Re: Metamethods: fallbacks or overrides?

["Thiago L." <fakedme@...>]

On 17/08/14 03:09 PM, Philipp Janda wrote:
> Am 17.08.2014 um 19:42 schrieb Dirk Laurie:
> > Originally, metamethods were called "fallbacks". The first name
> > change, to "tag methods", came with Lua 3.0. In Lua 5.0, the
> > name in the manual was changed to "metamethods", but "tag
> > methods" still appears the code (ltm.h).
> >
> > The idea, in the words of Roberto [1], is that:
> >
> >     In general, arithmetic metamethods are only called when
> >      Lua "does not know what to do".
> >
> > I.e. these metamethods are fallbacks. You no longer get an error,
> > you get the result of the metamethod.
> >
> > In Lua 5.2, metamethods were introduced that are more like
> > overrides. Lua does know what to do, but you can tell it to
> > something else.
> >
> > __len: preferred to the built-in code for the # operator
> > __pairs: preferred to the standard library "pairs" function
> > __ipairs: preferred to the standard library "ipairs" function
>
> Don't forget:
> __tostring
I thought __tostring was a fallback... (Lua doesn't know what to do, but 
instead of throwing an error it returns the memory address or a hash or w/e)
> __gc
> and to some extent
> __index
> __newindex
> __eq
>
> > Lua 5.3.0-alpha removes __ipairs, so it seems at this stage
> > that overrides is not a preferred direction for future Lua
> > development.
>
> In some cases overrides can't be avoided because Lua can't figure out 
> if an operation is invalid (e.g. #t always returns a value even for 
> non-sequences). The same is true for `tostring(v)`, `ipairs(t)`, and 
> `pairs(t)`. Given the above list I guess Roberto's quote really is 
> aimed at arithmetic metamethods only.
> I think it's pretty obvious the Lua authors want to remove `__ipairs` 
> while still allowing sequence-like objects with embedded nils (that's 
> why `__len` is involved). I honestly can't tell why.
>
> > [1] <http://lua-users.org/lists/lua-l/2012-06/msg00246.html>
>
> Philipp