Writing Macro for Positive Integers - scala

Question

Given the following types:
sealed trait PosIntCheckResult
case class LteZero(x: Int) extends PosIntCheckResult
case object NotConstant extends PosIntCheckResult
I'm trying to write a macro that checks whether the given Int is greater than 0.
import reflect.macros.Context
def getInt(c: Context)(value: c.Expr[Int]): Either[PosIntCheckResult, Int] = {
import c.universe._
value.tree match {
case Literal(Constant(x)) => if (x > 0) Right(x) else Left(LteZero(x))
case _ => Left(NotConstant)
}
}
But Any shows up for the x value:
Test.scala:29: type mismatch;
found : Any
required: Int
case Literal(Constant(x)) =>
if (x > 0) Right(x) else Left(LteZero(x))
How can I get the compiler to expect an Int rather than Any?

Answer 1

You just have to match on patterns where x is an Int:
case Literal(Constant(x: Int)) => //When using x here, it is an Int
Read about pattern matching on type and the other types of pattern matching in the docs.
You should also note that your macro will need to return an Expr in order to work. You can use reify to construct the desired Exprs at each case. Read about reify and def macros here. I'm not sure why this needs to be a macro, but if you're just learning the techniques, something like this might work:
object Macros {
def getInt(value: Int): Either[PosIntCheckResult, Int] = macro getIntImpl
def getIntImpl(c: Context)(value: c.Expr[Int]): c.Expr[Either[PosIntCheckResult,Int]] = {
import c.universe._
value.tree match {
case x#Literal(Constant(const: Int)) if const > 0 => reify(Right(c.Expr[Int](x).splice))
case x#Literal(Constant(_: Int)) => reify(Left(LteZero(c.Expr[Int](x).splice)))
case _ => reify(Left(NotConstant))
}
}
}