Interesting syntax but does it makes a bad API?

There's yet another interesting syntax in Scala for methods that look like this:

  def intOrErr(x: String): Int Or ErrorMessage = { ...

where it appears that the return type can really be one of two types. This actually isn't the case. It's Scala's syntactic sugar. It is equivalent to:

  def intOrErr(x: String): Or[Int, ErrorMessage] = { ...

but reads more nicely.

I came across this syntax while looking at a colleague's use of the Scalactic library. Production code returned an Or and he mapped over it. The mistake he made was what if the code had executed the unhappy path? The assertions in his map function would not have executed and the test would have given a green bar, the coder none the wiser that his code was running unhappily.

He should have used a fold but that's not immediately obvious (unlike Java code that throws an Exception where you must catch it or explicitly state that the method calling your code must deal with it).

I posted on this forum to get opinions as to how worrying this is. There's some interesting responses ranging from it's less than ideal to assertions in test code suck and there are libraries to help do it better.