Golua – A Lua 5.3 engine implemented in Go

Re:Golua – A Lua 5.3 engine implemented in Go by wahern on Friday November 16, 2018 @01:44PM (#18470899)

Why do you prefer that over

  lua_pushstring(L, "five");
  lua_pushnumber(L, 5);

The above is shorter and also significantly more efficient because you're not creating the intermediate table object. Even in Go, given Lua's coroutine and tailcall semantics (assuming they were implemented) the Go compiler would likely be forced to always heap allocate the argument list table. There's a reason PUC Lua is blazingly fast, and its partly the same reason why LuaJIT is so fast--the Lua stack protocol is a thin abstraction that easily admits a simple, efficient implementation yet still managing to provide an effective interface boundary that doesn't leak (beyond the fact of the protocol itself).

I don't think there's any good middle ground here. The best options are (1) Lua's explicit stack protocol or (2) some sort of type inference that permitted a direct call, like lua_tcall(L, "five", 5), into the VM. You can actually implement the latter in C somewhat easily (at least if you stick to simple data types) by using _Generic and __VA_ARG__ macros to generate the stack pushing code.

But I rarely see this done because it's a lot of magic for little gain. And where I have seen it done it's always been annoying because it's too leaky--invariably you have to fallback to the stack pushing code because there's a limit to the types of automagic type coercions you can accomplish between two drastically different languages. So you have to learn their magical wrapper routines in addition to the regular Lua API.

Many years ago I abused __VA_ARG__, __builtin_types_compatible_p, and libffi so I could invoke ad hoc functions as libevent callbacks without having to proxy the call through a function that unwrapped a pointer-to-void cookie. See http://25thandclement.com/~william/projects/delegate.c.html I still think it's kinda clever but I can't remember the last time I actually used it. Even in code bases already using delegate.c I ended up returning to the slightly more repetitive but transparent and idiomatic patterns for installing libevent events.

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Re:Golua – A Lua 5.3 engine implemented in Go by omaranto on Friday November 16, 2018 @04:37PM (#18472172)

I'm guessing the code was intended to be equivalent to:

    lua_newtable(L);
    lua_pushstring(L, "five");
    lua_pushnumber(L, 5);
    lua_settable(L, -3);

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Re:Golua – A Lua 5.3 engine implemented in Go by wahern on Friday November 16, 2018 @05:44PM (#18472547)

Ah, I see. So it's more a preference for a richer standard library, like,

  lua_newtable(L);
  luaL_setfieldi(L, -1, "five", 5);

rather than having to roll your own. Part of the function of the stack protocol is to minimize the number of unique functions needed to load and store values across language boundaries. So there's a single [low-level] function for communicating a number value in a type-safe manner, rather than a multitude of functions--one routine for setting a number value for a table string key, another for a number value for a table number key, another for a string value for a table number key, etc. The permutations explode, and unless the host language has some sort of generics capability then it adds significant complexity both to the interface and to implementation maintenance. (See Perl XS.) Alternatively you can reduce the number of permuted interfaces by using printf-style formatting strings and va_lists, but that's not type safe. (See Py_BuildValue.)

Notably, Lua 5.3 removed lua_pushunsigned, lua_tounsigned, luaL_checkint, luaL_checklong, and many similar routines as there was never an end to the number of precomposed type coercing, constraint checking auxiliary routines people demanded. They decided that the only way to win that game was to not play at all. But with C11 _Generic and a dash of compiler extension magic you can actually implement a richer set of type agnostic (but type safe) numeric load/store interfaces in just a handful of auxiliary routines. For better or worse, though, Lua doesn't target C11 nor wish to depend on compiler extensions. "Batteries not included" is a whole 'nother debate in the Lua universe.

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