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

[Xavier Wang <weasley.wx@...>]

2011/9/17 Dirk Laurie <dpl@sun.ac.za>:
> 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
>
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There is a problem. we can not use a tuple to call function, if a is a
tuple(1,2,3), so f(a) is the same as f(1,2,3) is normal, and f(a, a)
is the same of f(1,2,3,1,2,3) is normal, We can not do that if tuple
is just a userdata.

I think we can introduce the data type tuple in Lua 5.2, it can be
first-class type or not (I prefer not), I like the solution by
somebody in list, that use a special syntax for tuple, maybe use
var... is better. and makes tuple is only a range of stack.

there are two definition of tuple: a first-class datatype or a range
of stack. all has advantage and disadvantage, we can disscuss that,
but first we need a plan to add light-weight tuple to Lua language.