Overridden case class `toString`

[Bersier]

Compiler version

3.1.3

Minimized code

case class Foo(x: Double) extends (Double => Double):
  def apply(y: Double): Double = x * y


@main def main(): Unit =
  println(Foo(2.0))

Output

<function1>

Expectation

Foo(2.0)

[szymon-rd]

The toString is defined in the (Double => Double). If an implementation of toString is already provided, it's not generated for a case class and this is expected behavior:

trait Bar:
  override def toString = "Bar"

case class Foo(x: Double) extends Bar:
  def apply(y: Double): Double = x * y


@main def main(): Unit =
  println(Foo(2.0))

Outputs:

Bar

[Bersier]

I understand the behavior, but it's not ideal here. the toString behavior for functions is really a non-implementation (maybe the actual issue is that Function overrides toString).

It provides even less information than the Object toString method, which case classes do override (so, technically, there already is an exception to "If an implementation of toString is already provided, it's not generated for a case class").

It is also unexpected. It should at the very least be documented explicitly. The current documentation only says

A default toString method is generated, which is helpful for debugging.

This could be interpreted as saying: "There is always a useful toString method provided by case classes, which can still be overridden by the coder."

In any case, it is a language wart.

Edit: I'm also thinking that since it is the case class that extends Function, it would be more expected if case classes overrode whatever toString method is defined higher in the hierarchy. The coder would always be able to restore that behavior with an explicit implementation of toString that does some super call. But that's probably too big a semantic change at this point.