[twanvl]

> I'd be interested to see what a sufficiently strong QuickCheck specification of this problem would look like.

I would write something like this in haskell:

    spec :: [Integer] -> Property
    spec xs =
       length xs <= 2  ==>  fun (intercalate "," (map show xs)) == sum xs

This captures the three requirements, but not the implicit fourth requirement that the function throws an exception for other inputs.

[tom_mellior]

Nor does this exercise the trimming of the substrings, for example. This is good for testing the happiest path, I agree. I was interested in the tedious testing of all the unhappy paths.

[masklinn]

> not the implicit fourth requirement that the function throws an exception for other inputs.

You could probably generate invalid inputs by taking a list of strings as input. Though of course at that point the property test has to reimplement half the function.

That's an issue I often end up having with property tests: the oracle for interesting properties is as complex as the SUT, so you end up with two of them.