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Re: ipairs in Lua 5.3.0-alpha

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On 2014年8月19日 15:38:58 +0200
Jan Behrens <jbe-lua-l@public-software-group.org> wrote:
> extending ipairs in such way to accept functions (and possibly creating
> an easy to use API interface for ordinal iteration on functions AND
> tables) would encourage library programmers to create interfaces that
> do not distinguish between (function) iterators and (table) sequences.
With an API interface, I mean something like this:
#define luaL_geti(L, idx, i) ( \
 (*(i))++, \
 lua_type((L), (idx)) == LUA_TFUNCTION \
 ? (lua_pushvalue((L), (idx)), lua_call((L), 0, 1)) \
 : (lua_rawgeti((L), (idx), *(i))), \
 lua_isnil((L), -1) \
 ? (lua_pop((L), 1), LUA_TNIL) \
 : lua_type((L), -1) \
 )
(This is just a draft. A real implementation might want to check
types for safety and/or respect the __call metamethod.)
This would allow something like:
> function printcsv(seq)
> for i, v in ipairs(seq) do
> if i > 1 then io.stdout:write(",") end
> io.stdout:write(v)
> end
> io.stdout:write("\n")
> end
also in the C world:
static int printcsv(lua_State *L) {
 int i;
 luaL_checkany(L, 1);
 for (i=0; luaL_geti(L, 1, &i); lua_pop(L, 1)) {
 if (i > 1) fputs(",", stdout);
 fputs(lua_tostring(L, -1), stdout);
 }
 fputs("\n", stdout);
 return 0;
}
The resulting printcsv functions (both the Lua and the C variant) would
then accept either (function) iterators or (table) sequences. It
wouldn't matter for the caller.
Regards
Jan

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