Re: Safe navigation operator

["Juris Kalnins" <juris@...>]

On Mon, 23 Aug 2010 15:45:25 +0300, Cuero Bugot  
<cbugot@sierrawireless.com> wrote:

> > > local emptytable = {}
> > > local function Opt (t) return t or emptytable end
> > >
> > >   if config.Opt(network).Opt(server).url then
> > >     dosomething(config.network.server.url)
> > >   End
> > That does not make any sense. I do not know what I was thinking. Sorry
> > for the noise...
>
> Well at least it illustrate what I meant by 'context'. Opt() is "in the  
> middle path/table traversal" while the last ".url" is "the final  
> access". If metamethods __index / __newindex did provide this kind of  
> information then  it would be easy to implement safe navigation operator  
> with metatables
> Would that be something completely unrealistic to provide more  
> contextual information as parameters of the __index / __newindex  
> metamethods ?

If you don't care about perfomance, ...

======8<============================

v = {}
mt = {
    __index = function (t, k)
        local key
        local ret = {}
        if t and rawget(t, "_key") then
            key = t._key
            ret._container = t._container
        else
            key = {}
            ret._container = t
        end
        key[#key + 1] = k
        ret._key = key
        setmetatable(ret, mt)
        return ret
    end,
    __newindex = function (t, k, v)
        local key
        if t and rawget(t, "_key") then key = t._key else key = {} end
        key[#key + 1] = k
        io.write("assign ", v, " to")
        local sep = ' '
        for n,i in ipairs(t._key) do
            io.write(sep, i)
            sep = '.'
        end
        io.write(" in ", tostring(t._container), "\n")
    end
}

setmetatable(v, mt)

v.a.b.x.y.n.m = 1

======>8============================

Basically, you create proxy object that builds expression tree it is  
passing through
(in this case, navigation path), and use __newindex as a trigger for  
needed action.

To access value, you need to define all sorts of __eq, __tostring, etc  
methods that
vould return configuration value pointed to by the proxy.

Being able to handle chains of operations in a single metamethod would  
really simplify
such cases, e.g., to have

__newindex(container, value, key, ...)

But I do not see how it is possible to add this without making it  
imcompatible with
existing code.