Scala Language Macros Errors in Macros
Example
Macros can trigger compiler warnings and errors through the use of their Context.
Say we're a particularly overzealous when it comes to bad code, and we want to mark every instance of technical debt with a compiler info message (let's not think about how bad this idea is). We can use a macro that does nothing except emit such a message.
import reflect.macros.blackbox.Context
def debtMark(message: String): Unit = macro debtMark_impl
def debtMarkImpl(c: Context)(message: c.Tree): c.Tree = {
message match {
case Literal(Constant(msg: String)) => c.info(c.enclosingPosition, msg, false)
// false above means "do not force this message to be shown unless -verbose"
case _ => c.abort(c.enclosingPosition, "Message must be a string literal.")
// Abort causes the compilation to completely fail. It's not even a compile error, where
// multiple can stack up; this just kills everything.
}
q"()" // At runtime this method does nothing, so we return ()
}
Additionally, instead of using ??? to mark unimplemented code, we can create two macros, !!! and ?!?, that serve the same purpose, but emit compiler warnings. ?!? will cause a warning to be issued, and !!! will cause an outright error.
import reflect.macros.blackbox.Context
def ?!? : Nothing = macro impl_?!?
def !!! : Nothing = macro impl_!!!
def impl_?!?(c: Context): c.Tree = {
import c.universe._
c.warning(c.enclosingPosition, "Unimplemented!")
q"${termNames.ROOTPKG}.scala.Predef.???"
// If someone were to shadow the scala package, scala.Predef.??? would not work, as it
// would end up referring to the scala that shadows and not the actual scala.
// ROOTPKG is the very root of the tree, and acts like it is imported anew in every
// expression. It is actually named _root_, but if someone were to shadow it, every
// reference to it would be an error. It allows us to safely access ??? and know that
// it is the one we want.
}
def impl_!!!(c: Context): c.Tree = {
import c.universe._
c.error(c.enclosingPosition, "Unimplemented!")
q"${termNames.ROOTPKG}.scala.Predef.???"
}
