Extending Scala Case Class With NoStackTrace Leads To Unexpected toString Behaviour

Say you had an ADT similar to this:

import scala.util.control.NoStackTrace

sealed trait MyError extends NoStackTrace
final case class MyError1(message: String) extends  MyError
final case class MyError2(message: String) extends  MyError

This might seem weird to some. Why are we extending NoStackTrace ? This allows us to use the MyError as a return value through something like an Either:

def sanitiseInput(value: String): Either[MyError, ValidInput]

We can also use it as an error that can be thrown or raised into cats.IO, fs2.Stream, Monix.eval.Task or equivalent:

object IO {
  def raiseError[A](t: Throwable): IO[A] //`t` has to extend Throwable if we want to use this function.

  //other functions
}

You can also read about some related ideas in Make error ADTs subtypes of Exception.

Now if we use MyError in a test:

import scala.util.control.NoStackTrace

object MyErrorSuite extends weaver.FunSuite {

sealed trait MyError extends NoStackTrace
final case class MyError1(message: String) extends  MyError
final case class MyError2(message: String) extends  MyError

  test("error message") {
    expect.same(MyError1("error1"), MyError2("error2")) //this is an error
  }

}

the test output from Weaver-Test gets truncated somewhat:

[info] com.example.validation.extra.MyErrorSuite
[info] - error message 30ms
[info] *************FAILURES**************
[info] com.example.validation.extra.MyErrorSuite
[error] - error message 30ms
[error]   Values not equal: (src/test/scala/com/example/validation/extra/MyErrorSuite.scala:12)
[error]
[error]   com.example.validation.extra.MyErrorSuite$[MyError1]  |  com.example.validation.extra.MyErrorSuite$[MyError2]

All we get are the class names returned in the diff:

com.example.validation.extra.MyErrorSuite$[MyError1]  |  com.example.validation.extra.MyErrorSuite$[MyError2]

The diff we expected was something like:

 [MyError1]([error1]) | [MyError2]([error2]) //we can see that the class and error messages are different

When you create a case class it generates a toString implementation of the form: ClassName(field1Value, field2Value, ....).

So why are we loosing our toString implementation?

Cause

Let’s try a simpler example in the REPL:

case class MyError1(message: String)

scala> MyError1("Oh noes")
val res35: MyError1 = MyError1(Oh noes) //"Oh noes" is output

We can see that we do get the contents of all fields of the case class written out.

Let’s try extending NoStackTrace and see if it makes a difference:

import scala.util.control.NoStackTrace

case class MyError2(message: String) extends NoStackTrace

scala> MyError2("Oh noes")
val res36: MyError2 = MyError2 //no message output

We can see that although the class name is output the contents of the message field has not.

Interesting. This seems to be the cause of our issue in the test.

It turns out a case class doesn’t generate a toString method (and other implementations such has hashCode etc) if you already have a custom implementation for that method in a super type.

It’s not a bug, it’s a feature.

So where does our MyError2 class get a custom toString implementation from?

Lets have a look at the NoStackTrace class, since MyError2 extends that:

trait NoStackTrace extends Throwable {
  override def fillInStackTrace(): Throwable =
    if (NoStackTrace.noSuppression) super.fillInStackTrace()
    else this

  ...
}

No toString implementation here. Let’s follow the inheritance trail to java.lang.Throwable. Here, we see that it does have a custom toString implementation:

public String toString() {
    String s = getClass().getName();
    String message = getLocalizedMessage();
    return (message != null) ? (s + ": " + message) : s;
}

From the above implementation we can deduce that for MyError2 the getLocalizedMessage method returns null because we only get back the class name s as output: (MyError2) as opposed to: MyError2: message.

Let’s follow along to getLocalizedMessage to see how message is calculated:

public String getLocalizedMessage() {
    return getMessage();
}

and also to getMessage:

public String getMessage() {
    return detailMessage;
}

The detailMessage field is set through the many of the constructor methods for Throwable:

public Throwable(String message) {
    fillInStackTrace();
    detailMessage = message; //set
}

//or

public Throwable(String message, Throwable cause) {
    fillInStackTrace();
    detailMessage = message; //set
    this.cause = cause;
}

//or

public Throwable(Throwable cause) {
    fillInStackTrace();
    detailMessage = (cause==null ? null : cause.toString());  //set
    this.cause = cause;
}

//or

protected Throwable(String message, Throwable cause,
                    boolean enableSuppression,
                    boolean writableStackTrace) {
    if (writableStackTrace) {
        fillInStackTrace();
    } else {
        stackTrace = null;
    }
    detailMessage = message;  //set
    this.cause = cause;
    if (!enableSuppression)
        suppressedExceptions = null;
}

Since we have a field named message and not detailMessage, we don’t really override the value used by Throwable to generate its toString implementation.

Workarounds

If we renamed our message field in MyError2 to detailMessage we should be able to get our toString implementation working:

import scala.util.control.NoStackTrace

case class MyError2(detailMessage: String) extends NoStackTrace

scala> MyError2("Oh noes")
val res37: MyError2 = MyError2 //Doesn't work

Wow! That didn’t work either. Why though?

If we look at the definition of the detailMessage field on java.lang.Throwable we see that it’s private:

private String detailMessage;

This means we can’t override it from a sub class. Boo!

From our previous investigation we can see that all we need to do is override either getLocalizedMessage or getMessage or toString which are all public:

public String toString() {
    String s = getClass().getName();
    String message = getLocalizedMessage(); //message calculated from here
    return (message != null) ? (s + ": " + message) : s;
}


public String getLocalizedMessage() {
    return getMessage(); //message content retrieved from here
}


public String getMessage() {
    return detailMessage; //message content
}

Override getMessage or getLocalizedMessage

By overriding getMessage or getLocalizedMessage in our case class, we can get some form of toString-ery happening. While this is not ideal, it “works”.

import scala.util.control.NoStackTrace

case class MyError2(override val getMessage: String) extends NoStackTrace

scala> MyError2("Oh noes")
val res38: MyError2 = MyError2: Oh noes //We did it!

Override toString

If you want a more case classy toString implementation, you’re going to have to do it yourself:

case class MyError2(message: String) extends NoStackTrace {
  override def toString: String = s"MyError2($message)"
}

scala> MyError2("Oh noes")
val res39: MyError2 = MyError2(Oh noes) //we have case classiness

Now we can get our test to fail with a better error message:

import scala.util.control.NoStackTrace

object MyErrorSuiteTake2 extends weaver.FunSuite {

sealed trait MyError extends NoStackTrace {
  val message: String

  override def toString: String = {
    val className = getClass.getName
    s"$className($message)"
   }
}

final case class MyError1(message: String) extends  MyError
final case class MyError2(message: String) extends  MyError

  test("error message") {
    expect.same(MyError1("error1"), MyError2("error2"))
  }
}

Which results in:

info] com.example.validation.extra.MyErrorSuiteTake2
[error] - error message 38ms
[error]   Values not equal: (src/test/scala/com/example/validation/extra/MyErrorSuiteTake2.scala:20)
[error]
[error]   com.example.validation.extra.MyErrorSuiteTake2$[MyError1]([error1])  |  com.example.validation.extra.MyErrorSuiteTake2$[MyError2]([error2])

All this seems a bit tedious… as does extending the Exception hierarchy. If you do decide to go this route, hopefully this will help you stave off at least one of the issues with extending java.lang.Throwable and friends.