Re: Guessing the expected result type from an __index function

[Philipp Janda <siffiejoe@...>]

Am 13.05.2013 11:01 schröbte Jean-Luc Jumpertz:
> Le 13 mai 2013 à 09:38, Jerome Vuarand <jerome.vuarand@gmail.com> a écrit :
>
> > You don't always know at index time how the returned value will be
> > used. Consider for example:
> >
> > local obj = get_one_of_your_objects()
> >
> > local save = obj.save
> > [...]
> > With your hack, it's no longer possible to call the method in that way
> > (which I admit is very unusual).
>
> I am aware of this theoretical case. But it doesn't make much sense
> in general in an object-oriented code to extract a method function at
> some point and apply it later to an arbitrary object. And this is
> even more true when it concerns a bridge to an external compiled
> language. So I really don't consider this as an issue.

I've used something like this before[1][2], so it's not entirely 
theoretical.

  [1]:  http://lua-users.org/lists/lua-l/2010-02/msg00570.html
  [2]: 
https://github.com/siffiejoe/lua-prototype/blob/master/src/prototype.lua#L100

First-class functions are not nearly as useful if you can't get at the 
function values ...

> And it would be much better if some type of hint about the
> foreseeable usage of a field could be passed as an extra parameter to
> the index metamethod in a future version of Lua. This would allow the
> index metamethod to return different (but related) data for a same
> key depending of the field expected usage.

I'd rather see extra parameters to the __index metamethod for the 
following case:

    local v = matrix[ 1, 2 ]

Philipp