Re: Lua 5.2 wish item: Splicing

[Karel Tuma <ktuma@...>]

hi,

this is already easily doable,
table constructor just for tab = { func() }
fills the table with *all* the return values of a function.
all you have to do is "flatten" the introduced tree structure.
in lots of data describing i'm doing in lua, i've to use the "define and
normalize" paradigm to establish functionality i need.
note that flatten() can be called from a __newindex metamethod
defined on the environment of loadfile("config.lua").
also doing some sanitary checks during __newindex step might be good idea.

here's somewhat simplified example
----------------------------
function b()
        return "foo", "bar"
end

t = {
        a = 1,
        { b() },
        c = 2,
}

-- this must be done two-phase using temporary table,
-- modifying table keys while in ipairs() is just too tricky for us
function flatten(t)
        local tmp = {}
        for si,sv in ipairs(t) do
                for _,v in ipairs(sv) do
                        table.insert(tmp, v)
                end
                t[si] = nil
        end
        for _,v in ipairs(tmp) do
                table.insert(t, v)
        end
end
flatten(t)
for k,v in pairs(t) do
        print(k,v)
end
--------------------------
$ lua t.lua
1       foo
2       bar
c       2
a       1


On Thu, Nov 09, 2006 at 07:05:56PM +0100, Mark Hamburg wrote:
> When using Lua for more or less data description tasks, I'm frequently
> finding myself wishing for a splice operator that would allow one to use all
> of the results of a function when building a table. My particular example
> comes up when building view descriptions. For example, the following works
> in that makeChildren can return all of the items that it wants to:
> 
>     column{
>         spacing = 10,
>         makeChildren()
>     }
> 
> But the following does not work:
> 
>     column{
>         spacing = 10,
>         makeChildren(),
>         view{
>             title = "footer",
>         }
>     }
> 
> Now I only get the first child. Coding around this requires using temporary
> variables:
> 
>     local t = makeChildren()
>     t.spacing = 10
>     t[ #t + 1 ] = view{ title = "footer" }
>     column( t )
> 
> That approach doesn't work well when nested down within the description
> because I have to move the work with t out of the structural description
> which then makes the construction process harder to follow.
> 
> I'm thinking that something along the following lines would be useful:
> 
>     column{
>         spacing = 10,
>         $makeChildren(),
>         view{
>             title = "footer",
>         }
>     }
> 
> Where the $ means that we take all of the results from the function rather
> than just the first. (And if it has no results, we also reflect that.) I
> would also be quite open to other syntaxes.
> 
> Mark
> 
>