Re: [Feature Request?] __key

[Tim Hill <drtimhill@...>]

> On Jul 15, 2016, at 5:16 AM, Soni L. <fakedme@gmail.com> wrote:
> 
>> 
>> For example, what happens when __key starts returning different values each time it is used? What happens if it returns a very restricted range of keys (and hence stresses Lua’s hash collision processing)? How are collisions avoided for __key values? Remember that __key isn’t returning a hash, it’s returning the definitive unique identifier for that userdata, and Lua assumes these are guaranteed unique for a given type (the hash need not be, of course).
>> 
>> —Tim
>> 
> We can't have __hash because the reference manual doesn't specify how tables are implemented. For all we know, there could be a Lua table implementation that uses tries, arrays and binary search trees, instead of an array and a hashtable. __key provides a __hash-like interface without breaking such implementations.
> 
> Also, if implemented like __gc (a flag on the object), testing for __key is cheap.
> 

I think you missed my point. The contract for a table is that each value placed in a table has a unique unchanging identity within it’s type. For a string its just the string text, for a number the number etc. For userdata its the pointer. For __key to work, it must abide by this contract. The following problems then arise:

— What is the type of the return value? It can’t be any existing Lua type, since this is *already* used as the identity of a value of that type. In other words, if __key returns (say) 100 as the key, then that will collide with using the number 100 in the same table. Not good. In fact, the only thing that __key could conceivably return would be another userdata pointer. And if you analyze this carefully, you will see if doesn’t buy you any functionality over what you already have.

— If __key returns a different value when called multiple times on the same userdata, strange things will happen, possibly including Lua crashing, since this is unexpected behavior for any Lua value. Not good.

— If two distinct userdata’s return the same type+value for __key Lua will think they are the same value and bad things will happen if they are put in the same table, possibly crashing Lua. Not good.

—Tim