How can I consume a Scala macro/quasiquote for code templates? - scala

Question

I want to generate a bunch of objects at compile time that follow a simple pattern, so I wrote the following macro:
object MyMacro {
def readWrite[T](taName: String, readParse: String => T, label: String, format: T => String): Any = macro readWriteImpl[T]
def readWriteImpl[T: c.WeakTypeTag](c: Context)(taName: c.Expr[String], readParse: c.Expr[String => T], label: c.Expr[String], format: c.Expr[T => String]): c.Expr[Any] = {
import c.universe._
def termName(s: c.Expr[String]): TermName = s.tree match {
case Literal(Constant(s: String)) => TermName(s)
case _ => c.abort(c.enclosingPosition, "Not a string literal")
}
c.Expr[Any](q"""
object ${termName(taName)} extends TypeAdapter.=:=[${implicitly[c.WeakTypeTag[T]].tpe}] {
def read[WIRE](path: Path, reader: Transceiver[WIRE], isMapKey: Boolean = false): ${implicitly[c.WeakTypeTag[T]].tpe} =
reader.readString(path) match {
case null => null.asInstanceOf[${implicitly[c.WeakTypeTag[T]].tpe}]
case s => Try( $readParse(s) ) match {
case Success(d) => d
case Failure(u) => throw new ReadMalformedError(path, "Failed to parse "+${termName(label)}+" from input '"+s+"'", List.empty[String], u)
}
}
def write[WIRE](t: ${implicitly[c.WeakTypeTag[T]].tpe}, writer: Transceiver[WIRE], out: Builder[Any, WIRE]): Unit =
t match {
case null => writer.writeNull(out)
case _ => writer.writeString($format(t), out)
}
}
""")
}
}
I'm not sure I have the return value for readWrite and readWriteImpl correct--the compiler complains mightily about some assertion failure!
I'm also not sure how to actually consume this macro. First I tried (in a separate compilation unit):
object TimeFactories {
MyMacro.readWrite[Duration](
"DurationTypeAdapterFactory",
(s: String) => Duration.parse(s),
"Duration",
(t: Duration) => t.toString)
}
Didn't work. If I tried to reference TimeFactories.DurationTypeAdapterFactory I got an error saying it wasn't found. Next I thought I'd try assigning it to a val...didn't work either:
object Foo {
val duration = MyMacro.readWrite[Duration](
"DurationTypeAdapterFactory",
(s: String) => Duration.parse(s),
"Duration",
(t: Duration) => t.toString).asInstanceOf[TypeAdapterFactory]
}
How can I wire this up so I get generated code compiled like this:
object TimeFactories{
object DurationTypeAdapterFactory extends TypeAdapter.=:=[Duration] {
def read[WIRE](path: Path, reader: Transceiver[WIRE], isMapKey: Boolean = false): Duration =
reader.readString(path) match {
case null => null.asInstanceOf[Duration]
case s => Try( Duration.parse(s) ) match {
case Success(d) => d
case Failure(u) => throw new ReadMalformedError(path, "Failed to parse Duration from input 'Duration'", List.empty[String], u)
}
}
def write[WIRE](t: Duration, writer: Transceiver[WIRE], out: Builder[Any, WIRE]): Unit =
t match {
case null => writer.writeNull(out)
case _ => writer.writeString(t.toString, out)
}
}
// ... More invocations of the readWrite macro with other types for T
}

Answer 1

I don't think, that you can generate new identifiers using macros and than use them publicly.
Instead, try to replace object ${termName(taName)} extends TypeAdapter simply with new TypeAdapter and assign invocation of the macro to a val (as in your second example). You will then reference an anonymous (and generated) class stored in a val. Parameter taName becomes redundant.