Re: rethinking method calls with __mcall metamethod rather than __index/__call

[Mark Hamburg <mark@...>]

I strongly endorse this general concept and had been planning on  
writing something similar up. The set example is a really great  
example. The key point I'd thought about had to do with allowing one  
to construct object systems in which one could readily detect failure  
to use a colon when sending messages. It also allows for mixing  
efficient method inheritance with property accessors because __mcall  
could lead to a table of methods while __index could lead to a  
function that did the property accessor logic. Finally, it makes it  
easier to write methods that can know to what object they are being  
applied because they can't be accessed as "loose" functions. That's a  
help when writing efficient bridges to native code.

The semantics of __mcall would be something like the following (which  
essentially corresponds to the self opcode):

	function mcallprep( t, k )

		local mt = getmetatable( t )
		local mcall = mt and mt.__mcall

		if mcall then
			if type( mcall ) == 'table' then
				return mcall[ k ], t
			else
				local f, o = mcall( t, k )
				return f, o
			end
		end

		return t[ k ], t -- Uses standard lookup

	end

There should be discussion about whether this should first look at  
rawget( t, k ) before turning to the metamethods. I chose not to here,  
but that was essentially an arbitrary choice. The calling logic for  
the case where mcall isn't a table also potentially needs discussion  
but is designed to match with the expected behavior of the opcode it  
virtualizes. This also probably needs thought as to how one builds  
inheritance hierarchies.

Finally, one could argue that adding this feature essentially  
necessitates method currying and the obj:[ method ] extensions because  
otherwise one would lose the ability to call arbitrary methods based  
on a parameter or to cache the results of a method lookup.

Mark

P.S. I sometimes class all of this stuff together under the label  
"embracing the colon operator" -- i.e., stop wishing that period did a  
message send and embrace the colon operator.