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1 point by meric 5389 days ago | link | parent

blushes Yes, like that. Now I feel like when I program in lua I do so in an ivory tower.

    Lua 5.1.4  Copyright (C) 1994-2008 Lua.org, PUC-Rio
    > t_mt =  { __index = function(self, k)
    >>         return self.x + k
    >>     end,
    >>     __newindex = function(self, k, v)
    >>         self.y = k + v
    >>     end }
    > 
    > function foo(x)
    >>     return setmetatable({x = x, y = 0}, t_mt)
    >> end
    > 
    > bar = foo(5)
    > print(bar[10]) --> 15
    15
    > 
    > bar[20] = 5
    > print(bar.y) --> 25
    25


1 point by fallintothis 5388 days ago | link

  return setmetatable({x = x, y = 0}, t_mt)
I find this line really interesting. If I'm reading it right, you can alter the metatable (operator overloading) on the fly? So you could pass in a table as a parameter, set a metatable, play with some operators, then set a different metatable to overload operators differently?

In Python, the operators are kind of tied to specific classes. I guess you can go about casting and converting and inheriting and monkeypatching and such, but I don't think it's as straightforward as just setting a new metatable. Admittedly, I can't think of when I've ever needed to alter operator-overloading dynamically like that, so I've never really tried. Do you know if it's useful in Lua code? Or am I completely off-base here?

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1 point by meric 5388 days ago | link

Hmm, the only times I've used it as where I'd be casting objects in other languages... I've used it to 'cast' between 'tables' doing similar things. like `my1.rect` to `my2.rect`, if they both use .x, .y, .width and .height to store their coordinates. (Not sure that's a good practice because they can change.) Yes, you can alter metatables any time, but I don't do it that often.

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