Lua Five Alpha To Beta

wiki

Incompatibilities

Soon we will release Lua 5.0 beta. Here is a list of incompatibilities from 5.0 alpha to 5.0 beta. (Most of them relate to coroutines; this is an area that is still green in Lua.)

• the API to weak tables is back to the beginning: no more setmode/getmode; it uses the field "__mode" in the metatable.

• as we have anticipated, there are no more __gettable/__settable metaindices. They have been unified with __index/__newindex.

• Threads are a new type in Lua. (That was necessary to solve the problem of garbage-collecting coroutines.) The call

LUA_API lua_State *lua_newthread (lua_State *L);

leaves the new thread on the stack. All threads but the main one are garbage collectible. For coroutines, this is just what we need. Other multi-threaded systems (like LuaThreads??) may store threads in the registry, for instance, to avoid their collection.

• The API for coroutines changed too. "coroutine.create" now returns a coroutine (an object of type "thread"), and not a function. There is an explicit "coroutine.resume" to resume a coroutine. We feel that this API is simpler for most people to understand. The old functionality is still available as "coroutine.wrap", that creates a coroutine and "wraps" it inside a function (closure), that resumes the coroutine each time it is called.

• "coroutine.resume" can pass parameters to yield. "coroutine.create" (and "coroutine.wrap") now take only one argument, the coroutine body. The fist time you resume it, the extra arguments to "resume" (or the arguments to the closure, when you use "wrap") go as the parameters to the body. Next time you resume again the extra arguments go as the results from "yield". For instance:

x = coroutine.create(function (a,b,c) print(a,b,c) print(coroutine.yield()) print(coroutine.yield()) end)

coroutine.resume(x, 1, 2, 3) --> 1, 2, 3 coroutine.resume(x, 10) --> 10 coroutine.resume(x, 4, 5) --> 4, 5 print(coroutine.resume(x)) --> false cannot resume dead coroutine

• coroutine.resume works in "protected mode", like pcall. Its first return is true or false (absence or presence of errors). If true, the other results are the arguments to yield, or the return of the body. If false, the other argument is the error message. (The function returned by "wrap" does not work in protected mode.)

• the macro LUA_USERSTATE now opens an extra space *before* the area pointed by lua_State.

-- Roberto

New Features

Here is a list of novelties in 5.0 beta (compared to 5.0 alpha). They should create no incompatibilities in "normal" programs.

• The compiler does not change the order of operands for "commutative" operators. (I don't remember why, but I remember there was quite a discussion about that in the list.) (However, "a>b" is still translated to "b<a", and "a>=b" to "b<=a".)

• new facility to precompile Lua code from inside Lua (function stringdump).

• option "-l" in lua.c does a "require" (and therefore searches LUA_PATH), instead of a simple dofile.

• reorganization of object headers plus macros/documentation towards an incremental garbage collector. We plan to have the incremental collector in 5.1 (to be released as soon as possible, but only after 5.0 ;-).

• support for yields inside line/count hooks.