How to convert scala code block to string?

Question

I need to implement a testing function which checks compile-time error information for the "splain" plugin, part of this function needs to convert a codeblock into a string, e.g.:

def convert(fn: => Unit): String

// for testing

val code = convert {
  object I extends Seq {}
}

assert(code == "object I extends Seq {}")

Is this possible using standard scala features? Thanks a lot for your advice.

This function will enable verifications of compile-time messages of complex code that needs to be indexed and refactored by IDE often

Bowsprit

Which Scala version? Maybe you can achieve something with macros. Though I'm not sure you've given enough details about the context of why you need to do this ? – Lamblike

Answer 1

Yes, it's possible.

Li Haoyi's macro Text from sourcecode

def text[T: c.WeakTypeTag](c: Compat.Context)(v: c.Expr[T]): c.Expr[sourcecode.Text[T]] = {
  import c.universe._
  val fileContent = new String(v.tree.pos.source.content)
  val start = v.tree.collect {
    case treeVal => treeVal.pos match {
      case NoPosition ⇒ Int.MaxValue
      case p ⇒ p.startOrPoint
    }
  }.min
  val g = c.asInstanceOf[reflect.macros.runtime.Context].global
  val parser = g.newUnitParser(fileContent.drop(start))
  parser.expr()
  val end = parser.in.lastOffset
  val txt = fileContent.slice(start, start + end)
  val tree = q"""${c.prefix}(${v.tree}, $txt)"""
  c.Expr[sourcecode.Text[T]](tree)
}

does almost what you want:

def convert[A](fn: => Text[A]): String = fn.source

convert(10 + 20 +
  30
)

//10 + 20 +
//  30

Unfortunately,

if you have multiple statements in a {} block, sourcecode.Text will only capture the source code for the last expression that gets returned.

And since { object I extends Seq {} } is actually { object I extends Seq {}; () } the macro will not work in this case.

So let's write our own simple macro

import scala.language.experimental.macros
import scala.reflect.macros.blackbox

def convert(fn: => Any): String = macro convertImpl

def convertImpl(c: blackbox.Context)(fn: c.Tree): c.Tree = {
  import c.universe._

  val pos = fn.pos
  val res = new String(pos.source.content).slice(pos.start, pos.end)

  Literal(Constant(res))
}

Usage:

trait Seq

convert {
  val i: Int = 1
  object I extends Seq {}
  10 + 20 + 30
  convert(1)
}

//{
//    val i: Int = 1
//    object I extends Seq {}
//    10 + 20 + 30
//    convert(1)
//  }

Notice that arguments of def macros are typechecked before macro expansion (so convert { val i: Int = "a" }, convert { object I extends XXX } without defined XXX, convert { (; } etc. will not compile).

Decision

Thanks a lot! If I got lucky it will be in the test code of the next version of scalac – Bowsprit

It works, but with a caveat: The code must be parsed & typechecked. Or an error will be thrown before lihaoyi's macro can be invoked – Bowsprit

@Bowsprit In Scala all def macros are expanded after their arguments are typechecked. – Decision

@DmytroMitin, do you know a way to replace variables defined outside the convert method by their values? Like: val i = 5; convert(10 + i) // 10 + 5 – Abase

@MaxenceCramet Yes stackoverflow.com/questions/63132189 stackoverflow.com/questions/13767582 stackoverflow.com/questions/60806283 stackoverflow.com/questions/59387459 – Decision