Re: Implicit Table Indexing/Creation

[Gavin Kistner <gavin@...>]

On Feb 22, 2007, at 6:37 PM, William Ahern wrote:
> I thought I had spotted a patch awhile ago which came across this  
> list.
> It basically allowed for syntax like:
>
> 	local t, v;
>
> 	v = t.something.innerthing
>
> In this case v would be nil, and the interpreter wouldn't get upset.
> Likewise,
>
> 	t.something.innerthing = "foo"
>
> would implicitly create the table t.something.
>
> Is there a patch against 5.1 for this? Basically, amongst other less
> shallow reasons, I'd much rather be able to do
>
> 	proc.slaves.max	= 10
> 	proc.slaves.min	= 3
>
> rather than
>
> 	proc		= {}
> 	proc.slaves	= { max = 10, min = 3 }
>
> which is still nicer than
>
> 	proc	= {
> 		slaves	= {
> 			max	= 10,
> 			min	= 3
> 		}
> 	}
>
> Abstractly the namespace of the configuration is hierarchical, but
> usually its nice to pretend its flat. And to do that the language  
> needs
> to play along.

This doesn't exactly match your needs, but perhaps it's close enough?

Lua 5.1  Copyright (C) 1994-2006 Lua.org, PUC-Rio
> autotable = {
>>   meta = {
>>     __index = function(t,k)
>>       local auto = autotable()
>>       t[k] = auto
>>       return auto
>>     end
>>   }
>> }
> function autotable:new( )
>>   local t={}
>>   setmetatable(t,self.meta)
>>   return t
>> end
> setmetatable( autotable, { __call = autotable.new } )

> proc = autotable()
> proc.slaves.max = 10
> proc.slaves.min = 3
> for k,v in pairs( proc ) do print( k,v ) end
slaves  table: 0x30c260
> for k,v in pairs( proc.slaves ) do print( k,v ) end
max     10
min     3
> _ = proc.oops
> for k,v in pairs( proc ) do print( k,v ) end
oops    table: 0x30d080
slaves  table: 0x30c260


As you can see, you a) need to tell it when you want an autotable.  
(You can't just say "local t" and have dereferencing it automatically  
make it into t table. That's just crazy talk, IMO.) Also, simply  
asking for a property necessarily instantiates it.