Re: [Feature Request?] __key

["Soni L." <fakedme@...>]

On 15/07/16 04:02 AM, Tim Hill wrote:
> > On Jul 14, 2016, at 1:57 PM, Sean Conner <sean@conman.org 
> > <mailto:sean@conman.org>> wrote:
> >
> > It was thus said that the Great Soni L. once stated:
> >
> >  Oh, so to clarify (kind of making the bigint library up):
> >
> > bi = require "bigint"
> >
> > x = bi.make(2^128) -- syntax issues but I want something
> > y = bi.make(2^128) -- really really large
> >
> > t = {}
> > t[x] = "booya!"
> > print(t[y])
> > booya!
> >
> >  As it stands now, Lua will treat x and y as distinct values whereas Soni
> > doesn't consider them distinct values (because they're distinct 
> > uservalues).
> >
> >  Oddly enough, Common Lisp *may* treat to values as distinct, i.e. 2 does
> > not necessarily eq 2 in CL (they may eql and equal, but not eq).  Common
> > Lisp gets around this by using different forms of equality testing.
>
> Why isn’t this just an exercise in interning, and so just a property 
> of the (hypothetical) bigint library? That is, for any given big 
> integer X, the returned user data is always the same. This also makes 
> equality cheap. Of course there are implementation issues (several 
> occur to me), but it avoids the nasties around __key...
>
> 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.

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