[amalcon]

Technically, any data on your computer is represented as an integer. Integer subtraction is simply not guaranteed to be meaningful as a distance operation for all encodings.

For most data types that you're likely to index by (numbers, strings in alphabetical order, dates, tuples of these things), it's quite possible to build a meaningful distance operation which can be used for this. If you're indexing by something for which its difficult to do this (graphs, versioned strings), you should probably rethink your data structure.