The major difference between Lua 5.1 and Lua 5.2 is the environment a function operates in and this effects globals and modules in a rather big way. You can ch

The major difference between Lua 5.1 and Lua 5.2 is the environment a function operates in and this effects globals and modules in a rather big way. You can change the "global environment" a Lua function operates in quite easily in both Lua 5.1 and 5.2, but the underlying implementation is different between the two, and Mike Pall, the writer of LuaJIT, does not care for the new way globals are handled. There might also be issues with supporting Lua 5.3's integer type [1] in LuaJIT (now you use the C FFI to handle 64-bit integers). At this point, it seems there are two distinct versions of Lua, Lua and LuaJIT. I have some modules I wrote that support both Lua 5.1 and Lua 5.2, so I can still use them with LuaJIT if need be, but it's a pain (conditional code in both C and Lua) and it will only get worse if Lua 5.3 comes out [3]. [1] Up to Lua 5.2, Lua only supported one type of number (internally, a double [2]). Lua 5.3 now supports a distinct integer type, which has its implications (doubles don't wrap; you now get x.0 where before you would get x when printed, etc). The reason for 64-bit integer support is that doubles only support integer operations up to 2^53 or so. [3] [2] Unless you changed the internal representation of numbers in Lua, an easy change to make in the Lua sources. [3] I already had to work around 64-bit integers in some code dealing with process limits on a 64-bit system and Lua 5.1/5.2.