Quasiquotes for multiple parameter lists

Quasiquotation is an old idea (Miles Sabin notes the term in a 1937 Quine paper, for example) that’s now available in Scala (thanks to the efforts of Eugene Burmako and Den Shabalin), where it allows us to avoid the nightmarishly complicated and verbose code that’s required to construct abstract syntax trees manually in our macros.

Quasiquotes are a little like reification, but much more flexible about what kinds of things can be “spliced” into the tree, and where they can be spliced. For example, we couldn’t use reify in the following code, because there’s no way to splice in the name of the type member:

def foo(name: String): Any = macro foo_impl
def foo_impl(c: Context)(name: c.Expr[String]) = {
  import c.universe._

  val memberName = name.tree match {
    case Literal(Constant(lit: String)) => newTypeName(lit)
    case _ => c.abort(c.enclosingPosition, "I need a literal!")
  }

  val anon = newTypeName(c.fresh)

  c.Expr(Block(
    ClassDef(
      Modifiers(Flag.FINAL), anon, Nil, Template(
        Nil, emptyValDef, List(
          constructor(c.universe),
          TypeDef(Modifiers(), memberName, Nil, TypeTree(typeOf[Int]))
        )
      )
    ),
    Apply(Select(New(Ident(anon)), nme.CONSTRUCTOR), Nil)
  ))
}

This is an unreadable mess, and it’s not even a complete example—it depends on some additional utility code.

Compare the version with quasiquotes:

def foo(name: String): Any = macro foo_impl
def foo_impl(c: Context)(name: c.Expr[String]) = {
  import c.universe._

  val memberName = name.tree match {
    case Literal(Constant(lit: String)) => newTypeName(lit)
    case _ => c.abort(c.enclosingPosition, "I need a literal!")
  }

  c.Expr(q"new { type $memberName = Int }")
}

I know which version I’d prefer to maintain.

In general quasiquotation is this exactly this easy—you just use the q string interpolator to plop your compile-time tree pieces into the code. In some cases, though, the syntax required for quasiquoting isn’t quite the same as ordinary Scala syntax. Building a method with multiple parameter lists is one example. Suppose we’ve got the following representation of our parameter lists:

val paramss = List(
  List(newTermName("x") -> typeOf[Int], newTermName("y") -> typeOf[Char]),
  List(newTermName("z") -> typeOf[String])
)

And that we want to turn these into the following method:

def foo(x: Int, y: Char)(z: String) = ???

We could write the following for a single list with a single parameter:

q"def bar(${paramss.head.head._1}: ${paramss.head.head._2}) = ???"

But we can’t for example write this (or any obvious variation of this) to get both lists with all three parameters:

val quoted = paramss.map(_.map { case (name, tpe) => q"$name: $tpe" })

q"def foo..${quoted.map(ps => q"($ps)")} = ???"

This was confusing the hell out of me until yesterday Den pointed out the correct syntax:

val vparamss: List[List[ValDef]] = paramss.map(
  _.map { case (name, tpe) => q"val $name: $tpe" }
)

println(q"def foo(...$vparamss) = ???")

There are two things to note here. First, we need to use q"val $name: $tpe" instead of q"$name: $tpe" (even though the former isn’t valid Scala syntax for method parameters) in order to make it clear that we want a ValDef, not a regular old typed expression.

Next, we can’t wrap each parameter list in parentheses and then quote the lot with ..$. Instead we quote the list of lists with ...$ inside a single pair of parentheses.

Once you’ve seen this syntax, it makes perfect sense, but it’s maybe not exactly immediately obvious.