Error

An error type which is guaranteed to be useful.

In the Scala ecosystem, for better or worse, most errors end up getting converted to a subtype of java.lang.Throwable, usually java.lang.RuntimeException. This is convenient, because it interoperates well with the JVM ecosystem, but it has several notable drawbacks.

The primary draw back is that this valid instance of java.lang.RuntimeException,

scala> val e = new RuntimeException
val e = new RuntimeException
val e: RuntimeException = java.lang.RuntimeException

scala> e.getMessage
e.getMessage
val res0: String = null

scala> e.getCause
e.getCause
val res1: Throwable = null

One would be hard pressed to find a less helpful error. As amazing as it may seem, it is not uncommon to have exceptions like this in real world production code.

Error is attempting to be the nicer version of java.lang.RuntimeException, which is compatible with all existing error code and always gives you meaningful context about the error.

There are three common idioms in Scala code for handling errors.

• Create a custom Error ADT and then convert it to java.lang.Throwable when needs be (which is not always the case).
• Extend java.lang.RuntimeException (or sometimes java.lang.Exception ) and raise/throw the error.
• Create a new RuntimeException(<message>) and raise/throw that value directly.

Error aims to support all these cases.

It is an open trait, so custom Error ADTs can extend it. This gives error handling code the ability to be generic when needed, but still guarantees a reasonable error message.

Using the provided trivial implementation of Error, you can convert your ADT type into a Error when needed or you can just create instances of Error directly and raise them, similar to `new RuntimeException().

Error also supports annotating Error#causes, similar to java.lang.Throwable#getCause, but more expressive. Rather than having a single cause (which may be null) Error allows for an arbitrary number of causes.

Finally, since Error extends java.lang.RuntimeException, it can dropped into place with any code currently using java.lang.Throwable as the default bottom error type.