Following is a summary of how close Lua (5.1) is to being virtualizable:
Operation Example Works? Metamethods/comments --------- ------- ------ -------------------- Numbers Arithmetic a + b Yes __add, __sub, __mul, __div, __pow, __unm Equality a == b Limited[*] __eq Ordering a < b Limited[*] __lt, __le Booleans Truth value if a then No Would add overhead to the idiom 'if table[key] then' Boolean Logic not a, a and b No Strings Concatenation a .. b Yes __concat (see below) Get length string.len(a) No[!] Other utility string.find, etc. No[!] Tables Indexing a[b] Yes __index Write to index a[b] = c Yes __newindex Length #a Mostly __len (but requires userdata before 5.2) Iterating for x in pairs(a) No Fix is trivial [#] raw operations rawget(a,k) No array functions table.insert(a,b) No Functions Invoking a(x, y) Yes __call [+] Threads Resuming coroutine.resume(a) No[!] File handles io.type(a), ... No[!] See also I/O library abstraction [@] Misc String repr. tostring(a) Limited __tostring (not honored by C lua_tostring) type function type(a) No
[*] Lua defines an equality or inequality among different types to always yield false. The __eq metamethod also always returns true for values that are raw equal, but this doesn't support the behavior of the value NaN=0/0, where typically NaN ~= NaN.
[+] There are a couple of places in Lua where the __call metamethod is not checked. One of these is the __index metamethod itself (this affects FuncTables).
[!] Native types can have metatables, allowing you to using : notation (eg. aString:find(aPattern)), which is virtualizable. Only some types like strings have the metatable defined by default, but it can be added via debug.setmetatable.
[#] See GeneralizedPairsAndIpairs for virtualizing pairs/ipairs/next. Also see LuaFiveTwo.
[@] LuaList:2008-07/msg00345.html, LuaList:2005-05/msg00178.html
Advantages of virtualization:
Disadvantages:
RiciLake adds:
a .. b where one of a and b is a number and the other one is an object with a __concat metamethod, the number will be converted to a string before the metamethod is called. Also, .. is right-associative, but a has priority if both a and b have __concat metamethods. So presented with a .. b .. c, where all three have metamethods, the sequence will be: ameta.__concat(a, bmeta.__concat(b, c)). Furthermore, in the case of a .. 34 .. 56 the result will be ameta.__concat(a, "3456"). Some of these might be surprising.