struggling to print out attribute names of case classes with com-lihaoyi.github.io/PPrint/

Question

I really like the promise of this library for pretty printing case class instances! But I want my output to include attribute names of my case classes, and so far i am coming up short.

Here is my most recent attempt:

import fansi.Str
 object Testo extends App {

case class Person(name: String, age: Int, address: Address)
case class Address(street: String, city: String)

val person = Person("John Doe", 30, Address("123 Main St", "New York"))
val list = List(person, person)
val map = Map("foo" -> list)
val map2 = Map("foo" -> list, "bar" -> person)

val pprint2 =
  pprint.copy(
    additionalHandlers = {
      case p: Product => {
        val stringo: Seq[String] = p.productIterator.toSeq.map {
              case (name, value) => s"$name = ${pprint.apply(value)}"
          }

        val result = p.productIterator.toSeq.map(item => "x")
        pprint.Tree.Literal(stringo.mkString(""))
      }
    }
  )

pprint2.pprintln(map2)
val result: Iterator[Str] = pprint2.tokenize(map2, showFieldNames = true)

val xx = result.map(_.toString()).mkString("")
System.out.println("xx:" + xx)

}

I get a crash like this->

Connected to the target VM, address: '127.0.0.1:63150', transport: 'socket'
Exception in thread "main" scala.MatchError: Person(John Doe,30,Address(123 Main St,New York)) (of class Person)
        at Testo$$anonfun$1.$anonfun$applyOrElse$1(Test.scala:32)
        at scala.collection.immutable.Stream.map(Stream.scala:418)
        at Testo$$anonfun$1.applyOrElse(Test.scala:32)
        at scala.PartialFunction$Lifted.apply(PartialFunction.scala:228)
        at scala.PartialFunction$Lifted.apply(PartialFunction.scala:224)
        at pprint.Walker.treeify(Walker.scala:50)
        at pprint.Walker.$anonfun$treeify$3(Walker.scala:78)
        at scala.collection.Iterator$$anon$11.nextCur(Iterator.scala:486)
        at scala.collection.Iterator$$anon$11.hasNext(Iterator.scala:492)
        at pprint.Renderer.rec(Renderer.scala:29)
        at pprint.PPrinter.tokenize(PPrinter.scala:165)
        at pprint.PPrinter.pprintln(PPrinter.scala:141)
        at Testo$.delayedEndpoint$Testo$1(Test.scala:42)
        at Testo$delayedInit$body.apply(Test.scala:15)
        at scala.Function0.apply$mcV$sp(Function0.scala:39)
        at scala.Function0.apply$mcV$sp$(Function0.scala:39)
        at scala.runtime.AbstractFunction0.apply$mcV$sp(AbstractFunction0.scala:17)
        at scala.App.$anonfun$main$1$adapted(App.scala:80)
        at scala.collection.immutable.List.foreach(List.scala:431)
        at scala.App.main(App.scala:80)

Any ideas on how to do this, most welcome !

Answer 1

The problem is not related with the library. Your code is throwing MatchError

This class implements errors which are thrown whenever an object doesn't match any pattern of a pattern matching expression.

It is saying that none of the cases you have, match against to the object you are pattern matching.

In the line

val stringo: Seq[String] = p
  .productIterator // returns a Iterator[Any]
  .toSeq // converts Iterator[Any] to Seq[Any]
  .map {
    case (name, value) => // Any doesn't match with Tuple2(x, y)
      s"$name = ${pprint.apply(value)}"
  }

As I detailed in the comments, Product.productIterator returns a value of type Iterator[Any]. When you write a case like

case (x, y) => //...

It is expecting a tuple of 2 elements. I guess you want to get the name of the element and also the value. In that case, you can use the method Product.productElementNames combined with productIterator.

case p: Product => {
  val nameAndValue = p.productElementNames.zip(p.productIterator)
  val stringo: Seq[String] = nameAndValue.toSeq.map {
    case (name, value) => s"$name = ${pprint.apply(value)}"
  }
  pprint.Tree.Literal(stringo.mkString(", "))
}

I think doing that will produce the expected output