Re: [Proposal] ## operator

[Tom N Harris <telliamed@...>]

On Saturday, November 22, 2014 11:28:01 PM Thiago L. wrote:
> On 22/11/14 10:04 PM, Tom N Harris wrote:
> > So what you're really saying is if you want to track the number of items
> > in a table, the best way is to code your own item-counting class using
> > metamethods.
> Doesn't solve the problem for arbitrary input.

Then the problem is overloading plain tables for uses beyond its intrinsic 
contract. You're asking for a specific policy (items must be countable) but 
want to accept tables that don't promise that policy. If you need countable 
tables, then only accept countable tables. Or convert uncounted tables to 
countable tables when encountered, or fast-path countable tables but fall back 
to slow iteration of uncountable tables.

That's what the comparison to Java elucidates. Advanced behavior comes about 
by creating classes. In Java they're necessary and unavoidable, so you put 
needed policy in methods without giving it a second thought. But Lua classes 
are optional so it's easy to overlook when a class, or something class-like, 
would be prudent. Java doesn't have a built-in associative array; it's 
implemented as a class. Lua doesn't have a built-in countable container; it 
must be implemented with metamethods.

Saying "but what do I do if the user passes a plain table to my Lua function?" 
is like saying "but what do I do if the user passes a plain Array to my Java 
function?" except you're told explicitly what happens by the type system.

-- 
tom <telliamed@whoopdedo.org>