dynamic string interpolation

Question

I would like to pretty-print a Product, such as a case class, so I create the following trait:

  trait X extends Product {
    def fmtStrs =
      productIterator map {
        case _ : Double => "%8.2f"
        case _ => "%4s"
      } map (_ + separator) toSeq
    override def toString = {
      new StringContext("" +: fmtStrs : _*) f (productIterator.toSeq : _*)
    }
  }

This uses string interpolation as described in the ScalaDoc for StringContext.

But this won't compile, with this cryptic error:

Error:(69, 70) too many arguments for interpolated string
      new StringContext("" +: fmtStrs : _*) f (productIterator.toSeq : _*)

Is this a bug, or limitation of a macro? Note that doing the following works fine, so I suspect this may be related to the variable argument list:

scala> val str2 = StringContext("","%4s,","%8.2f").f(1,23.4)
str2: String = "   1,   23.40"

Rather bizarrely, if you change the f interpolator to the s interpolator, this appears to complie (and work) ... — Luciano
– Luciano, Commented Nov 3, 2016 at 15:19

Alexey Romanov · Accepted Answer · 2016-11-03 18:50:36Z

2

The reason f is a macro is so that it can give you an error when types of format specifiers and arguments don't match, and this isn't possible to check by looking at ("" +: fmtStrs : _*) and (productIterator.toSeq : _*), so it isn't particularly surprising this doesn't work. The error message could be clearer, so let's see what exactly happens.

If you look at the implementation of f (it took me some time to actually find it, I finally did by searching for the error message), you'll see

c.macroApplication match {
  //case q"$_(..$parts).f(..$args)" => 
  case Applied(Select(Apply(_, parts), _), _, argss) => 
    val args = argss.flatten
    def badlyInvoked = (parts.length != args.length + 1) && truly {
      def because(s: String) = s"too $s arguments for interpolated string"
      val (p, msg) =
        if (parts.length == 0) (c.prefix.tree.pos, "there are no parts")
        else if (args.length + 1 < parts.length)
          (if (args.isEmpty) c.enclosingPosition else args.last.pos, because("few"))
        else (args(parts.length-1).pos, because("many"))
      c.abort(p, msg)
    }
    if (badlyInvoked) c.macroApplication else interpolated(parts, args)

With your call you have a single tree in both parts and argss, and parts.length != args.length + 1 is true, so badlyInvoked is true.

s doesn't care what its arguments look like, so it's just a method and your scenario works.

edited Nov 3, 2016 at 18:50

answered Nov 3, 2016 at 18:36

Alexey Romanov

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1 Comment

Luciano Over a year ago

Thanks; for the record, just ended up calling java.lang.String.Formatter directly instead of using string interpolation.

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