TypeIdent(cls)), cls.primaryConstructor), Nil))
closure.asExpr
The problem, mainly, is I am unable to declare the type parameter X
, and its reference in the first value parameter and the functions return type. I have noticed some reflection functions are annotated as experimental
.
CodePudding user response:
Try
import scala.annotation.experimental
import scala.quoted.*
inline def newPoly: PolyFunction = ${newPolyImpl}
@experimental
def newPolyImpl(using Quotes): Expr[PolyFunction] = {
import quotes.reflect.*
val name: String = "$anon"
val parents = List(TypeTree.of[Object], TypeTree.of[PolyFunction])
def decls(cls: Symbol): List[Symbol] =
List(
Symbol.newMethod(
cls,
"apply",
PolyType(List("X"))(_ => List(TypeBounds.empty), polyType => {
val typeParam = polyType.param(0)
MethodType(List("a"))(_ => List(typeParam), _ => typeParam)
})
)
)
val cls = Symbol.newClass(Symbol.spliceOwner, name, parents = parents.map(_.tpe), decls, selfType = None)
val applySym = cls.declaredMethod("apply").head
// argss=List(List(TypeTree[TypeRef(NoPrefix,type X)]), List(Ident(a)))
val applyDef = DefDef(applySym, argss => Some(argss(1)(0).asInstanceOf[Term]))
val clsDef = ClassDef(cls, parents, body = List(applyDef))
val closure = Block(List(clsDef), Apply(Select(New(TypeIdent(cls)), cls.primaryConstructor), Nil))
closure.asExprOf[PolyFunction]
}
Usage:
newPoly
//scalac: {
// class $anon extends java.lang.Object with scala.PolyFunction {
// def apply[X](a: X): X = a
// }
// new $anon()
//}
Scala 3.2.1
Method Override with Scala 3 Macros