Macro to access source code text at runtime

问题:

Is there already or is it possible to have a Scala macro that gives me access to the text of the source? For instance I would like to write code like this:

val list = List(1, 2, 3)
val (text, sum) = (list.sum).withSource{(source, sum) => (source, sum)}
// would return ("list.sum", 6)
(list.sum).withSource{(source, sum) => println(s"$source: $sum"}
// prints list.sum: 6

回答1:

Do you really want a source code or Tree is enough?

For Tree you could use prefix of Context like this:

import scala.language.experimental.macros
import reflect.macros.Context

implicit class WithSourceHelper[T](source: T) {
  def withSource[R](f: (String, T) => R): R = macro withSourceImpl[T, R]
}

def withSourceImpl[T, R](c: Context)(f: c.Expr[(String, T) => R]): c.Expr[R] = {
  import c.universe.{reify, Apply}

  val source = c.prefix.tree match {
    case Apply(_, List(s)) => s
    case _ => c.abort(c.enclosingPosition, "can't find source")
  }

  reify{ f.splice.apply(c.literal(source.toString).splice, c.Expr[T](source).splice) }
}

Usage:

scala> val (x, y) = (1, 2)
x: Int = 1
y: Int = 2

scala> {x + y}.withSource{ (s, r) => s"$s = $r" }
res15: String = x.+(y) = 3

scala> val list = List(1, 2, 3)
list: List[Int] = List(1, 2, 3)

scala> val (text, sum) = (list.sum).withSource{(source, sum) => (source, sum)}
text: String = list.sum[Int](math.this.Numeric.IntIsIntegral)
sum: Int = 6

scala> (list.sum).withSource{(source, sum) => println(s"$source: $sum")}
$line38.$read.$iw.$iw.$iw.list.sum[Int](math.this.Numeric.IntIsIntegral): 6

回答2:

I was not able to reuse withSource directly to just print the source and value and return the value. The withSource macro cannot be utilized from the same object itself (so I cannot just add my slightly modified version of withSource right in that file) and I am not able to call withSource from a subclass of WithSourceHelper, limiting reuse through inheritance.

In case anyone is interested, here is a complement to Senia's answer to just log the value with the source and return the value so the rest of the computation can occur.

def logValueImpl[T](c: Context): c.Expr[T] = {
  import c.universe._
  val source = c.prefix.tree match {
    case Apply(_, List(s)) => s
    case _ => c.abort(c.enclosingPosition, "can't find source")
  }
  val freshName = newTermName(c.fresh("logValue$"))
  val valDef = ValDef(Modifiers(), freshName, TypeTree(source.tpe), source)
  val ident = Ident(freshName)
  val print = reify{
    println(c.literal(show(source)).splice + ": " + c.Expr[T](ident).splice) }
  c.Expr[T](Block(List(valDef, print.tree), ident))
}

I then define it as an implicit conversion on def p = macro Debug.logValueImpl[T]. I can then use like this:

List(1, 2, 3).reverse.p.head 
// prints: immutable.this.List.apply[Int](1, 2, 3).reverse: List(3, 2, 1)

The funny part is that I can apply it twice:

List(1, 2, 3).reverse.p.p

And it will show me what the logValueImpl macro did:

{
  val logValue$7: List[Int] = immutable.this.List.apply[Int](1, 2, 3).reverse;
  Predef.println("immutable.this.List.apply[Int](1, 2, 3).reverse: ".+(logValue$7));
  logValue$7
}

It seems to work with other macros as well:

f"float ${1.3f}%3.2f; str ${"foo".reverse}%s%n".p`
//prints:
{
  val arg$1: Float = 1.3;
  val arg$2: Any = scala.this.Predef.augmentString("foo").reverse;
  scala.this.Predef.augmentString("float %3.2f; str %s%%n").format(arg$1, arg$2)
}: float 1.30; str oof%n

Even more interestingly if I used showRaw instead of show I can even see the tree of the expanded macro, which may come handy to figure out how to write other macros.