Testing exceptions in Scala - scala

Question

I want to test (in Intellij Idea testing framework) the following code:
def eval(t: Term): Term = t match {
case t if isVal(t) => t
case If(t1,t2,t3) if True == eval(t1) => eval(t2)
case If(t1,t2,t3) if False == eval(t1) => eval(t3)
case Succ(t) if isNum(eval(t)) => Succ(eval(t))
case Pred(t) => eval(t) match {
case Zero => Zero
case Succ(v) if isNum(v) => v
case _ => throw TermIsStuck(Pred(eval(t)))
}
case IsZero(t) => eval(t) match {
case Zero => True
case Succ(v) if isNum(v) =>False
case _ => throw TermIsStuck(IsZero(eval(t)))
}
case _ => throw TermIsStuck(t)
}
For a reference you can see this repository. So I wrote the following test:
test("testEval") {
Arithmetic.term(new Arithmetic.lexical.Scanner("if iszero pred pred 2 then if iszero 0 then true else false else false")) match {
case Arithmetic.Success(res,next) => assert(eval(res) == True)
case Arithmetic.Failure(msg,next) => assert(false)
}
Arithmetic.term(new Arithmetic.lexical.Scanner("pred succ succ succ false")) match {
case Arithmetic.Success(res,next) => assert(Arithmetic.eval(res) == TermIsStuck(Succ(False)))
case Arithmetic.Failure(msg,next) => assert(false)
}
}
Getting the error:
Succ(False)
fos.Arithmetic$TermIsStuck: Succ(False) ...
How can I test this catching the error? Indeed this test should be passed...

Answer 1

You are in Scala you are not supposed to throw exception around, but in scalatest you can do something like this:
a[yourException.type] should be thrownBy yourFunction()
You can do a better version of that,
case class Error(exception: Exception){
throw exception
???
}
Or something like
case object Error
And then you check it like:
Error shouldBe yourFunction()

Answer 2

Looks like you're not using any widespread testing framework for scala (such as scalatest or specs2) - an obvious recommendation would be to use some.
However, if you'd like to stick with "no-framework" approach, one option would be to use good old try .. catch:
val term = Arithmetic.term(
new Arithmetic.lexical.Scanner("pred succ succ succ false")
)
try {
val evaluated = term match {
case Arithmetic.Success(res,next) => Arithmetic.eval(res)
case Arithmetic.Failure(msg,next) => assert(false)
}
} catch {
case TermIsStuck(Succ(False)) => // do nothing, or explicitly mark test as successful
case _ => assert(false) // any other exception should fail the test
}
However, I think the implementation is partially broken - it never returns Arithmetic.Failure, but throws exception. I think the intent of Arithmetic.Failure was exactly to capture such cases. If that's the case, then better implementation would be to replace throw TermIsStuck with Arithmetic.Failure(TermIsStuck(...)).