> 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.

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.

> 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.