value semantics and __hash metamethod (?)

[Lorenzo Donati <lorenzodonatibz@...>]

Hi all!

Recently I needed to use "objects" (tables holding pairs of numbers) as 
indexes in a table, but using value semantics, not object identity.

I searched the wiki and the list archives and I understood (I hope :-) ) 
that the usual technique in such cases is to map the object to a unique 
string representation (unique in the sense that every distinct value 
must have a distinct representation and viceversa) and use that 
representation as the index.

However, coming from a Java world, I realized that this could be 
conceptually also be solved introducing a new metamethod (say __hash) to 
"override" default hash calculation.

I wonder why there is no such metamethod. Would it cause problems or 
impact too heavily (code complexity or speed?) on current Lua 
implementation. Or is it just not so useful as I think?

In Java I used to introduce "value classes" almost everywhere (Java 
parlance for classes of objects with value semantics), so I'm curious 
why in Lua, given that there is an __eq metamethod, there is no way to 
override hash calculation (since for value semantics equality and hash 
computation should behave coherently).

Are objects with value semantics in Lua not so heavily used as table 
indexes?

I'm really curious to know how people solve this problem usually.
Am I missing something, or there are other techniques to achieve the 
same result? Although I find the string representation technique valid 
and simple, there are cases IMHO where it would introduce relevant 
overhead, especially when the internal "object" representation cannot be 
the same as the "string key" representation, so they cannot be "merged"

Thanks!

-- Lorenzo