Re: hash metamethod?

["Alex Davies" <alex.mania@...>]

Hey Joshua,

> There is one problem with my proposed scheme that bothers me a bit.  The 
> question is: what would a table return as its default __hash and __eql 
> methods?  On one hand I'd want it to treat the table by value, calling 
> __hash and __eql on all of its individual members, so that tables with the 
> same contents hash the same.

I just want to say definitely not :).  I don't think even a second should be 
spent considering that - this is Lua, not Python.  We're not afraid to call 
two different tables just that - different.  (Don't get me wrong, I think 
Python's great - but I do think that this as default behaviour of its kind 
of weird/pointless, and I cannot see how it could be implemented remotely 
efficiently.)  Similarly simply adding tables as keys (oftenly done) would 
be very slow -potentially many comparisons of equality have to be done on 
each key lookup - tables are all we have, lets keep them fast.

Anyway, a patch for a simple __hash would not be hard to make.  Only a 
couple of places would have to be modified.  Again though, you have to 
remember that tables are integral to Lua's performance - anything that 
bloats or slows them really has to be considered very carefully.  And with 
that in mind, Lua has no extra space in the nodes in its tables to store the 
hash of each key. So whenever the table gets resized, table keys would have 
to be checked for a __hash metamethod and recalled (how does Python handle 
it?).  Along with being slow, this could also cause problems in the future 
in the unlikely case that the gc starts resizing tables.  (Actually I lie - 
if you want to 8 byte align the Node data structure you could fit a hash 
value in there).

Personally I do think a __hash metamethod would be a large improvement where 
Lua is used largely as a standalone language.  As an extension/scripting 
language not quite so important.

Anyway, good luck keep us informed. :)

- Alex