Re: [manual] chaining metamethods

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

On 27/01/16 03:04 PM, Stephan Hennig wrote:
> Hi,
>
> the sentence
>
>    Note that queries for metamethods are always raw; the access
>    to a metamethod does not invoke other metamethods.
>
> , new to the Lua 5.3 manual, is misleading.  The second half of the
> sentence can be interpreted as: An access to a metamethod, which can be
> a table as described later, will not respect that table's metatable.
> Which is not true and which would make chaining metamethods impossible.
> What does not invoke other metamethods is retrieving metamethods from
> the metatable itself.
An access TO a metamethod is raw. The access TO a metamethod is what you 
use to get to the metamethod.

__pairs for example, calls the value using __call.

The access to a metamethod is provided through the metatable. The access 
of a metamethod is provided through the metamethod itself.
> -- A custom __gc metamethod that prints something on the screen.
> local function my__gc(t)
>    print(tostring(t), ' is garbage collected.')
> end
> -- Watch garbage collection of a table via __gc metamethod.
> local t = {}
> local mt = { __gc = my__gc }
> setmetatable(t, mt)
> print(t)
> t = nil
> collectgarbage()
> -- Try again with an empty metatable.  Does Lua respect the
> -- metatable's metatable?
> local t = {}
> local mt = {}
> local mtmt = { __index = { __gc = my__gc } }
> setmetatable(mt, mtmt)
> setmetatable(t, mt)
> print(t)
> t = nil
> collectgarbage()
> -- OK, Lua is disrespectful (to metatables).
>
> I'd suggest something along these lines:
>
>    Note that queries for metamethods are always raw; when
>    looking-up metamethods, Lua doesn't consider a metatable's
>    metatable.  Lua considers metatables of metamethods, though.
>    In fact, chaining metamethods is at the heart of many
>    approaches to object-oriented programming in Lua.
>
> Best regards,
> Stephan Hennig

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