Scala macro for printing code?

Question

I want to do something like this:

def assuming[A](condition: => Boolean)(f: => A): A = {
  require(condition, /* print source-code of condition */)
  f
}

Sample Usage:

def fib(n: Int) = n match { // yes, yes, I know this is not efficient
  case 0 => 0 
  case 1 => 1
  case i => assuming(i > 0) { fib(i-1) + fib(i-2) }
}

Now, for example, if you call fib(-20), I want it to throw an exception with a message like Assertion failed: -20 > 0orAssertation failed: i > 0

Answer 1

A non-approving macro dude is one of the main use cases that you implement to learn how to use macros?

Well, that’s what I thought too.

By "glean snippets" in my other answer, I meant what specs2 does in its macro s2 .

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object X {
  import reflect.macros.blackbox.Context
  def impl[A: c.WeakTypeTag](c: Context)(p: c.Expr[Boolean])(body: c.Expr[A]) = {
    import c.universe._
    def treeLine(t: Tree): String = lineAt(t.pos)
    def lineAt(pos: Position): String = if (pos.isRange) pos.lineContent.drop(pos.column - 1).take(pos.end - pos.start + 1) else "???"
    val msg =
      if (p.tree.pos.isRange) {  // oh, joy
        treeLine(p.tree)
      } else {
        /*
        Console println s"content ${p.tree.pos.lineContent}"
        Console println s"column ${p.tree.pos.column}"  // alas, that the column of the point of the top of the tree, e.g., < in "a < b".
        val len = body.tree.pos.start - p.tree.pos.start
        p.tree.pos.lineContent drop (p.tree.pos.column - 1) take len
        */
        // OK, I get it: positions are a big mystery. Make woo-woo ghost noises.
        // What we do know is the start of the apply, which must have a close paren or brace in front of it to match:
        // apply(condition)(body)
        showCode(p.tree)
      }
    q"require($p, $msg) ; $body"
  }
  def x[A](p: Boolean)(body: =>A): A = macro X.impl[A]
}

, ​​ :

object X {
  import reflect.macros.blackbox.Context
  def impl(c: Context)(p: c.Expr[Boolean]) = {
    import c.universe._
    def lineAt(pos: Position): String = if (pos.isRange) pos.lineContent.drop(pos.column - 1).take(pos.end - pos.start + 1) else "???" 
    val msg = lineAt(c.macroApplication.pos)  // oh, joy
    q"require($p, $msg) ; new { def apply[A](body: =>A): A = body }"
  }
  def x(p: Boolean): { def apply[A](body: =>A): A } = macro X.impl
}

x(10 < 5)(println("hi")): requirement failed: (10 < 5)(p. .

Answer 2

:

http://www.scala-lang.org/api/2.11.0/scala-reflect/#scala.reflect.api.Printers

scala> show(q"-1 < 0")
res6: String = -1.$less(0)

scala> showCode(q"-1 < 0")
res7: String = (-1).<(0)

, . ​​

Answer 3

Scala 2.11.x, - showCode. Scala. :

scala> import reflect.runtime.universe._
import reflect.runtime.universe._

showCode(q"3.14 < 42")
res1: String = 3.14.<(42)

Scala show, :

scala> show(q"3.14 < 42")
res2: String = 3.14.$less(42)

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