table.concat and metamethods

[Dirk Laurie <dirk.laurie@...>]

The manual says:

> table.concat (table [, sep [, i [, j]]])
> Given an array where all elements are strings or numbers, returns
>      table[i]..sep..table[i+1] ··· sep..table[j].

It says nothing about what happens with elements of other types.
Can one make them work with the aid of metamethods?  Let's try.

        function Concat(x,y) return tostring(x)..tostring(y) end
        debug.setmetatable(false,{__concat = Concat})
        t={true,false}
        print(t[1]..','..t[2])
    true,false
        print(table.concat(t),'))
    stdin:1: invalid value (boolean) at index 1 in table for 'concat'

So the standard implementation does not use metamethods.  A glance
at `ltablib.c` confirms that `addfield` is used, a function with no
other task in the 5.2.1 source code.

It is not hard to patch `ltablib.c` to use `lua_concat` instead.
If no metatables are supplied, the behaviour of `table.concat` is
still the same, except that the error message changes to

    attempt to concatenate a boolean value

However, assigning just this one metamethod:

        getmetatable"".__concat = Concat

makes `table.concat` work for anything, because `sep` (even when
defaulting to "") causes the metamethod to be invoked except when
Lua "knows what to do".  And the debug library is not used.

I append my proof-of-concept code (I do not have the temerity to upload
it to the Wiki LuaPowerPatches page) as a patch to only `ltablib.c`.

Should this be default behaviour?  Now that the table library respects
__len, it is maybe not too far-fetched to suggest that it might also
respect __concat.

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