Ah, that would be specific to the job. If your doing a sum, maybe 0, or if you're building a list, it'd be an empty list. I suspect that a good default would just be whatever the first value generated by the mapper is, in which case the first value of any key wouldn't be folded, but rather just set in the hash table. But that is all job specific. If you can recommend a better solution, I'm all ears.
Furthermore, I should have explained the types better. As described, the HashFold framework would require the output of the map, the arguments of the fold, and the output of the fold all be the same type. This isn't as limiting as it sounds and if it turns out to be limiting for some problem domains, there are solutions to each of those.
