Scala: macro to create an instance from a class body

Question

I am building a DSL in Scala, and for that, I need to store \"instances\" of a class (Parent in this case), except these \"instances\" must be re-created several ti

Answer 1

Unfortunately macros will not help.

Macro annotations (which expand before type checking) can't annotate code blocks:

@constructMacro {
  func(1)
  func(userVar)
}

is illegal.

Def macros (which expand during type checking) have their arguments type checked before macros are expanded. So

constructMacro {
  func(1)
  func(userVar)
}

doesn't compile:

Error: not found: value func
      func(1)
Error: not found: value func
      func(userVar)

That's the reason why macro shapeless.test.illTyped accepts a string rather than code block:

illTyped("""
  val x: Int = "a"
""")

rather than

illTyped {
  val x: Int = "a"
}

So the closest you can implement is

constructMacro("""
  func(1)
  func(userVar)
""")

def constructMacro(block: String): ParentWrapper = macro constructMacroImpl

def constructMacroImpl(c: blackbox.Context)(block: c.Tree): c.Tree = {
  import c.universe._
  val q"${blockStr: String}" = block
  val block1 = c.parse(blockStr)
  q"""construct(new Parent {
    ..$block1
  })"""
}

---

You can annotate a variable

@constructMacro val x = {
  func(1)
  func(userVar)
}

//         becomes
// val x: ParentWrapper = construct(new Parent {
//   func(1)
//   func(userVar)
// })

@compileTimeOnly("enable macro paradise to expand macro annotations")
class constructMacro extends StaticAnnotation {
  def macroTransform(annottees: Any*): Any = macro constructMacroImpl.impl
}

object constructMacroImpl {
  def impl(c: whitebox.Context)(annottees: c.Tree*): c.Tree = {
    import c.universe._
    annottees match {
      case q"$mods val $tname: $tpt = { ..$stats }" :: Nil =>
        q"$mods val $tname: ParentWrapper = construct(new Parent { ..$stats })"

      case _ =>
        c.abort(c.enclosingPosition, "Not a val")
    }
  }
}

Answer 2

If I understood correctly, you just need to change object to def in the ObjectA block:

class User {
  var userVar = 13
  def makeParent = new Parent {
    func(1)
    func(userVar)
  }

  construct(makeParent)
}

and it'll do what you want.