An interface to define custom pretty print to any objects?

[ndac-todoroki]

Sometimes I want to print custom outputs from certain classes.

data class Foo(val key: Int, val value: List<Int>) {
   override fun toString(): String {    // pp() doesn't use this!
      return "#Foo($key => $value)"
   }
}

I wish I can override what pp() uses to format class instances, for example by implementing some interface.
(overriding fun pp() is an option too, but I think it is not the best, because the implementer needs not to forget newlines)

[derlin]

What do you think of defining an annotation, @ppAtomic, that would instruct pp to call toString on instances of this class ?

[snowe2010]

Hm. The main point of pretty-print is to avoid the toString method, because toString methods usually do not do a good job of rendering the actual structure. Once dipping down into the toString, there's then no way to jump back out to rendering things 'correctly' (like the List in this example), without either duplicating things, or messing up the formatting.