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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 __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