Re: Think different
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- Subject: Re: Think different
- From: Daurnimator <quae@...>
- Date: 2015年8月31日 14:36:59 +1000
On 31 August 2015 at 10:56, Soni L. <fakedme@gmail.com> wrote:
> Third: Stacks.
>
> There are 2 ways to do a stack object in Lua: Either you use tables (PITA),
> or you use coroutines: (code may or may not be correct - other examples
> available online)
>
> local function stackinternals(op, x, ...)
> if op == "push" then
> local a, b = coroutine.yield()
> return stackinternals(a, b, x, ...)
> elseif op == "pop" then
> local a, b = coroutine.yield(...)
> return stackinternals(a, b, select(2, ...))
> elseif op == "peek" then
> local a, b = coroutine.yield(...)
> return stackinternals(a, b, ...)
> end
> end
> function newstack(...)
> local co = coroutine.wrap(function(...)
> local a, b = coroutine.yield()
> return stackinternals(a, b, ...)
> end)
> co(...)
> return co
> end
>
> (It's also A LOT faster than a table-based implementation, and if done
> in-thread instead of using coroutines it's even faster!)
This is an interesting one.
I was playing around creating different things using this style, and
discovered a real need for a 'settop' like function from lua.
That is, a function that removes trailing vararg values.
I was surprised to find that it's hard/slow to implement in lua alone
for arbitrary `n`,
It's very simple to write from C:
```C
#include <lua.h>
#include <lauxlib.h>
static int settop(lua_State *L) {
int n = luaL_checkinteger(L, 1);
luaL_argcheck(L, n >= 0, 1, NULL);
luaL_checkstack(L, n+1, NULL);
lua_settop(L, n+1);
return n;
}
int luaopen_settop(lua_State *L) {
lua_pushcfunction(L, settop);
return 1;
}
```
It wouldn't be a bad addition to the lua base library.
A few interesting uses of it:
- acts like parenthesis (...) discarding unwanted arguments without
temporaries in a call: `foo("bar", settop(1, somestring:gsub(".",
qux))))`
- but with the benefit of working with any number of arguments
instead of just 1
- getting a list of nils for preallocating a table: `{settop(100)}`
(see http://lua-users.org/lists/lua-l/2009-06/msg00438.html)