Re: ipairs in Lua 5.3.0-alpha

[Jan Behrens <jbe-lua-l@...>]

On Tue, 19 Aug 2014 19:15:47 +0200
Jan Behrens <jbe-lua-l@public-software-group.org> wrote:

> On Tue, 19 Aug 2014 18:01:00 +0200
> Jan Behrens <jbe-lua-l@public-software-group.org> wrote:
> 
> > On Tue, 19 Aug 2014 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.)
> 
> And here the "real" implementation (which is doing type-checks,
> respecting __call, and which does not evaluate lua_type for every
> iteration step):
> 
> ==================================================
> #define LUA_ITERTYPE_FUNC 1
> #define LUA_ITERTYPE_META 2
> #define LUA_ITERTYPE_RAW  3
> 
> typedef struct {
>   lua_State *L;
>   int itertype;
>   int i;
> } luaL_Iterator;
> 
> static void luaL_iterinit(lua_State *L, luaL_Iterator *iter, int idx) {
>   iter->L = L;
>   if (luaL_getmetafield(L, idx, "__call")) {
>     iter->itertype = LUA_ITERTYPE_FUNC;
>   } else if (lua_type(L, idx) == LUA_TFUNCTION) {
>     lua_pushvalue(L, idx);
>     iter->itertype = LUA_ITERTYPE_FUNC;
>   } else {
>     if (luaL_getmetafield(L, idx, "__index")) {
>       lua_pop(L, 1);
>       iter->itertype = LUA_ITERTYPE_META;
>     } else {
>       luaL_checktype(L, idx, LUA_TTABLE);
>       iter->itertype = LUA_ITERTYPE_RAW;
>     }
>     lua_pushvalue(L, idx);
>   }
>   iter->i = 0;
> }
> 
> static int luaL_iternext(luaL_Iterator *iter) {
>   lua_State *L = iter->L;
>   int i = ++iter->i;
>   switch (iter->itertype) {
>   case LUA_ITERTYPE_FUNC:
>     lua_pushvalue(L, -1);
>     lua_call(L, 0, 1);
>     break;
>   case LUA_ITERTYPE_META:
>     lua_pushinteger(L, i);
>     lua_gettable(L, -2);
>     break;
>   case LUA_ITERTYPE_RAW:
>     lua_rawgeti(L, -1, i);
>     break;
>   default:
>     abort();  // should not happen
>   }
>   if (lua_isnil(L, -1)) {
>     lua_pop(L, 2);
>     return 0;
>   }
>   return i;
> }
> ==================================================
> 
> 
> The invocation is a bit more clumsy (but avoids unnecessarily
> evaluating lua_type and luaL_getmetafield for every iteration step):
> 
> static int printcsv(lua_State *L) {
>   luaL_Iterator iter;
>   int i;
>   luaL_checkany(L, 1);
>   for (luaL_iterinit(L, &iter, 1); (i = luaL_iternext(&iter)); lua_pop(L, 1)) {
>     if (i > 1) fputs(",", stdout);
>     fputs(lua_tostring(L, -1), stdout);
>   }
>   fputs("\n", stdout);
>   return 0;
> }
> 
> 
> If that's too clumsy, one could also do something like this:
> 
> #define luaL_loop(L, iter, idx, i) \
>   for ( \
>     luaL_iterinit((L), &(iter), (idx)); \
>     (i = luaL_iternext(&(iter))); \
>     lua_pop((L), 1) \
>   )

Sorry, this should have been:

#define luaL_loop(L, iter, i, idx) \
  for ( \
    luaL_iterinit((L), (iter), (idx)); \
    (*(i) = luaL_iternext(iter)); \
    lua_pop((L), 1) \
  )

for the example below to work.

> 
> And then write:
> 
> static int printcsv(lua_State *L) {
>   luaL_Iterator iter;
>   int i;
>   luaL_checkany(L, 1);
>   luaL_loop(L, &iter, &i, 1) {
>     if (i > 1) fputs(",", stdout);
>     fputs(lua_tostring(L, -1), stdout);
>   }
>   fputs("\n", stdout);
>   return 0;
> }
> 
> 
> It still works fine:
> 
> % lua52 -l printcsv
> Lua 5.2.3  Copyright (C) 1994-2013 Lua.org, PUC-Rio
> > printcsv{"a", "b", "c"}
> a,b,c
> > printcsv(assert(io.open("testfile", "r")):lines())
> line 1 in testfile,line 2 in testfile
> 
> 
> Regards
> Jan
>