Ultimately the OP simply wanted to be able to index a data structure using a user defined n-tuple key. So far I've only heard two suggestions: 1) radically change the typing semantics of Lua with constant tables, or 2) implement the abstract interface using nested tables and a function accessor.
I suggested a library that would generate a userdata “tuple key” for a given table sequence. This required neither changes to the language nor nested tables, though it did need some C code to generate the userdata tuples.
—Tim
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