How can I splice in a type and a default value in Scala quasiquotes?

Question

I'm trying to make a type-provider that gives updated case classes.

How might I splice in a type and a default value (or omit the default value)?

def impl(c: Context)(annottees: c.Expr[Any]*): c.Expr[Any] = {
    import c.universe._
    import Flag._

    val result = {
      annottees.map(_.tree).toList match {
        case q"$mods class $name[..$tparams](..$first)(...$rest) extends ..$parents { $self => ..$body }" :: Nil =>
          val valType    = //TODO
          val valDefault = //TODO
          val helloVal   = q"""val x: $valType = $valDefault"""
          q"$mods class $name[..$tparams](..$first, $helloVal)(...$rest) extends ..$parents { $self => ..$body }"
      }
    }
    c.Expr[Any](result)
  }

I've tried:

I've tried simply val valType = q"String", but then I get an error as if a default value was not found: not enough arguments for method apply

I've also tried splicing in a val defined as typeOf[String], and I've also tried splicing lists of ValDefs into my q"$mods class... (like I've seen done into a q"def... in some similar questions on this site), but in each case there is a typer error.

Any tips? Thanks very much for looking.

Answer 1

You can use the tq interpolator in the definition of valType to create the type tree.

The rest is a little trickier. It seems to work just fine if you define the extra parameter directly:

q"""
  $mods class $name[..$tparams](
    ..$first,
    val x: $valType = $valDefault
  )(...$rest) extends ..$parents { $self => ..$body }
"""

But when you define $helloVal and then plug that in you end up without the default parameter flag. You could write a helper like this:

def makeDefault(valDef: ValDef) = valDef match {
  case ValDef(mods, name, tpt, rhs) => ValDef(
    Modifiers(
      mods.flags | DEFAULTPARAM, mods.privateWithin, mods.annotations
    ),
    name, tpt, rhs
  )
}

Now you can write the following:

val valType    = tq"String"
val valDefault = q""""foo""""
val helloVal   = makeDefault(q"val x: $valType = $valDefault")

And everything should work as expected.