scala.reflect.macros: the importance of 'c'

[Andrei-Pozolotin]

following example in scala 2.12.4:
https://docs.scala-lang.org/overviews/macros/bundles.html

1. the following compiles fine:

import scala.language.experimental.macros
import scala.reflect.macros.whitebox.Context
object Macros {
  def mono : Unit = macro MacrosImpl.mono
  def poly[ T ] : String = macro MacrosImpl.poly[ T ]
}
class MacrosImpl( val c : Context ) {
  import c.universe._
  def mono : c.Tree = q"${println( 1 )}"
  def poly[ T : c.WeakTypeTag ] : c.Expr[ String ] = c.Expr[ String ]( q"${}" )
}

2. but after renaming val c : Context -> val ccc : Context compilation fails:

import scala.language.experimental.macros
import scala.reflect.macros.whitebox.Context
object Macros {
// error: bundle implementation has incompatible shape: required: : MacrosImpl.this.<none>.Expr[Unit] or   
  def mono : Unit = macro MacrosImpl.mono
// error: bundle implementations cannot have implicit parameters other than WeakTypeTag evidences
  def poly[ T ] : String = macro MacrosImpl.poly[ T ]
}
class MacrosImpl( val ccc : Context ) {
  import ccc.universe._
  def mono : ccc.Tree = q"${println( 1 )}"
  def poly[ T : ccc.WeakTypeTag ] : ccc.Expr[ String ] = ccc.Expr[ String ]( q"${}" )
}

so, c must stand for something really important? :-)

[milessabin]

See macro-compat and discussion here.

[hrhino]

If I scroll down far enough, is there a decision not to change this, or is the note that it's "probably fixable" still valid?

Yeah I think it's fixable. nme.c is only used in two places, and in both of them we can apply a bit smarter logic that wouldn't hardcode the name of the context parameter.

I got bit by this last week, when I tried using a macro bundle.

[Andrei-Pozolotin]

oh, no, not just me here going bonkers? :-)

So apparently noone has ever written a macro bundle that uses a different name for the context

I could not believe this too

[hrhino]

I think it's more like "no one has ever successfully written a macro bundle..." like that.

[som-snytt]

The problem is that xeno-by said he'd take a look after scala.world, but no one ever really comes back from scala.world.

I've converted a macro to bundle, by cut/paste. The c is for "cargocult."