Re: Cons, tuple and __slots__ -- a simple mechanism to implement them in Lua

[Dirk Laurie <dpl@...>]

On Fri, Sep 16, 2011 at 10:12:50PM +0200, Javier Guerra Giraldez wrote:
> 
> as much as i'd like to see real tuples in Lua (as the type of ... ), 

Let me take this wish seriously, if only because I hate the kludgy
look of `select'#'`.

Syntax problems:
  - We can use `(1,2,3)` for a tuple with elements 1,2,3, but we can't
    use `(...)` to turn `...` into a first-class value because the
    notation already means something else (the first element of `...`).
  - We can use `[1,2,3]` for a tuple with elements 1,2,3, and by
    analogy with tables would then want to write func[1,2,3], not
    func([1,2,3]).  But how does Lua know that we want to invoke
    __call rather than __index?
Semantic problems:
  - Suppose `a` is a tuple with elements 1,2,3, and `f` a function.
    Is `f(a)` supposed to be equivalent to `f(1,2,3)`?
  - `...` is a name for a group of entries on the local stack.
    It is not a first-class value any more than `a` is a first-class 
    value when you say `local a`.  

Solution:

Syntax:
  - `tuple(...)` turns `...` into a first-class value.
  - `totuple(a)` turns a proper list into a tuple.
Semantics:
  - `tuple` is a userdata, with metamethods to implement __index,
    __len, __concat (maybe more?)
Implementation:
  - a nice little add-on module written in C

Wait — surely someone has done almost exactly this already?

Dirk