Re: Building recursive argument lists in a shorter way?

[Andrew Starks <andrew.starks@...>]

On Tuesday, September 24, 2013, Dirk Laurie  wrote:
> 2013/9/23 Andrew Starks <andrew.starks@trms.com>:
>
> > I have many setter methods that handle properties that represent an
> > inventory of one or more objects of the same class (to borrow from
> > OOP).
> >
> > So that I don't need to write a "set_prop" and "set_props" function, I
> > write this quite a bit:
> >
> > local function f(arg, ...)
> >     if (...) then
> >         return arg, f(...)
> >     else
> >         return arg
> >     end
> > end
>
> I assume something non-trivial is being done too, i.e
>
> local function f(arg, ...)
>     if (...) then
>         return arg, f(...)
>     else
>         arg = complicated_function_of(arg)
>         return arg
>     end end

Not really and it wouldn't be the first time I've labored under an ignorance that others shed in chapter 1 of PiL. :P This example might be better:

function f:add_one_or_more_props(args,...)

--do stuff to add props 

if (...) then

  self:add_one_or_more_props(...) 

else

  return self

end 

end

Or another variant: 

function f:add_one_or_more_props(args,...)

  if not args then return self end

  --do stuff

   f:add_one_or_more_props(...) 

end

I think that it's a much simpler question than I communicated and the second version might be my answer, although erroring on the first call, if it doesn't contain arguments, is not possible...

-Andrew

 

> So you seem to be using recursion only for the sake of
> being able to return a vararg list. If you need to stick to
> interpreted Lua, you can do that with coroutines, but it is
> easier IMHO to write an API function for this case.
>
> Such a function is included in the `xtable` module that
> John Hind and I wrote:
>
> https://github.com/dlaurie/xtable
>
> That module itself has not proved to be popular, but you might
> want to extract just 'map'. All it does is to return a variable-length
> list of a function or table lookup applied to all the elements of the
> vararg list.
>
> /* tuple.map(tbl,table_or_monadic_function,...) */
> static void tuple_call(lua_State *L) {
>   int i, top=lua_gettop(L);
>   for (i=2; i<=top; i++) {
>     lua_pushvalue(L,1); lua_pushvalue(L,i); lua_call(L,1,1);
>     lua_replace(L,i);
>   }
> }
> static void tuple_index(lua_State *L) {
>   int i, top=lua_gettop(L);
>   for (i=2; i<=top; i++) {
>     lua_pushvalue(L,i); lua_gettable(L,1);
>     lua_replace(L,i);
>   }
> }
> static int tuple_map(lua_State *L) {
>   if (lua_type(L,1)==LUA_TFUNCTION) tuple_call(L);
>   else if  (lua_type(L,1)==LUA_TTABLE) tuple_index(L);
>   else return 0;
>   return lua_gettop(L)-1;
> }